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Holocene stratigraphy and sedimentary processes in Santa Barbara Basin: Influence of tectonics, ocean circulation, climate and mass movement

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Holocene stratigraphy and sedimentary processes in Santa Barbara Basin: Influence of tectonics, ocean circulation, climate and mass movement

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Content HOLOCENE STRATIGRAPHY AND SEDIMENTARY PROCESSES IN SANTA BARBARA BASIN: INFLUENCE OF TECTONICS, OCEAN CIRCULATION, CLIMATE AND MASS MOVEMENT by Scott Ellis Thornton A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fullfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Geological Sciences) October, 1981 UMI Number: DP28558 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMI Dissertation Publishing UMI DP28558 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES. CALIFORNIA 9 0007 / This dissertation, written by Scott Ellis Thornton under the direction of h..is.. Dissertation Com mittee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillm ent of requirements of the degree of D O C T O R O F P H I L O S O P H Y Dean 'ATIDN COMMITTEE Chairman DEDICATION This dissertation is dedicated to the memory of my father Charles Sherman Thornton, Jr., who imparted his ambition, curiosity, and zeal for the pursuit of the unknown to me in no small measure. In addition, this dissertation is dedicated to the memory of my former dissertation committee member, the late Dr. Richard 0. Stone, whose direction, intellectual advisement, encouragement and fine sense of humor has forever left its mark on me. ACKNOWLEDGMENTS Choice of a mentor has a large effect on one’s success. It is for that reason that I gratefully acknowledge the role which Dr. Donn S. Gorsline, my advisor and mentor, has played in my intellectual, academic and personal growth during the period of my education and research for the Ph.D. His charisma, good nature, and sense of humor have had as much effect on me as our academic relationship. I have tapped his boundless mental library in the course of research and drawn on his personal energy for encouragement and scientific leadership. For this I am eternally thankful. Dr. B.W. Pipkin and Dr. W. Weber also contrib uted significantly as members of my dissertation reading commi ttee . Many colleagues in the sedimentation and micropaleon tology laboratories have provided assistance in both shipboard collection of samples and laboratory analysis of the data. I am particularly indebted to Richard Herrera for his shipboard skill and zealousness, and his timeless patience in performing many aspects of laboratory analysis. Brian Edwards taught me nearly all that I know about logistics in shipboard collection of box core samples, and their subsequent treatment back on land. Kevin O'Toole, John Liestman and many others assisted on the R.V. Velero IV in shipboard collection of data. Several colleagues helped me interpret seismic profiles from the basin. In particular, Steven Crissman and Michael Ploessel of McClelland Engineers, Ventura, lent their expertise in interpretation of mass movement features in I | the Santa Barbara Channel. Dr. Michael Field of the U.S.G.S. made several suggestions on seismic interpretation which proved very useful, and showed me similar features from other slope settings in offshore northern California. Dr. Thomas Nardin provided a great deal of basic advice on seismic interpretation and discussed mass movement features which he had interpreted from other slope settings. Discussions with Tom on numerous general aspects of seismic stratigraphy, sediment budgets, turbidite deposition, slope sedimentation, echo character and hemipelagic sedimentation influenced my thinking a great deal. Discussions with Dr. W.R. Normark of the U.S.G.S. were important in interpreta tion of Quaternary faulting, mass movement and turbidite deposi tion. Simple geotechnical properties were analyzed by Elisa beth Karlstrom and Patricia Garcia. Dr. Frances Hein introduced me to the literature on geotechnical properties, and discussions with her on the importance of such proper ties in interpreting mass movement were invaluable. Dr. E.W. Hamilton of the Office of Naval Research provided advise on the collection of marine samples for geotechnical analysis, and provided suggested readings on the subject. Most of the textural data was collected and reduced by Rick Herrera (sedimentation analysis by pipetting) and Pat Garcia (settling tube). Analysis of calcium carbonate and organic carbon contents were performed by James Threl- fall , Cynthia Callaway, and Susan Rasmussen. Dr. David Drake lent his expertise on suspended sediment transport, general setting of the basin and the origin of the gray layers. Professor Dr. Hans-Eric Reineck of Germany aided in the analysis of sedimentary structures with invaluable advice based on many years of studying sedimentary struc tures. Dr. James Coleman of Louisiana State University encouraged my pursuit of the question of how the debris flows are generated and move, and consideration of his work on the subject materially affected this portion of my dissertation. Dr. Richard Hoover of Exxon Production Research provided advice on interpretation of sedimentary structures and acoustic signatures in Santa Barbara Basin based on their research there, and drew my attention to literature on the subject. Discussions with others of the Seismic Stratigraphy group at Exxon Production Research were invaluable in the light they shed on my seismic i interpretations. Dr. R.H. Yerkes of the U.S.G.S. and Dr. G.A. Davis of U.S.C. provided advise on the tectonics and structural geology of the adjacent mainland area of the transverse ranges. Dr. R.L. Kolpack generously provided unpublished textural data for samples of the mainland shelf and Dr. Brian Edwards of the U.S.G.S. provided textural data from the Hueneme Sill area. Peter Day provided textural data from the insular shelves of the Northern Channel Islands. Financial support is gratefully acknowledged from the I 1Tenneco Foundation for supplying fellowship support for one year. Fellowship and assistantship support were provided by the Sea Grant Program for one year, as well as miscella neous expenses. Support for all aspects of the research was provided by the National Science Foundation Grants 0CE76-00156, 0CE78-20453, OCE80-20290 to Drs. Gorsline, Douglas and Ku. Finacial support for acquisition of LANDSAT images was provided by the Department of Geological Sciences Graduate Research Fund. A very special note of thanks is due my wife, Mary Lou Cotton, for her patience with my numerous sleepless nights thinking about my research, absences at sea while collecting the data, and absence of mind while writing the dissertation. Her love, understanding and advice made the task much easier and more enjoyable. SI ABLE OF CONTENTS DEDICATION....................................................ii ACKNOWLEDGMENTS ............................................ iii LIST OF T A B L E S .............................................. .. LIST OF FIGURES...............................................xi ABSTRACT................................................... xvii Chapter page I. INTRODUCTION .......................................... 1 General Statement ................................ 1 Purpose..............................................2; Study Area........................................... 3 Stratigraphic, Structural and Tectonic Setting of the Santa Barbara Basin ................ 8 Current Circulation and Sediment Transport . . 24 Climate and Terrigenous Sediment Delivery to the B a s i n ................................... 37 Mass Movement in Marine Sediments ........... 41 Previous Studies in Santa Barbara Basin . . . 55 II. METHODS................................................. 64 General Statement .............................. 64 Shipboard Methods .............................. 65: Analytical Methods .............................. 76 Point C o u n t s ................................... 76 Textural Analysis ............................ 78 Calcium Carbonate and Organic Carbon . . . 79j Piston Core Extrusion and Sampling .... 81! Box Core Extrusion and Sampling.............. 83 Radiography......................................86 Water Content, Saturated Bulk Density and Porosity.................................92 Atterberg Limits Determination ........... 94 Computer-assisted Contouring and Data Base 98 III. SURFICIAL SEDIMENT CHARACTERISTICS .............. 102 Results........................................102 Moment Measures .............................. 102 Calcium Carbonate and Organic Carbon . . . 125 Sand Grain Constituents ..................... 131 Discussion......................................143 IV. SEISMIC, STRATIGRAPHIC AND GEOTECHNICAL RESULTS . 155 Seismic Reflection Data ....................... 155 Holocene Stratigraphy ......................... 196 The Laminated Zone and Gray Layers .... 196 Slope Area G ...............................211 Mass Movement Scarps and Deposits ......... 216 Bottom Photographs: the Depositional Surface................................230 Down-core Textural Data ....................... 235 Geotechnical Properties ....................... 266 V. SUMMARY AND DISCUSSION.......................... 294 Surficial Sediment Characteristics and Sedimentary Processes .................. 294 Tectonic Deformation and Sediment Creep in Slope Area G .............................299 Seismicity and Mass Movement.................304 Mass Movement Scars and Deposits: Structure, Stratigraphy and Geotechnical Properties.309 Basin and Slope Stratigraphy.................314 REFERENCES...................................................331 ix LIST OF TABLES Table 1. Characteristics of Mass Movement Zones and Slope Area G........................................... 2. Chronology of gray turbidite layers, gray flood layers and olive gray turbidite layers. . . LIST OF FIGURES F igure page 1. Index map of basin outlines in northern portion of California Continental Borderland............... . 4 2. Bathymetry of Santa Barbara Basin and vicinity. . 6 3. Structural provinces of the Santa Barbara Embayment........................................... 10 4. Flake tectonics model of Transverse Ranges with velocity diagram for flakes and plates......... 23 5. Patterns of surface turbidity caused by suspended sediment, based on interpretation of LANDSAT, Band 5 imagery of January 20-21, 1973.......... 27 6 . Cells of suspended sediment in surface waters simplified from turbidity and current patterns. 28 7. Currents inferred from turbidity pattterns for January 20-21, 1978............................... 29 8. Plume originating from Santa Clara and Ventura Rivers and associated features interpreted from LANDSAT imagery for January 21, 1973........... 30 9. Generalized surface water currents after Kolpack (1971 ).............................................. 32 10. Current vectors for the Upwelling Period from 1975-1 976 LANDSAT interpretations............... 34 11 . Histogram of Santa Barbara regional rainfall normalized to deviations above or below mean. . 40 12. Mass transport types, mechanical behavior, transport and sediment support mechanisms, structures and acoustic characteristics. . . . 50 x i ! 13. 14. 15. 1 5. 17. t i 18. 19. 20. i 21 . 22. 23. 24. 25. I 26. ! 27. Flow behavior, flow type and sediment support mechanisms for sediment gravity flows............. 52 Bo X C O res collected for this study as well as most Of the previously collected box cores deeper than the shelf break 6 6 Pis ton cores collected for this study as well as previously collected Pi ston cores. 6 8 Sei smic lines collected for this study, as well as previously collected lines in Hueneme Sill ar•ea..70 Sei smic lines from U.S. G.S. open-file report on environmental hazard s in Lease Sale 48 examined in western and central Santa Barbara Basin. » • • 72 SYMAP computer contour map of mean phi d iameter of surface (0-2 cm) sedimen ts. 104 SYMAP computer contour map of phi skewness of surface sediments (0-2 cm) . 1 0 6 SYMAP computer contour of phi sorting of surface sediments (0-2 cm) expressed as standard deviation.......................................... 108 SYMAP computer contour map of percentage sand of surface sediments (0-2 cm)...................... 111 SYMAP computer contour map of percentage mud (silt plus clay) of surface sediments (0-2 cm). . . . 113 SYMAP computer contour map of percentage sand of surface sediments (0-2 cm) for the areas having less than 10% sand............................... 115 SYMAP computer contour map of percentage silt of surface sediments (0 -2 cm)...................... 118 SYMAP computer contour map of percentage clay for surface sediments (0-2 cm)...................... 121 SYMAP computer contour map of the ratio of sand to mud of surface sediments (0-2 cm)..............123 SYMAP computer contour map of percentage calcium carbonate of surface sediments (0-2 cm). . . . 126 xii 28. SYMAP computer contour map of percentage carbon of surface sediments (0 -2 cm). organic • • • • • 129 29. Percentage mineral grains in the sand fraction. 133 30. Percentage mica in the sand fraction. 135 31 . Percentage foraminifera in the sand fraction • • • 137 32. Percentage planktic foraminifera in the fraction................................. sand 139 on on Percentage radiolaria in the sand fraction. • • • 141 34. Sources and areas of similar sedimentary processes in surficial sediments of Santa Barbara Basin and adjacent shelves.............................. 150 35. Location of Seismic Lines. . . 156 36. 3.5 kHz High-resolution Seismic Profile Line A-B. 161 37. 3.5 kHz High-resolution Seismic Profile Line C-D. 163 38. 3.5 kHz High-resolution Seismic Profile Line E-F. 165 39. 3.5 kHz High-resolution Seismic Profile Line G-H. 167 40. 3.5 kHz Seismic Line I-J. . . 170 41 . 3.5 kHz Seismic Line K-L. . . 172 42. 3.5 kHz Seismic Line M-N. . . 175 43. 3.5 kHz High-resolution Seismic Line 0-P 178 44. 3.5 kHz High-resolution Seismic Line Q-R 181 45. 3.5 kHz High-resolution Seismic Line S-T 183 46. 3.5 kHz High-resolution Seismic Line U-V 188 47. 3.5 kHz High-resolution Seismic Line W-X 190 : I 48. Mapped faults, mass movement areas and sand wave ! field in Santa Barbara Basin.......................193! 49. X-radiograph of piston core AHF 27375.............. 199 xii i 50. X-radiograph of 57-96 cm portion of piston core AHF 2787 5 201 51. Distribution of laminated zone of varved sediments and gray layers of the slope and basin floor. . 209 52. X-radiograph of box core AHF 28282.................. 212 53. X-radiograph of piston core AHF 17640 from Slope Area G................................................ 214 54. X-radiograph of piston core AHF 27366 from Flow F..217 55. X-radiograph of piston core AHF 27367 from mud flow deposit of Flow E................................... 220 56. X-radiograph of piston core AHF 28031 from scarp area of mud flow, Flow A........................... 223 57. X-radiograph of piston core AHF 28082 from deposit of mud flow, Flow A................................. 225 58. X-radiographs of box cores AHF 27144, 27386 and 27891 from mud flows................................228 59. Bottom photographs of various features on the basin floor, the present depositional surface. . . . 233 60. Down-core texture and moment measures for piston core AHF 27370 in the north slope-Montalvo Trough corridor......................................238 61. Down-core texture and moment measures in box core AHF 28089 on south slope of Montalvo Trough. . 240 62. Down-core texture and moment measures for piston core AHF 27366 in the mud flow deposit of Flow F...................................................... 242 i 63. Down-core texture and moment measures for piston core AHF 27367 in mud flow deposit of Flow E. . 244 64. Down-core texture and moment measures for piston core AHF 27375 in the laminated zone of the central basin floor............................... 2481 65. Down-core texture and moment measures for box cores AHF 27391 and 27393 from the mud flow deposit, Flow C................................................ 251 66. Down-core texture and moment measures of box core AHF 27866 from the scarp of mud flow deposit, Flow A................................................ 254 67. Down-core texture and moment measures of box core AHF 27890 from deposit of mud flow, Flow A. . . 256 68. Down-core texture and moment measures for piston core AHF 28081 from the scar of mud flow, Flow A...................................................... 259 69. Down-core texture and moment measures for piston core AHF 28082 from the mud flow deposit of Flow A...................................................... 261 70. Down-core texture and moment measures for piston core AHF 28077 downslope from mud flow deposit of Flow A.............................................264 71. Percentage water content for 0-2 cm box core samples...............................................263 72. Porosity in 0-2 cm box core samples...................270 73. Saturated bulk density of 0-2 cm box core samples..272 74. Atterberg limits from piston core AHF 27866 from mud Flow F............................................275 75. Atterberg limits of piston core AHF 27868 from Slope Area G, 700 m south of fault................277 76. Atterberg limits and activity of piston core AHF 28081 from scarp area of Flow A................... 281 77. Atterberg limits and activity for piston core AHF 28082 on mud flow deposit of Flow A.............. 283 78. Activities of piston cores AHF 27866, 27867 and 27370................................................ 286 79. Organic carbon versus plasticity index for 0-2 cm box core samples across Santa Barbara Basin and northern shelf.......................................289 80. Plasticity chart for piston cores from various mass movement features of Santa Barbara Basin slopes................................................ 292 xv 81. 82. 83. Epicenters of earthquakes in the Santa Barbara Channel from 1934-1 975............................. 305, I I Generalized vertical sequence of sedimentary j structures in mud flow deposits of Santa Barbara ! Basin..................................................316 Vertical sequences of sedimentary structures in turbidites of Santa Barbara Basin...............325 xvi ABSTRACT In Santa Barbara Basin, the northernmost basin in the California Continental Borderland, slopes comprise 96% of the 2300 square kilometer basin area deeper than the shelf break and provide a test area for establishing slope sedimentary processes. Holocene stratigraphy, shallow structure and surficial sedimentary patterns reflect a multitude of processes: 1) mass movement; 2) turbidite sedimentation; 3) tectonic deformation and gravity faulting; 4) current winnowing and transport of mud; 5) a !laminated section of hemipelagic sediments, flood layers and turbidites preserved by anoxic water conditions. Proximity to the Santa Clara River-Ventura River point source of terrigenous sediment results in a dominance of terrigenous input over pelagic, biogenic input and produces a terrigenous-dominated hemipelagic stratigraphic record. Clayey silt is derived from this point source of suspan- ! sates and transported by the Anacapa Current to the basin floor via the Montalvo Trough-North Slope Corridor, best demonstrated by a map of percentage silt in surficial sediments. In the absence of a significant sand fraction over 95% of the basin, silt is the diagnostic size fraction x v i i indicative of sediment transport through suspended sediment transport driven hy surface currents and nepheloid layer flow. Where the Anacapa Current enters the basin over the Hueneme Sill, current winnowing results in a residual, surficial blanket of poorly sorted silty sand, part of which is a sand wave field. The sand waves average 20 m in amplitude and 300 m in wavelength, are oriented slope-par allel in an east-west direction and are explainable by the increased flow regime produced by thinning of the Anacapa Current as it moves across the slope. In the western central basin floor, a "hemipelagic core" is evidenced by grain counts, texture, calcium carbonate content and represents the area in the basin where pelagic and lithoge- nous sedimentation are most balanced. Percentage silt and grain counts define a suspended sediment pathway from the northwest of the basin which is produced by a current- driven plume. This influx is presently covering the relict Conception fan with silty hemipelagic sediment. Mass movement presently covers at least 13$ of the basin. Mud flows documented by seismic profiling and characteristic sedimentary structures are the most common mass movement process. Mud flow deposits typically show an increase in clast frequency towards the top, an increase in activity upward, laminations at the base, elutriation features, and minor folds and faults. A generalized vertical sequence of structures for a typical mud flow serves as a model for other mud flow deposits of world ocean slopes. A 200 square kilometer area on the Montalvo Trough-North Slope Corridor is typified by sediment creep, gravity faulting and some reverse faulting interpreted from seismic profiles. The reverse faulting incorporates Holocene displacement and documents the effects of fore shortening of the Montalvo Trough by the tectonically-in- duced north-south compression of this portion of the western Transverse Ranges. Mass movement occurs on low slope gradients of less than 2.5 degrees and as little as 0.06 degrees over the lower slope. Mud flows, slumps and slides are typically around 10 square kilometers in areal extent, and are usually less than 5-6 meters thick. Scar areas and deposits are readily interpreted from seismic profiles to yield flow morpholo gies typical of these types of mass movement. Plasticity characteristics provide differentiation of the various mass movement features which are probably related to differences in clay content, sedimentation rates and organic carbon content. Holocene stratigraphy in the central basin floor consists of laminated sediments produced by the seasonal nature of terrigenous input from the river point source and xix preserved by anoxie bottom water conditions. This laminated zone has been mapped and roughly coincides with the 550 m isobath in response to oxygen levels of 0.1 ml/1 previously reported for near-bottom water. Turbidites and flood layers are interspersed with the laminated intervals in this zone, and can be traced upslope out of the laminated zone to the northeast of the basin center through the Montalvo Trough-North Slope Corridor. The mapped outline of the gray layer turbidites and gray flood layers is explainable from the sediment transport pathway inter preted from surficial sediment characteristics showing the significant imprint of the Anacapa Current on suspended sediment transport. Most of the terrigenous sediment which has entered the basin during the Holocene has passed through the Montalvo Trough-North Slope Corridor as turbi dites, flood layers or suspended sediment transport during non-flood years. The apparently high sedimentation rates resulting from these three sedimentary processes explain the presence of sediment creep and gravity faulting on low slope gradients. Vertical sequences of sedimentary struc tures and vertical textural variation allow differentiation of six types of muddy turbidites, which provide refinement of muddy turbidite models. A chronology determined by varve counting in one laminated zone piston core correlated with an adjacent box core provides absolute dates of gray layer turbidites, gray flood layers and olive gray turbidites. Gray layer turbidites occur at a frequency of 1/120 years and all gray layers (flood + turbidite) occur at a frequency of l/59> which probably reflect the influ ence of roughly 100 and 50 year floods, respectively. The absolute sedimentation rate is 173 cm/1000 years at this site. Synthesis of the various sedimentary processes opertive on the slopes of this basin leads to a slope to basin floor model for canyonless slopes. Turbidity currents, suspended sediment transport and mass wasting on the lower slope are the characteristic processes. Distribution of surface and bottom currents, in addition to tectonism, will alter the distribution and morphology of resulting sedimentary deposits. xx i Chapter I INTRODUCTION GENERAL STATEMENT Analysis of processes which operate over the entire extent of a single, modern sedimentary basin requires an integrated approach incorporating all known variables. In a tectonically complex and currently active setting such as the California Continental Borderland, the influence of structure and tectonics will exert fundamental control on the shape and bathymetry of the present depositional surface of an individual basin. Contemporary tectonics causes synsedimentary and postdepositional deformation of the sedimentary pile, producing large and small structures. Sediment delivered to the margins of a basin will be displaced by a number of transporting processes. Currents will transport and redeposit sediments along trajectories determined by persistent patterns of ocean circulation. Delivery of sediment denuded from continental drainage basins will be governed by climate and relief of these drainage basins. Once deposited on basin slopes, sediments can be remobilized by a broad spectrum of mass movement mechanisms under the influence of gravity. Deformational structures resulting from mass movement have been exten sively documented in sedimentary rocks, but inadequately analyzed in recent depositional analogues. Once deposited on the basin slopes and basin floor, sediments will be subjected to a variety of diagenetic processes related to the physical properties of the water column as well as those of the sediments themselves. All these processes occur over the extent of a given sedimentary basin, sometimes simultaneously. It is thus not always a simple imatter to define which process or set of processes is responsible for a sedimentary product. PURPOSE It is the purpose of this study to define the extent and nature of processes affecting distribution and character of Holocene sediments on the slopes and basin floor of Santa Barbara Basin, in the California Continental Borderland. In order to attain this goal, a large collection of sediment samples and seismic lines has provided a broad coverage used as the data base from which to make infer ences. One particular set of sedimentary processes, the continu of mass movement processes, is particularly impor tant in transporting sediments already deposited on subma rine slopes. The general mechanics of transport and deformation during mass movement are, as yet, poorly understood. Consequently, the role of mass movement will be emphasized with the goal of inferring the nature of the mass movement process from its sedimentary deposits and the physical setting of the slope along which sediment was transported. This is resolved in a model of slope deposi tion in a fine-grained sedimentary basin under the influ ence of compressional tectonics. STUDY AREA Santa Barbara Basin is the northernmost basin of the California Continental Borderland, a complex checkerboard of basins, ridges and islands extending offshore from Point Conception to Baja California (Figure 1). The basin is bounded on the north by the mainland from Point Conception to Point Hueneme, on the south by the Northern Channel Islands platform, and is open to the west and east (Figure 2). Sills restrict exchange of open ocean water on the west, where the sill is at a water depth of 475 m, and on the east, where Hueneme Sill forms a broad area at a minimum depth of 225 m (Figure 2). Water depths between the Northern Channel Islands are approximately 50 m. Santa Barbara Basin is elongate and west-trending in contrast to other basins on the California Continental Borderland, which trend northwest. The basin is a maximum of 116 km long from sill to sill and 35 km wide at its Pt. Conception *%• Santo Borbora SANTA ARBARA BAS Los .Angeles P4°N SANTA SANTA \ \M 0 N IC A CRUZ ' \B A S IN b a s in AN PEDRO BASIN 120® W XSAN NICOLASw CATALIN BASIN ^ \ b a s i^ 118® W ! Figure 1: Index map of basin outlines in northern portion of California Continental Borderland. 4 widest point from shelf break to shelf break off western Santa Rosa Island. The basin lacks an active submarine canyon, but beyond the eastern limit of the basin, Hueneme Submarine Canyon heads within 2 km of the mainland and extends into Santa Monica Basin to the southeast. Two small submarine valleys extend along the northwest slope off Gaviota (Figure 2) trending northwest, but do not constitute significant depressions into the slope, unlike submarine canyons along the remainder of the southern California mainland conti nental slope. A ridge and trough offshore from Santa Barbara, the Montalvo Ridge and Montalvo Trough (Figure 2) are surface expressions of structural control on basin bathymetry (Fischer, 1972). Montalvo Ridge is less than 120 m deep at its crest. The basin center, reaching a maximum depth of 589 m, lies to the west of the Montalvo Ridge and Montalvo Trough. Irregularities in the contours at 450, 500, and 550 m are evident along the northern slope of the basin contiguous to the basin center. The shelf break along the northern and southern limits lies at approximately 90 m water depth. Below the shelf break, the areal extent of the basin is 2,292 square kilometers. Only a small portion of the basin center is Figure 2 Bathymetry of Santa Barbara Basin and vicinity. Contour in meters. Contour interval = 50 m. 6 i o :• o 119 3 0 S A N T A B A R B A R A T C H A N N E L B A T H Y M E T R Y nautical ml 5 1 0 1 5 20 25 SANTA BARBARA kllomtler i bolhymtlrv In m t l t r t s : Ventura r» ■ River Montalvo VENTURA BASIN CENTER River SANTA CRUZ ISLAND — — - j SANTA ROSA ISLAND •P ( l \\W & v-\ i ,A l ■- ' 120 *00’ 119*30'. relatively flat, and that portion slopes very gently to the south. This flat basin floor, covering an area of 56 square kilometers, constitutes only 2.4% of the basin area. Thus, 97.6% of the basin area consists of slope areas. Slopes are then the dominant physiographic surface within the basin on which sediment can be deposited. Compared to the gradients of the slopes on the remainder of the California Continental Borderland, the slopes of Santa Barbara Basin are some of the most gentle (Emery, 1960). Although a small amount of coastal drainage exists from onshore canyons between Point Conception and Pitas Point, the Ventura and Santa Clara rivers at the extreme east end of the basin (Figure 2) are the predominant sources of sediment delivery to the basin (Fleischer, 1972). They debouch onto the broadest section of mainland shelf, which extends from Goleta Point to Point Hueneme. STRATIGRAPHIC, STRUCTURAL AND TECTONIC SETTING OF THE SANTA BARBARA BASIN Santa Barbara Basin is located in the western Trasverse Ranges along the southern California uplift (Yerkes and i i others, 1980). Usually included with the basin in analysis; of structural framework and tectonics are the Santa Inez | Mountains to the north, the Channel Islands to the south, Point Conception and San Miguel Island to the west and the , Ojai area to the east (Vedder and others, 1969). Collectively, this area has been termed the Santa Barbara Embayment (Reed and Hollister, 1936). The Santa Barbara Embayment comprises the westernmost portion of the Tranverse Ranges and is essentially the uplifted, found ered, submarine extension of the Ventura Basin, a topographic and structural depression containing more than j 18,000 m of upper Cretaceous and Cenozoic strata (Fischer, 1972; Ingle, 1980; Vedder and others, 1969). . The structural and tectonic setting of the Santa Barbara i 1 Basin is, thus, a continuation of onshore structure within the greater Santa Barbara Embayment. The continuity of onshore and offshore geology has been demonstrated by Fischer (1972), who divided the Santa Barbara Embayment into six major blocks delineated by structure, stratigraphy or geologic evolution (Figure 3). The structural grain of the basin is west-trending like the Transverse Ranges, and is presently typified by west-trending folds and reverse faults with great structural relief in the thick Tertiary- Quaternary sedimentary sequence (Yerkes and others, 1980). The geologic history of the Santa Barbara Embayment can be traced back into the Cretaceous, and is characterized by recurrent tectonism interrupted by periods of relative quiescence (Vedder and others, 1969). Figure 3 Structural provinces of the Santa Barbara Embayment. From Fischer (1976). Su w ' H S Z - Son C o r * ' ° n o S F SM I 6 + 5-6 • 4-5 rrrri SCALt in MILES O 3-4 . Earthquake iworm (S^I< 1970 1 2 0 * 0 0 ’ 119*00' The basement terrane underlying the Santa Barbara Embayment has been inferred by Yeats (1981) to be Francis can-like rocks termed the Catalina Schist. Strata depos ited on top of this basement reflect the interplay between tectonics and depositional events through the Paleogene and Neogene basin-filling episodes (Ingle, 1980). The paleo- bathymetry and depositional history of the area have been most recently synthesized by Ingle (1980) utilizing the upper depth limits of benthic foraminifera and associated |1ithostratigraphy to produce 1ithostratigraphic- chronos- tratigraphic plots which yield estimated rates of uplift, subsidence and sediment accumulation for separate paleoen- vironmental sedimentary facies. Filling of the Santa Barbara Embayment was slow during the late Cretaceous through early Eocene when lower bathyal (greater than or equal to 2,000 m) basin plain and distal submarine fan deposits (Jalama, Sierra Blanca, Anita, Juncan, Canada-Pozo and Matilija formations) accumulated at j ! j rates between 20-50 m/million years balanced by equally slow subsidence (Ingle, 1980). After this period, massive wedges of outer, middle and supra-fan sediments filled the trough from the north and east at rates of 200-300 m/million years during the middle Eocene (Cozy Dell and Sacate formations). During the late Eocene, sedimentation 12 accelerated to 500 m/million years as subsidence decreased to less than 100 m/million years, accompanied westward transgression of slope, shelf and littoral facies over proximal fan and base-of-slope deposits (Gaviota forma tion). Over this area, a tectonic pause was associated with widespread nonmarine deposition (Sespe formation), some littoral deposition (Alegria formation), and erosion during the Oligocene. This tectonic pause was supplemented by sea level lowering which was a part of the global !eustatic event caused by refrigeration (Kennet and Shack- leton, 1976; Ingle and others, 1976) or by adjustment of mean depth, volume and freeboard of the world ocean basin caused by volume changes of the mid-ocean ridge system (Pittman, 1973). Termination of nonmarine deposition by abrupt and widespread subsidence at rates between 150-500 m/million years during the latest 01igocene-early Miocene (Ingle, 1980) concomitant with initiation of equally dramatic tectonic episodes throughout the circum-Pacific (Dott, 1969) produced shelf-littoral to slope deposition (Vaqueros and Rincon formations). This rapid subsidence continued, resulting in a series of effectively silled i |middle bathyal Miocene basins, momentarily deficient in terrigenous sediment (Ingle, 1930). This sedimentary i | setting permitted relatively undiluted diatom deposition | ! from highly productive surface waters to produce the idiatomaceous muds and diatomites of the Monterey, lower 13 Sisquoc and lower Santa Margarita formations. Low oxygen levels in these middle to late Miocene basins prevented destruction of primary laminations in the Monterey forma tion by prohibiting development of significant megainverte brate faunas capable of bioturbation. The analogy between the Miocene Monterey Formation deposition and the present-day Santa Barbara Basin in terms of preservation of laminations due to lower oxygen content of basinal waters is quite useful (Ingle, 1980; Soutar and others, 1981). During the late Miocene to early Pliocene time, tectonic reorganization of the Miocene borderland basins in the northern borderland began (Emery, 1960; Yerkes and others, 1965; Moore, 1969; Ingle, 1980; Nardin, 1981) with increased subsidence to lower bathyal depths in some synclinal areas. Early Pliocene mudstones and distal sands (middle-upper Sisquoc and upper Santa Margarita formations) covered the diatomites as increased terrigenous influx occurred through local fan lobes and fine-grained hemipe lagic sedimentation (Ingle, 1980). In the middle Pliocene, a major flexing of the northern borderland took place, marked by rapid uplift at rates of 400-1,000 m/m.yr. of anticlinal interbasin ridges and borderland margins (Ingle, 1980). This resulted in dramatic increases in sedimenta tion rates (greater than 2,000 m/m.yr.) and subsidence (greater than 1,000 m/m.yr.) in nearshore synclinal depocenters such as the Ventura Basin. The Ventura Basin was filled to capacity by the late Pleistocene as indicated by rapid reductions in sedimentation rates and subsidence (both less than 1,000 m/m.yr.). A major late Pleistocene tectonic episode (the Pasadenan Orogeny)then deformed the northern borderland margins, basin sills and interbasin ridges to their present configuration, initiating modern depositional patterns (Fischer, 1972; Ingle, 1980; Nardin, 1981 ) . The structural and tectonic grain of the Transverse Ranges Province, which includes the Santa Barbara Basin, is west-trending in marked contrast to the predominantly northwest-trending structural grain of the remainder of the southern California Continental Borderland and onshore southern California. Formation of this and other basins of the Borderland can be explained in terms of plate interac tions from Cretaceous to Holocene time (Blake and others, 1978; Crouch, 1979; Crowell, 1974; Howell and others, 1980). The fundamental mode of basin formation within a transform regime can be explained by understanding the nature of crustal deformation between branches of anasto mosing strike-slip faults (Crowell, 1974). Pull-apart basins or tipped-fault wedges will be produced where right-lateral slip faults converge or diverge (Crowell, 1974). The underlying cause of this right-slip and subse quent basin formation in the southern California area appears to have been the collision of the Pacific and North American lithospheric plates along the East Pacific Rise 29 million years ago (Atwater, 1970) and subsequent transform motion along the San Andreas Fault system. As a result of this collision and shift to transform motion, the basins of the California Borderland reached their present configura tion in late Miocene to early Pliocene time (Howell and others, 1980; Nardin, 1981). The structural and tectonic grain of the Santa Barbara Basin and the rest of the western Transverse Ranges points to a more complex and different tectonic history compared to the rest of the Borderland. Prior to Eocene time, little can be said about the structural grain of the Santa Barbara Embayment. Although the assumption has been made that the area had the same west-trending structural grain (Ingle, 1980), there is little hard supportive evidence. Indeed, there is some suggestion of north-south to north west-southeast late Cretaceous sedimentary basins in the area (Colburn, 1971) reflecting tectonic control of deposi tion and a different structural grain from the present (Fischer, 1972). Late Cretaceous to Paleocene oblique subduction in central and northern California has been invoked to explain basin formation to the north (Howell and others, 1980), but the implications of this oblique subduc tion are unclear for the Santa Barbara Embayment. Perhaps some element of this oblique subduction active to the north was active in the Santa Barbara Embayment and would explain the remnant north to northwest-trending structure and depositional patterns. Throughout Eocene time, a broad stable forearc basin persisted in the California Conti- ;nental Borderland in response to renewed high-angle subduc tion (Howell and Link, 1979; Howell and others, 1980). Reconstructions of distribution of upper Eocene rocks at the end of the Eocene by Yeats (1968) depict a west- trending structural grain in the Santa Barbara Embayment, with the Northern Channel Islands displaced to the east relative to their present position. After a period of tectonic quiescence in the Oligocene, from late Oligocene to early Miocene the Santa Barbara Embayment subsided accompanied by volcanism which peaked in late early Miocene (Ingle, 1980). Once again, the struc tural grain was presumably west-trending. The early-middle Miocene volcanism and basin formation in the Santa Barbara Embayment was symptomatic of a circum-Pacific period of basin formation and volcanism ultimately caused by adjust ment of plate margins and a change in the direction and rate of subduction (Dott, 1969). This, in turn, produced back-arc spreading along the western Pacific, initiation of translational tectonics in California and produced the California Borderland topography (Atwater, 1970; Blake and others, 1978; Packham and Falvey, 1971) and very similar Neogene stratigraphies throughout the northern circum-Pa- cific marginal basins (Ingle, 1981). Some evidence of this translational tectonics can be found in the offshore Oxnard Plain where pre-late Miocene northwest-trending faults are jevident in seismic profiles (Greene, 1976). Thus, by the early Miocene, borderland topography had begun to form. A series of effectively silled basins seems necessary to explain the laminated diatomites of the Monterey Formation deposited in lower to middle Miocene (Ingle, 1980, 1981) although similar diatomaceous lamina tions also form where the oxygen minimum layer intersects slopes underlying upwelling areas of high productivity (Calvert, 1964; 1966; Donegan and Schrader, 1981; Soutar and others, 1981). Yeats (1978) claims acceleration of subsidence rates after the Miocene and is supported by paleobathymetric evidence from the Ventura Basin (Ingle, 1980). These higher post-Miocene subsidence rates are found only in the basins clustering around the big bend in the San Andreas Fault as it crosses the Transverse Ranges (Yeats, 1978). He attributes this phenomenon to the strengthening of the lithosphere below the Transverse Ranges as it cooled after the overriding of the East Pacific Rise (Yeats, 1978), a cooling process probably still occurring at present (Henyey, 1976). The implication is that the strengthening of the lithosphere may explain the north-south compression and consequent subsidence in the Santa Barbara Embayments at the beginning of the Pliocene. An alternative, but not necessarily contradictory hypothesis involves the origin of the big bend of the San Andreas Fault. Davis and Burchfiel (1973) have noted the coincidence of this bend with the intersection of the Oarlock-Big Pine Fault, which they interpret as an intra continental transform structure. A second bend, to the south, also coincides with the intersection of a possible transform structure, the Santa Cruz-Malibu-Sierra Madre Fault (Davis and Burchfiel, 1973)- Thus, it is tempting to speculate that the two bends in the San Andreas Fault are related to, or caused by these two transform, left-slip structures. This provides a setting for basin deformation akin to that envisioned by Crowell (1974). In this setting, however, the Santa Barbara Embayment lies between two left-lateral transform structures cross-cutting the right-lateral San Andreas. If the displacment along the 19 San Andreas is greater than that along the left-lateral transform structures, then deformation from the San Andreas rounding the two bends would be expected to be important. The more west-trending Garlock-Big Pine and Santa Cruz-Mal- ibu-Sierra Madre faults merely set the west-trending northern and southern limits for north-south compression within the entire block. However, the more southerly Santa Inez Fault, rather than the Big Pine Fault is normally taken as the northern limit of the structural area, but has | the same left-lateral displacement (Fischer, 1976) and thus may be related to the transform movement along the Garlock-Big Pine Fault. The eastern limit of the Santa Barbara Embayment is normally assigned to the San Gabriel Fault, not the San Andreas, but it has the same right lateral sense of movement, probably reflecting uptake of San Andreas transform movement by splaying-out of the fault. Ultimately, regardless of the intricacies of fault movement, the onset of compression in the Santa Barbara Embayment is due to the opening of the Gulf of California and the attachment of Baja California to the Pacific plate with subsequent transform movement along the San Andreas (Atwater, 1970). t Once initiated, the north-south compression within the Santa Barbara embayment has continued to the present (Fischer, 1972; Yeats, 1981; Yerkes and others, 1980). Fischer (1972) dates the onset of compression and uplift as middle late Pliocene along the eastern Ventura Basin in a tectonic pattern similar to the present. Present-day tectonism in the Santa Barbara Embayment is dominated by north-south compression north of a diagonal line extending from Point Conception through Santa Barbara Basin to the Santa Monica Mountains (Fischer, 1972). Between this line and the Northern Channel Islands platform to the south, extensional structures, typified by the present central basin trough dominate (Fischer, 1972). The eastern and central Santa Barbara Basin region is dominated by seismi- cally and geologically active west-trending reverse faults and folds of great structural relief in a thick Tertiary- Quaternary sequence (Yerkes and others, 1980). Current, ongoing deformation is expressed as earthquakes with fault-plane solutions indicative of reverse to left-rev erse-oblique faulting correlative with known reverse faults, west-trending folds and usually high uplift rates of 2-10 mm/yr based on uplift of dated marine terraces (Lee and others, 1979; Sarna-Woycicki and others, 1979; Yerkes and Lee, 1979; Yerkes and others, 1980). Yeats (1981) presents a rather unique and intriguing scenario for explaining the Quaternary tectonics of the Transverse Ranges, including Santa Barbara Basin. In his synthesis, microearthquake focal mechanisms are suggestive of a flattening with depth of north-dipping thrust faults which results in a mid-crustal detachment in which the central Transverse Ranges move south relative to the mantle. The central Transverse Ranges and western Mojave Desert comprise tectonic flakes of brittle crystalline rocks underlain by ductile schist which constitutes the zone of decollement. He infers that the Transverse Ranges flake presently moves west-northwest relative to the western Mojave flake, south- southwest relative to the Pacific Plate, and north-northwest relative to the Great Valley-Sierra Nevada (Figure 4;Yeats, 1981). Another consequence of his scheme is a convergent plate boundary underlying Santa Barbara Basin (Figure 4) which coincides with Fischerfs (1972) line demarking compression within the area . The age of this current tectonic scheme may be the same as the age of the break through thrust faults (Yeats, 1981). Thus this tectonic system is no older than 2 m. yrs. and may be younger (Yeats, 1981). This tectonic model seems to fit the deformational and earthquake fault- plane solutions within the Santa Barbara Basin (Yerkes and others, 1980) and may explain the ongoing compression. In addition, it could explain the mid-Pleistocene compres- TECTONIC BOUNDARIES CRUST MANTLE TRANSFORM N V CONVERGENT ^ DIVERGENT KM PELONA SCHIST Figure 4: Flake tectonics model of Transverse Ranges with velocity diagram for flakes and plates. Numbers are displacements in mm/yr. Letters represent flakes of plates: A, America; G, Great Valley-Sierra; M, Mojave; P, Pacific; T, Transverse Ranges. Note convergent crustal boundary beneath Santa Barbara Basin. 23 sional event within the north portion of Santa Barbara Basin (Fischer, 1972) which appears to correlate with deformation caused by the Pasadenan Orogeny in Santa Monica and San Pedro basins to the south for the same age (Nardin, 1981). CURRENT CIRCULATION AND SEDIMENT TRANSPORT The California Current System constitutes the eastern boundary current of the north Pacific gyre that flows to the south as a broad, shallow (less than 200 m), sluggish, meandering and complex surface stream along the west coast of North America (Wooster and Reid, 1963). Off southern California, a primary feature of this current is a semiper- 1 manent cyclonic gyre centered over the south California Continental Borderland (Wyllie, 1966). Along the coast is a countercurrent flowing to the north at depths greater than 200 m which sometimes surfaces, and is called the Davidson Current (Reid and others, 1958). A great deal of variability occurs within the California Current as meanders and eddies spin off and decay downstream (Bernstein and others, 1977). As wind stress is the primary forcing mechanism for this current’s flow (Chelton, j 1980; Hickey, 1979; Reid and others, 1958), seasonal j variation in the exact path of the California Current | System is due to variation in wind stress. The result of i this seasonal variation in wind stress is a set of three successive current ’ ’seasons” within the southern California Borderland (Pirie and Stellar, 1977). From July to October, the southward-flowing California Current dominates current patterns during the Oceanic Season. From October through mid-February, the Davidson Current dominates with net northward flow of water along shore, but continued southward flow of the California Current offshore during the Davidson Current Season (Pirie and Stellar, 1977). Finally, during the Upwelling Season, upwelling prevails in certain parts of the Borderland from mid-February to July, although the surface current patterns are quite similar to those in the Oceanic Season. During all these seasons, the cyclonic gyre is present in the northern Borderland, although its seaward edge moves further offshore during the Oceanic and Davidson seasons than during the Upwelling Season. The surface current circulation in the California Borderland has important implications for transport of suspended sediment derived from rivers. Based on analysis of LANDSAT, Band 5 images for January 20-21, 1973 after floods, Thornton (1981) was able to trace plumes of suspended sediment over the entire Borderland (Figure 5). When simplified, these suspended sediment patterns outlined 25 three cells of suspended sediment transport in surface waters of the Borderland, two of which, cells A and B, are important for Santa Barbara Basin (see Figure 6). In addition, surface currents inferred from the patterns of turbidity due to suspended sediment loading show a great deal of fine detail, but confirm the importance of the cyclonic gyre within the Borderland (Figure 7). The two major sources of terrigenous sediment for the entire California Borderland, the Santa Clara and Ventura Rivers, act as a point source for suspended sediment, most of which is probably deposited in Santa Barbara Basin (Drake and others, 1972; Thornton, 1981). The influence of these two rivers can be seen clearly as they formed a single plume in 1973 (Figure 8). Within Santa Barbara Basin, more detail on surface current circulation is available from a drift card experi ment (Kolpack, 1971) and from LANDSAT analysis from 1972-1976 (Davis, 1980; Pirie and Stellar, 1977). While the surface current patterns are variable from one oceano graphic season to another, results from the May to December, 1969 drift card experiment display some of the important patterns within the basin (Figure 9). Throughout the year, the net flow through the basin is westward with the west-flowing Anacapa Current (Drake and others, 1972) SantJL Mari 0 River 120 Son Antonio Cr Santo Inez R c / T yfcnturoR Son)0 a q , a R Santo Borfeflr Ventura Santa . n Monica Santa SGR An0 R Clear Cleor Clear Cleor SBI • Clear T u rb id A reo s 120 W Clear Figure 5: Patterns of surface turbidity caused by suspended sediment, based on interpretation of LANDSAT, Band 5 imagery of January 20-21, 1978. Abbreviations of localities in Santa Barbara Basin vicinity are: PA, Point Arguello; PC, Point Conception; PD, Point Dume; SMI, San Miguel Island; SRI, Santa Rosa Island; SCI, Santa Cruz Island; AI, Anacapa Island. From Thornton (1981). 27 I Figure 6 S U SP E N SA TE C ELLS Cells of suspended sediment in surface waters simplified from turbidity and current patterns. From Thornton (1981). 28 Figure 7: Currents inferred from turbidity pattterns for January 20-21, 1978. Relative thickness of streamlines denotes relative intensity of currents. From Thornton (1981 ) . 29 Sonia Borbpr January 21, 1978 Santa Clara R Goleta Pt ^ Port Hueneme ( ') . ^Anacapa I p J S a n ta Cruz l.j Santo Rosa I. Figure 3: Plume originating from Santa Clara and Ventura Rivers and associated features interpreted from LANDSAT imagery for January 21, 1978. Stippling concentration denotes relative intensity of turbidity patterns. From Thornton ( 1981 ) . 30 entering across the Hueneme Sill as a permanent feature which is important in transporting river-derived suspended sediment to the west across the Santa Barbara Basin (Thornton, 1981; Figure 8). A central basin gyre (Figure 9; Figure 10) which is best-developed during the Upwelling Season tends to trap suspended sediment in Santa Barbara Basin (Drake, 1972). Because most rainfall in southern California occurs between November and March, the surface current patterns during the decaying Davidson Current Season and the onset of the Upwelling Season will be most important in redistributing suspended sediment derived from runoff (Thornton, 1981). Deeper flow within the basin is less well understood. At 200 m depth, the flow is to the west across the basin (Wyllie, 1966). Flow of basin water deeper than 200 m is probably largely controlled by sill depth. Because the eastern sill is at 245 m and the western sill is at 475 m, one can presume that water below 245 m must be derived from the west, which has been supported by Emery (1954) on the basis of sill depth and dissolved oxygen content. This point is important, because inflow of deeper water into the basin would be derived from the Pacific Intermediate Water, which lies in the oxygen minimum layer (Emery, 1960; Hulsemann and Emery, 1961). The oxygen content of this deep basin water is then rapidly depleted by organisms and Figure 9 Generalized surface water currents after Kolpack ( 1971). Note Anacapa Current. 32 L>Pt. Arguello SURFACE CURRENTS ,!Pt. Conception - , ~ W . • .Coo Oil Pt.SQnt(J Barbaro c i i t Anacapa I. 3 4 ° * San Miguel Island j Santa Rosa I. Santa Cruz I. Figure 10: Current vectors for the Upwelling Period from 1975-1976 LANDSAT interpretations. From Pirie and Steller ^1977)• 34 30' 20 SANTA BARBARA j » o ' £ p - /VEN TUR A <00 SANTA CRUZ ISLANO 00' «0. * > o SANTA ROSA ISLANO 1 2 0 * 0 0 ' 119*30' 20*30' U O bacterial decay of organics, to produce disaerobic bottom conditions in the basin below sill depth (Hulsemann and Emery, 1961). Very few measurements of bottom current velocities in Santa Barbara Basin exist. Bottom current velocities of up to 28 cm/sec ’ ’near bottom” and maximum of 14-17 cm/sec at 330 m water depth have been reported (Soutar and others, 1981). For a three-day record collected just above sill depth at 430 m, an average current velocity of 6 cm/sec has been reported (Sholkovitz and Gieskes, 1971). Near-bottom currents measured on the Santa Barbara-Oxnard shelf and slope vary from 21-36 cm/sec and are related to tidal forcing rather than the California Current System (Drake and others, 1971). The only indications of the direction of deep-water movement within the basin are based on transmissometry, which indicates a counterclockwise gyre deeper than 300 m (Drake, 1972). The vertical water movement and the resulting recharge time for the basin are not well-established. On the basis of oxygen consumption, Rittenberg and others (1955) have estimated a replenishment of at least once every two years. Hidaka (1961) suggested that the entire water column is recharged by upwelling. Sholkovitz and Gieskes (1971) observed that basinal waters were almost completely replaced in less than a month during an upwelling event in May, 1970. The frequency of these events is not entirely clear, but most of the evidence points to a recharge every year or two. Smaller scale events have been recorded in Santa Barbara Basin waters. Sholkovitz and Soutar (1975) found evidence of turbidity current transport of seawater near the sill depth (440-480 m) to the center of the basin (590 m) based on anomalously high oxygen and nitrate content of central basin water and sediment trap material. This mechanism of seawater transport by turbidity currents had been previ ously suggested, but not so conclusively proven (Emery and others, 1962). Chung (1973) and Hammond (unpublished data) have observed maxima at depth in radon profiles which could be explained by lateral injection of seawater, possibly by a turbidity current or low density suspended sediment transport along isopycnals (Drake, 1971). CLIMATE AND TERRIGENOUS SEDIMENT DELIVERY TO THE BASIN The coastal southern California area lies within a warm, Mediterranean , semi-arid climatic region with average annual rainfall of approximately 26 cm/yr. The occurrence of rainfall exhibits a strong seasonality, with most rain falling between December and March. The seasonality of rainfall is important in terms of sediment delivery to the basin by rivers, as the rates of fluvial erosion within drainage basins are greater from areas with a strongly seasonal climate than non-seasonal climates (Fournier, 1969; Wilson, 1973)- For Mediterranean climates, a crudely parabolic curve exists for plots of sediment yield versus precipitation for drainage basins, with a peak sediment yield at a mean annual precipitation of 127-165 cm (Wilson, 1973)- Consequently, given the low annual precipitation relative to these optimal levels for maximum sediment delivery to the basin, one might expect a low sediment yield. However, denudation of the adjacent drainage basins depends on other factors as well. Relief and rates of uplift exhibit a strong effect on rates of denudation in drainage basins (Ahnert, 1970). The adjacent rugged topography, with relief of as much as 2000 m within the Santa Clara River drainage basin is suscep tible to rapid erosion. In addition, continued Holocene uplift of the adjacent Transverse Ranges (Sarna-Wojcicki and others, 1979) maintains the extreme relief and tends to increase denudation rates. The extreme relief, continued uplift and consequently steep gradients of the headwaters of the rivers draining into Santa Barbara Basin would be expected to produce a great deal of sand to the offshore, a characteristic of transform margins with mountainous coasts (Inman and Nordstrom, 1971). The relationship between regional rainfall and subse quent sedimentation rates has been examined by Soutar and Crill (1977). Amount of rainfall varies a great deal from one year to another (Figure 11). It is obvious from such a graph that there are peaks of rainfall for certain years, such as 1862, 1864, 1890, 1910, 1941, 1952, 1958 and 1969 and it is tempting to suggest that there is a periodicity to the rainfall records. Statistical analysis of the rainfall record, however, fails to demonstrate such a periodicity (Soutar and Crill, 1977). For the rainfall index, there is no discernible difference in the pattern of rainfall through time and that of a random pattern (Soutar and Crill, 1977). One is merely left with the generaliza tion that there are significantly greater years of rainfall for eight years over the 107 year record. Thus, a crude frequency evolves by dividing the high rainfall years by 107 years of record, or one very wet year per 13.4 years. In addition to this frequency, a second frequency may be evident for the two years of highest rainfall, 1941 and j 1884, which would yield a 53.5 year frequency rainy season. While these generalizations stand for this period of record, a 13.4 year and 53.5 year flood frequency, the pattern is statistically random (Soutar and Crill, 1977). STANDARO UNITS SANTA BARBARA AREA R A IN F A LL 3*00 1-00 -1.00 -3*00 1900 1970 I960 1950 1940 1930 19SO 1910 1900 1090 1000 1070 10GO Figure 11: Histogram of Santa Barbara regional rainfall normalized to deviations above or below mean From Soutar and Crill (1977). By comparing varve measurements on a set of box cores dated by the Pb-210 method, Soutar and Crill (1977) tested the relationship between rainfall and sedimentation rate. They concluded that the correlation was best when a model was used which introduced a time lag between periods of high rainfall and unusually thick varves of four years. The delay in sediment reaching the basin floor after rainfall is best explained by storage of the fine sediment on the shelf and subsequent resuspension over a period of years until all the excess fine-grained material is trans ported to the deep basin floor (Drake and others, 1971). Although a measurable flood layer began to form in the deep basin 4-6 months after the 1969 floods, continued resuspen sion of additional silt and clay stored on the shelf would be expected for several years (Drake, 1972). MASS MOVEMENT IN MARINE SEDIMENTS Mass movement refers to the movement of material "under the influence of gravity but without the active aid of any transporting medium" (Sharpe, 1938). In the marine environment, it is operative on the continental slope and rise, and appears to be ubiquitous on large portions of slopes and rises of all continental margins of the world. As an example of its extent, Embley (1980) has estimated that at least 40% of the continental rise off eastern North America is covered by a veneer of mass movement deposits. 41 Mass movement is but one of a number of sedimentary processes responsible for transporting terrigenous sediment across the continental slope into the deep sea. Numerous examples of mass movement have been documented on all submarine slopes which have been sufficiently surveyed. Early recognition of the importance of slumps in ; generating turbidity currents focused on the 1929 Grand Banks earthquake (Heezen and Ewing, 1952; Heezen and Drake, 1964). The importance of rotational slumping on the ! i continental slope off the eastern United States was later recognized (Embley and Jacobi, 1977; Emery and Uchupi , 1972; Rona and Clay, 1967; Rona, 1969; Stanley and Silver- berg, 1969 ; Uchupi, 1967). Elsewhere throughout the Atlantic, translational slides have been documented on a wide variety of slopes (Embley and Jacobi, 1977; Jacobi, 1976; Malahoff and others, 1980; Moore and others, 1970; Summerhayes and others, 1979). A third important type of mass movement, debris flows, appear to be a very important i t depositional process capable of transporting sediments over l great distances, and were first recognized in the Atlantic (Embley, 1976, 1980). Debris flow deposits consisting exclusively of silt and clay, termed mud flows, have since been recognized in the Mediterranean (Stanley and Maldo- Inado, 1981). Mud flows appear to be particularly important off areas of high sedimentation rates, such as the Missis- sippi Delta (Coleman, 1976; Prior and Coleman, 1973; Prior and others, 1979). Mass movement phenomena are not restricted to the Atlantic ocean margins. In the Gulf of Mexico, in addition to mud flows on the upper Mississippi delta (Coleman, 1976), features on the Mississippi Fan which traveled considerable distances were termed slumps (Walker and Masingill, 1970), but appear to have the morphologic characteristies of mud flows. Lehner (1969) demonstrated I ; jextensive mass movement on the upper Gulf of Mexico slope. In the Pacific, Normark (1974) delineated a slide sheet off Baja California which moved down a 3 degree slope in late Pleistocene. A particularly significant finding within the California Continental Borderland has been that small-scale mass movement features may be more important than previ ously recognized (Edwards, 1979; Field and Clark, 1979; Field and Edwards, 1980). Mass movement features are found on all slopes in the Borderland (Field and Edwards, 1980) on continental slopes (Haner and Gorsline, 1978) as well as jinsular slopes (Field and Clark, 1979; Nardin and others, I 11979a)• Mass movement in the Pacific is also operative in trench settings (Moore and others, 1976). Despite the extensive study of mass movement features on continental slopes of the world, relatively little is known 43 about the mechanics, sedimentary structures, exact triggering mechanism or even what to call specific mass movement types. A wide spectrum of triggering mechanisms has been recognized. Mass movement and slope failure can be triggered by oversteepening of a slope by depositional, erosional or tectonic means, horizontal accelerations caused by earthquakes, tidal or surface wave action, gas charging, overloading of the sediment with an accompanying increase in pore pressure, and leaching by ground water (summarized in Nardin and others, 1979b). In order to generate a slope failure, a slope must obviously exist. For a packet of sediment on a slope, the magnitude of the downslope component of gravitational force is directly proportional to the declivity of the slope (Field, 1981). The slope can be quite gradual. Numerous examples have been documented where slope failures occur at less than 4 degrees (Field and Edwards, 1981; Haner and Gorsline, 1978; Lewis, 1971) and as low as 0.5 degrees (Prior and Coleman, 1978). At the other extreme, slope sediments can be theoretically stable at gradients of nearly 25 degrees, less than the angle of internal friction (about 30 degrees), provided there are no external forces and the sediment is normally consolidated (Morgenstern, 1967). Thus, declivity of the slope is less important than other factors in controlling slope stability (Field, 1981). Declivity is important to the abundance of slope failures, but is subordinate to sedimentation rate as a controlling factor (Field, 1981). If the sedimentation rate is high, slope failure will occur regardless of the slope gradient (Field, 1981). In general, the steeper the slope, the more frequent and abundant slope failures will be, given that sediment accumulation is occurring and that a triggering mechanism occurs (Moore, 1961; Field, 1981). Shear strength and consolidation state are both more important than declivity in controlling slope failure, and both are related to sedimentation rate (Moore, 1961). Consol idation state of marine sediments is particularly important in considerations of slope stability. The case where sediment is underconsolidated, that is, where excess pore pressure exists due to inadequate expulsion of pore water or gas can be produced by at least four mechanisms: 1) rapid sedimentation rate; 2) gas in sediments, usually by reduction of organic material at depth to produce methane; 3) injection of artesian water or gas pressure from strata underlying the deposit; or 4) residual pore pressure from the loss of overburden (Booth, 1979; Bryant and others, 1967; Sangrey, 1977). In areas of high sedimentation rates of material containing high concentra tions of reducible organic matter, such as off the Missis sippi Delta, production of methane by reduction of organic matter appears sufficient to support methane in the bubble phase, and results in significantly lower shear strength for these apparently underconsolidated fine-grained sediments (Whelan and others, 1976). Regardless of the cause of underconsolidation, the high pore pressures in the sediment decreases the shear strength and increases the susceptibility of the sediment mass to fail (Terzaghi, 1956; Morgenstern, 1967). At this point, the exact relationships which cause submarine slope failure are poorly-defined and require considerably more submarine sampling, geotechnical analysis and refinement of the mechanical relationships. One approach to determining whether a marine slope will fail has been the calculation of safety factors (e.g., Booth, 1979). This model, the infinite slope model, states that the stability of a sediment mass is inversely related to the excess pore pressure, the angle of internal friction with respect to the effective stresses, and the slope angle. Stability is directly related to the sediment thickness and the submerged bulk density (Booth, 1979). Unfortunately, this model assumes that only gravity acts on the mass and ignores horizontal accelerations from earth quakes or other triggering mechanisms (Booth, 1979; Hein and Gorsline, 1981). Consequently, it is unrealistic for areas with considerable seismicity, such as the California Continental Borderland. Another simple approach utilizes the infinite slope model, but introduces an acceleration term, which can be taken as earthquake acceleration (Morgenstern, 1967). Using this approach, on a slope of 5 degrees off northern California at a distance of 30 km from a magnitude 7 earthquake, the earthquake affect is 3-4 times greater than the influence of gravity on sediment stability (Lee and others, 1981; cited in Field, 1981). Using a similar approach, Edwards and others (1980; cited in Field, 1981) demonstrated slope instability on slopes of 2-4 degrees for horizontal accelerations of only 0.13-0.17 g on the mainland slope south off Los Angeles. While the exact relationships between geotechnical properties, triggering mechanisms and consequent slope failure need much more refinement, it seems clear that earthquake-in duced horizontal accelerations are capable of causing slope failure and mass movement even on very slight slopes. It is tempting to speculate that, in seismically active continental margin slopes, no slope is slight enough to preclude slope failure and initiation of mass movement in areas of high sedimentation rates (100-1000 cm/1000 yr; Field, 1931). Classifications of types of mass movement are signifi cant in that they strive to relate observed deformational structures and morphologies to a mechanical framework. As such, these classifications are genetic, while based on descriptive features of inferred mechanical origin. Considerable controversy still exists as to which classifi cation schemes are most correct (see Nardin and others, 1979b and Lowe, 1979, for example). The controversy stems from an incomplete knowledge at present as to the mechan ical nature of deformation in marine sediments mobilized by mass movement, and can probably only be resolved by two [simultaneous approaches: 1) further documentation of i geotechnical properties from known, recent mass movement features; and 2) more laboratory studies under simulated hydrostatic pressures with analogous materials to known transported marine sediments where scale problems have been adequately solved. Nonetheless, some very good attempts have been made to date which will be useful in this study to infer the general mechanics of deformation and movement from sedimentary structures and morphology. i i | Using the analogy between subaerial and submarine mass movement processes and deposits Dott (1963) developed the first good classification of subaqueous gravity deposi- tional processes. Subsequently, numerous classifications have been proposed and refined which cover all or most mass movement processes and their products (Carter, 1975; Lowe, 1979; Middleton and Hampton, 1973, 1978; Nardin and others, 1979b). The two most significant classifications at present are those by Nardin and others (1979b), which is actually a refinement of Middleton and Hampton's (1978) classification, and that of Lowe (1979) which is based on a different rheological approach. Both classifications are shown for comparison (Figure 12 and Figure 13). The main difference between the two classifications lies in the definition and rheological interpretation of debris flows, jwhich will be important for this study. Lowe (1979) recognizes, quite correctly, that mud flows, a type of debris flow, are supported by cohesion of a clay-water i i matrix slurry. Thus, he terms these cohesive debris flows or mud flows. I ; Many characteristics of mud flows are relevant to this study. In particular, the shear during deformation is distributed throughout the sediment mass, and the strength of the mass is principally due to the cohesion from clay jcontent (Lowe, 1979; Nardin and others, 1979b). Buoyancy iwithin the mud flow can support large blocks up to boulder i ? Isize (Hampton, 1979), resulting in pebbly mudstones which | 'have been recognized in the stratigraphic record as having i | been produced by mass movement rather than by glacial i sedimentation (Crowell, 1957; Dott, 1961). Mud flows with i pebbles or blocks often give the appearance that the j 4 9 ! Figure 12: Mass transport types, mechanical behavior transport and sediment support mechanisms structures and acoustic characteristics. From Nardin and others (1979). Sediment Gravity Flow Slide Mass Transport Processes Mechanical B ehaviour Transport Mechanism and Sediment Support Acoustic Record Characteristics Sedimentary Structures Rock FaU Glide Slump Liquified Flow Fluidized Flow Turbidity Current Freefall and subordinate rolling of individual blocks or clasts along steep slopes Strong bottom return, poor internal return, hyperbolae. Grain supported conglomerates, disorganized, open network variable matrix. Debris Flow- Mud Flow § £ Inertial ^ jS Viscous u i Plastic Limit Plastic Liquid Limit Shear failure along discrete shear planes with little internal deformation or rotation Shear failure accompanied by rotation along discrete shear surfacea with little internal deformation Shear distributed throughout the sediment m » r i \ Strength is principally from cohesion due to clay content. Additional matrix support may come from buoyancy. Cohesionless sediment supported by dispersive pres sure. Flow may be in inertial (high concentration) or viscous (low concentration) regime. Usually re- , quires steep slopes. / Internal reflectors essentially continuous and undeformed but may show some contortion. Hummocky surface locally, hyperbolae if very irregular. Low-angle faults may be visi ble. Strong basal shear surface reflector. Reflectors dip into slope if slumped. Essentially undeformcd, continuous bedding although some plastic deformation may be present particularly at the toe or base. Plow structures, folds, tension faults, joints, slick- ensides, grooves, rotational blocks if slumped. Sea floor reflectors may be hyperbolic, irregu lar, or smooth; may also be prolonged. Few, if iny, internal reflectors (transparent). Mounded or lense shaped. Blunt distal termi nations. Matrix supported, random fabric, clast size variable, matrix variable. Rip ups, rafts, in verse grading and flow structures possible. Massive, a-axis parallel to flow and imbricate upstream, inverse grading near base. Cohesionless sediment supported by upward displace ment of fluid (dilatance) as loosely packed structure collapses, settling into a more tightly packed frame work. Requires slopes in excess of 3°. Individual flow deposits generally loo thin to be resolved acoustically. Generally, as with turbidites, sequences of deposits may pro duce reflectors but pattern is unknown. dcwatering structures, sandstone dikes, flame- load structure, convolute bedding, homogen ized sediment Cohesionless sediment supported by the forced upward motion of escaping pore fluid. Thin (<IOcm) and short-lived. / Bouma series Even layered, continuous, onlapping reflectors. Discontinuous or hyperbolic reflectors if channels present. Supported by fluid turbulence cn f l o w BEHAVIOR FLOW m * t SEO IM EN T S I P IM R T M E C H A N IS M FLUID FLUIDAl FLOW TURBIDITY CURRENT FLUIDIZED FLOW FLUID TURBULENCE ESCAPING PORE FLUID (FULL SUPPORT) LIQUEFIED FLOW ESCAPING PORE FLUID (PARTIAL SUPPORT) PLASTIC (BINGHAM) DEMIS FLOW GRAIN FLOW M U D F L O W OR C O H E S IV E DEBRIS FLO W DISPERSIVE PRESSURE j MATRIX STRENGTH | MATRIX OENSITY Figure 13: Flow behavior, flow type and sediment support mechanisms for sediment gravity flows. From Lowe (1979). pebbles are "floating" in the muddy matrix, and that matrix support of the elasts is evident. In addition, pebbly mud flows often exhibit "inverse grading" eharaeterized by an upward increase in either or both clast size or percentage of clasts (Fisher, 1971). Explanations for this phenomenon have included kinetic sieving (Middleton, 1970), dispersive pressure (Bagnold, 1954) or lift forces from fluid dynamic boundary effects (summarized in Naylor, 1981). A new mechanism has been proposed by Naylor (1981), in which the strength loss (sensitivity) that clay suffers upon deforma tion results in the fact that the lowest, most strongly sheared layers of a muddy debris flow are weakest and, thus, support relatively small clasts. In this model, coarse lag deposits would be expected to occur upslope of inversely graded deposits (Naylor, 1981). Actual measurements and observations of the causes of particle support are valuable in explaining reverse grading and the floating of coarse debris by mud flows. Pierson (1981) has monitored a subaerial mud flow on slopes of 5-7 degrees at Mt. Thomas, New Zealand. About two-thirds of the weight of large particles was supported by buoyancy and about one-third by static grain to grain contact within zones of little or no shear in Mt. Thomas debris flows. Grain to grain contact was replaced by turbulence and dispersive pressure in boundary shear zones of low velocity flows and in high velocity, turbulent debris flows. Surprisingly, cohesive strength of the clay/silt/water interstitial fluid provided less than 2% of the force suspending particles greater than 1 cm in diameter (Pierson, 1981). In this noncohesive setting, support mechanisms may be quite different, however, than in presu mably cohesive marine mud flows. Laboratory experiments on debris flows lend valuable insight into the mechanisms and sedimentary character of movement. Johnson (1970) and Hampton (1972) have shown that a rigid plug of sediment overlies a laminar flow field or is "sandwiched” between laminar zones depending on the distribution of shear stress. Hampton (1972) has also shown that turbidity currents may be generated by debris flows by the generation of a turbulent cloud at the top of experimentally produced debris flows. Because shear and flow duration depend on competence of material within a tdebris flow, zones of variable competence may produce layers of various grain sizes either normally and/or inversely graded within a particular flow (Hampton, 1975). Forms of mass movement are often intimately associated. A continuum of forms of slope failure and mass movement was first recognized for the generation of a turbidity current from the Grand Banks slump (Heezen and Ewing, 1952). Depending on various conditions on the slope, such as I 54 gradient, texture, geo technical properties and magnitude of the triggering event, slides, mass flows and fluidal flows likely form a continuum of mass movement processes (Middleton and Hampton, 1976). In addition, several different flow mechanisms may operate simultaneously or in sequence at a particular site of slope failure (Lowe, 1979). In interpreting deposits from mass movement, one must recognize the importance of multiple transport mecha nisms (Lowe, 1979). PREVIOUS STUDIES IN SANTA BARBARA BASIN Numerous studies have taken place in Santa Barbara Basin largely because of the unique laminated character of the bottom sediments and the low oxygen conditions within the deep basin below sill depth. These studies have beea sedimentological, paleontological or geochemical in scope and will be reviewed to set the background of previous relevant work performed in the basin. Emery (1960) was the first to suggest that the lamina tions found in cores from the deep part of the basin represented varves of yearly deposition in an anoxic basin, based on close correspondence between varve counts and i radiocarbon dating. In addition, he suggested the possi bility that varve thicknesses might correlate with varia- 55 tions in past rainfall and tree rings (Emery, 1960), a contention later tested by Soutar and Crill (1977). Two laminae constitute a varve in these laminated sediments, the darker lamina consisting of detrital silt and clay transported to the basin by winter rains, and a lighter- colored diatom-rich lamina representing planktic produc tivity in the remainder of the year in the surface waters which support high organic productivity (Hulsemann and Emery, 1961). Four types of strata were recognized in the deep Santa Barbara Basin floor: 1)laminae; 2) disturbed laminae; 3) homogeneous; and 4) turbidites (Hulsemann and Emery, 1961). Gray silt and clay layers which exhibited fining-upwards textural patterns were interpreted as turbidity current deposits. The thickness and percentage of gray layers increase towards the north and grain size increases towards the north, indicating that the turbidity currents originated from that direction (Hulsemann and Emery, 1961). They were able to correlate gray layers across the basin from north to south. Laminated sediments were interpreted to record periods of low oxygen conditions in basin waters and homogenous sediments presumably recorded periods of higher oxygen in basin waters (Hulsemann and Emery, 1961). Dissolved oxygen of basin waters in the sub-sill area (475 m) was less than 0.4 ml/1 and the central portion of the low oxygen zone had oxygen contents of 0.1 ml/1 or less (Emery and Hulsemann, 1962). Emery and Hulsemann (1962) suggested that internal waves, one of which they recorded in the basin at 375 m, might be responsible for the oxygenation of the bottom waters. Several studies concerning the micropaleontology of Santa Barbara Basin have been made. Harman (1964) examined foraminifera in grab samples of surface sediment and piston cores. He found several species of benthic foraminifera in the low oxygen zone of the basin with thin, fragile test walls, which are rare in oxygenated environments. The thin tests led him to conclude that the low oxygen conditions decreased the ability of foraminifera to secrete calcium carbonate. Berger and Soutar (1970) examined the preserva tion of plankton shells in box cores, and concluded that selective dissolution of calcareous assemblages explained differences in distributions above and below sill depth. They suggested that the thin-walled foraminifera studied by Harman (1964) may have been present above sill depth, but were easily dissolved. In addition, Berger and Soutar i (1967, 1970) demonstrated minimum turnover times of 2-3 weeks for the living planktonic foraminifera in the basin. Kling (1977) examined radiolaria in box cores collected by Soutar in Santa Monica and Santa Barbara Basin, and dernon- : strated different dominance of species between the two basins, which could reflect the different circulation regimes of the two basins within the California Current. Heusser (1978) examined pollen assemblages in a long piston core and inferred climatic fluctuations requiring the replacement of wet, cool conditions by warmer, drier climate culminating at approximately 5700 year B.P. (before present). In what is probably the most significant micro- paleontologic investigation in Santa Barbara Basin, Pisias (1978, 1979) examined radiolaria down-core in the same core as Heusser (1978), as well as surface sediment samples throughout offshore California and western Mexico. Pisias |(1978) analyzed radiolarian assemblages, and, using a paleotemperature transfer function, concluded that inter vals in the core representing 800-1800, 3600-3800 and 5400-8000 years B.P. record warmer sea surface temperatures than at present. Using data from the analysis of radiola rian assemblages, inferred sea surface temperatures, inferred dynamic height anomalies and existing hydrographic data from the California Current, Pisias (1979) developed reconstructions of the California Current for selected time !intervals. i i ! Based on a set of very carefully collected, vented box core samples, Soutar along with many other investigators examined numerous sedimentological and geochemical aspects of the laminated sediments representing the last few hundred years. Most importantly, a close correspondence between dates determined using the Pb-210 method and actual 58 varve counts conclusively established the annual nature of the varves consisting of paired laminae (Koide and others, 1972; Soutar and Crill, 1977). A mat-forming organism found within the laminated zone is partly responsible for the density difference between the dark "winter” laminae and lighter "summer” laminae (Soutar and Crill, 1977). In addition to establishing laminae thickness patterns caused by rainfall (discussed elsewhere in introduction), Soutar and Crill (1977) investigated the relationship between ! |”interruptive" sediment layers and earthquake shocks without conclusive results. Knowing that the sediments in the box cores contained a valuable historical record , mercury concentrations, lead concentrations, concentrations of halogenated hydrocarbons and concentrations of bomb-pro duced Fe-55 were all examined in light of anthropogenic contamination within the sedimentary record (Horn and others, 1974; Krishnaswami and others, 1973; Young and others, 1973). Fish scales found in the same box cores j reflect known historical estimates of population and allows estimates of fish biomass in the past (Soutar and Isaacs, 1974) . Studies of sediment types in the basin have yielded significant results. In particular, the role of pelleting in sedimentation and the fluxes of radioisotopes proved to be particularly significant. Sediment collected in sediment traps during the rainy season consisted of clay flocculates with abundant silt particles, while that collected after the rainy season had passed consisted almost entirely of fecal pellets (Soutar and others, 1977). Several types of sediment traps were deployed in March and April of 1978 as a part of an intercalibration experiment (Dymond and others, 1981). Dunbar and Berger (1981) showed that fecal pellets constituted more than 60% and perhaps as much as 90% of collected material and that pellet transport :can account for approximately one-half of the sediment flux to the basin floor, including the terrigenous supply. It is thus likely that terrigenous detritus produced by runoff is quickly cleared from the water by pelleting and thereby restricted to nearshore basins because of the high settling velocity of pellets and short residence time in the water column. It should be noted, however, that the period of trap deployment was during the Upwelling Period (Pirie and Steller, 1977), so biological activity within the surface mixed layer would have been enhanced and pelleting might well have been much greater than the remainder of the rainy season. Analysis for isotopes of uranium, thorium, radium, lead and polonium on sediment trap material revealed that the measured fluxes of Pb-210, Po-210 and Th-228 were greater than the predicted flux (Moore and others, 1981). They explain this inconsistency by the intensified scavenging in the water column and the fact that sediments initially deposited on the shelf are subsequently resus pended and transported. The floods of 1969 were the focus of significant study concerning the transport of suspended sediment in Santa Barbara Basin. Following the floods, suspended sediment was transported within the water column, concentrated along density interfaces largely determined by temperature contrasts (Drake, 1971). Flood sediment was initially I deposited on the Santa Barbara-Oxnard Shelf and subse quently resuspended by wave and current action to be transported into the deep basin as a measureable flood layer of reddish oxidized material (Drake and others, 1972; Drake, 1972). Similarities in the mineralogy of the gray layers within piston cores of the central basin led Fleischer (1972) to conclude that they were flood layers, not turbidites. Because the source of sediment incorpo rated into turbidites would be the same as that of flood material, similarity of mineralogy does not preclude a turbidite origin for the gray layers (D.E. Drake, pers. comm.) . Important diagenetic changes take place in marine sediments, especially within an anaerobic basin setting. Organic nitrogen is lost preferentially over organic carbon by preferential destruction and loss of ammonia from proteinaceous compounds over more refractory carbon-bearing compounds (Mulhern, 1976). This results in a general increase in C:N ratios with depth (Emery and Rittenberg, 1952; Emery, 1960). This process results in considerable concentrations of ammonia in Santa Barbara Basin sediments in the anaerobic zone (Rittenberg and others, 1955). By itself, organic carbon content decreases with depth in Santa Barbara Basin laminated zone sediments (Heath and others, 1977). Within the uppermost one to two meters of basin sediment, organic material is oxidized as a result of sulfate reduction (Sholkovitz, 1973). Below this zone, methanogenic bacteria degrade organic material to produce significant concentrations of methane (Barnes and Goldberg, 1976). Furthermore, the production rate of methane corre lated directly with the amount of organic matter in the basin (Doose and Kaplan, 1981). An excess of methane is not accumulating in Santa Barbara Basin because a balance is reached between production by methanogenic bacteria and consumption by sulfate-reducing bacteria (Barnes and Goldberg , 1976). In terms of sedimentary environments and depositional processes within the basin, no single investigator has synthesized the processes operative for the entire basin, with the exception of Fischer (1972, 1976). His data base was primarily seismic reflection data in the basin deeper than the shelf break and is most relevant to the Eocene to Pleistocene as summarized in the introductory section on tectonics. Edwards and Gorsline (1978) found evidence of current scour and winnowing on the Hueneme Sill area at the east side of the basin, at approximately the same position as an area identified as landslide deposits by Green (1976). The first report of mass movement in the basin involved an area on the north slope defined as a "slide” by Duncan and others (1971; cited in U.S. Geological Survey, 197^). Subsequently, an environmental assessment conducted before an oil lease sale resulted in a map of mass movement features (Crissman and Ploessel , 1979; Ploessel and others, 1979) which was later revised (Richmond, 1980). In an attempt to understand exactly what processes produced these mass movement features, attempts were made to analyze Atterberg limits (Hein and Gorsline, 1981) as well as seismic reflection data, piston and box core samples and bottom photographs (Hein and Thornton, 1979; Thornton and Crissman, 1979; Thornton, 1980, 1981). It is at this point, where areal distribution of the features is estab lished, but little else, that this study commences. Chapter II METHODS GENERAL STATEMENT The goal of shipboard and analytical methodology for this study was to typify the depositional surface, define the morphologies and lineations within the uppermost 10-50 meters from acoustic records, analyze the small-scale sedimentary structures within the uppermost 6.0 meters, characterize some basic down-core variations in sedimentary characteristics and, with the combined knowledge of these aspects of the sediment pile, to portray the three-dimen sional shape of particularly interesting deposits. Several different shipboard and analytical techniques were neces sary to fulfill this goal. In essence, because the sedimentary record must be sampled remotely, one strives to collect substantial sample material in order to accurately depict the stratigraphic record represented by the upper most several meters of the unconsolidated basin. 64 SHIPBOARD METHODS Between 1976 and 1980, the R.V. Velero IV occupied and sampled 68 box coring stations, 25 piston coring stations and 600 line kilometers of seismic data in the Santa Barbara Basin (data locations shown on Figure 14, Figure 15, and Figure 16, respectively). Positional accuracy of station locations within the Santa Barbara Channel using LORAN-C is normally 400 m, and resolution is within 5% of the distance to the target. Drill ships and drilling rigs in the channel provided numerous additional targets for radar in addition to the LORAN-C grid. These additional targets improved the positional accuracy in some parts of the channel to 100 m. Station positions were taken when the box corer or piston corer was on the bottom at the time of core entry, evidenced by the winch tensiometer. Box core subsamples were taken following the methodology of Edwards (1979), with the exception that no benthic macrofauna samples were taken. Longitudinal slab samples j | were taken with 18 cm by 2 cm plastic sampling pieces 30 cm long or 60 cm long, depending on box core penetration. Surface textural samples were carefully scooped from the | 0-2 cm portion of the box core sample with a plastic spoon and were placed in bags. Square 10 cm by 10 cm plastic sampling tubes 60 cm long were inserted into the box core Figure 14: Box cores collected for this study as well as most of the previously collected box cores deeper than the shelf break. Noples • • Pt. Conception SANTA BARBARA . 27 28 SCALE (km) Vtntura NRiver 27154 27l ?5 . 72 • 29290 27146 27145 7 C 4 27147 27i 52 2 7 1 5 6 /4 7 1 6 3 | • • • / ' 28869 * •27780 *52, K*^>, * « 27140 e n t u r a *8282 28279 • v •27886 •27890 27885 • 27141 27143 27148 28268 26^75 28267 27173 28283 • *27753 27151 27158 ' 7159 27160 27I4\.„. / 28248* 27149 27150 28278. 2 7 1 4 2. 27748 25413 27891 27743 7<J0^2T893 27745 28277 24825 25414 • 24829 25415 25419 I ,27744 28254 28253 2774 26985 28233 * • 28251 25416 * 25417 • 25416 tr 24845* / u 4824 25 San Miguel I. 24819 420 ’ .25421 24860 25427# Santa Rosa I 254 2 4 Anacapa 24819 CT> Figure 15: Piston cores collected for this study as well as previously collected piston cores. iPt. Cpnceptiqi SCALE O tm ) 28081 • •28082 28077 Santo Rosa I. Figure 16 : Seismic lines collected for this study, as well as previously collected lines in Hueneme Sill area . 70 PI. Conctpl ! Figure 17 Seismic lines from U.S.G.S. open-file report on environmental hazards in Lease Sale 48 examined in western and central Santa Barbara Basin. 72 —3 UJ Conception LINES Coal Oil Pt San Miguel I. onto Rosa I Santa Rosa I. sample for down-core analysis of the texture, calcium carbonate content and organic carbon content. Square 10 cm by 10 cm plastic sampling tubes 60 cm long were taken for microfaunal analysis not used in this study. For analysis of water content, bulk density and porosity, samples were taken from the box core sample surface and bottom using 25 mm by 25 mm stainless steel cylinders. These samples were then capped on both ends by plastic squares secured by rubber bands, and placed in sea water to prevent dessica- jtion, and to preserve the original properties (E.L. I Hamilton, pers. communication). Piston core samples were cut at approximately one meter intervals, capped, taped, (labelled and stored vertically on board ship. Positioning for seismic lines was taken at the beginning and end of each line and at 15 minute intervals between. I The Sxhip's speed was held as constant as possible at 6 knots (11.1 km/hr). Both 3.5 kHz high-resolution seismic reflection profiling and air gun, deeper-penetration seismic profiling were performed simultaneously. The high-resolution profiles were taken using an Edo Western model 515H 3.5kHz piezoelectric transducer, model 248C transceiver and booster in conjunction with an Edo Western dry paper recorder. All profiles were recorded with a 0.25 sec sweep duration, a 0.5 msec pulse length and fast paper feed. The seismic system is capable of yielding vertical resolution of 0.75 m and penetration up to 50 msec (two-way travel time, about 40 m). The vertical exaggeration, depending on ship’s speed, varied between 10:1 and 15:1 (Dobrin, 1976; Sheriff, 1976). In addition to seismic lines collected for this study, U.S.G.S. open-file high- resolution seismic lines were examined in a close grid spacing in western and central Santa Barbara Basin, which were collected by McClelland Engineers (Figure 17). One jmust continually bear in mind the considerable vertical exaggeration when interpreting structures, slope features and correlating acoustic reflectors. While the vertical exaggeration can cause interpretive problems, the compressed horizontal scales which result make subtle features more easily recognizable. For air gun, deeper- penetration seismic reflection profiling, a Bolt model 600B air gun, McClelland Engineer’s single channel, 20 element hydrophone, amplifier, variable bypass filter and GIFFT wet-paper recorder were utilized. This air gun was fitted with a 10 cubic inch chamber pressurized at 1600-2000 psi and was towed at a depth of approximately 30 m. A one i | second sweep duration was utilized, which allowed the gun to be fired at a rate of 1/3 sec. The maximum stored !energy and fundamental frequency of the 10 msec bubble i pulse in this arrangement are 3140 joules and 21 Hz, respectively. Most of the energy is present in frequencies less than 500 Hz. On the bandpass filter, the frequency used was 50-2000 Hz. Resolution to 2.5 m and penetration up to 500 msec (about 400 m) was achieved with this system. Vertical exaggeration varied between 10:1 an 15:1, depending on ship’s speed. For this study, all of the seismic-reflection records used are single channel analog data. Normally, computer processing would attenuate unwanted reflection and random noise, correct for amplitude jchanges not associated with reflectivity, improve resolu tion and migrate reflections to their proper spatial positions. However, computer processing was not performed on this data. ANALYTICAL METHODS Point Counts Point counts were made of sand constitutents for the surface (0-2 cm) samples. Bulk samples were carefully wet-sieved on a 230 mesh (63 micron) sieve and oven dried at 90 degrees C. Constituent grains were identified as mineral grains (with subcounts for mica and opaque mineral s) , spicules, radiolarians, planktic foraminifera, benthic foraminifera, carbonate fragments (foraminiferal fragments, mollusk fragments, micro- mollusk fragments, and corroded, unidentifiabls carbonate grains), fecal pellets, plant fragments, fish scales and ostracodes. The main goal of examining the sand fraction was to ascertain terrigenous versus biogenic sources of sediment for this readily-exam ined portion of the total sediment size spectrum. Point counts were performed by splitting dry sand fractions which resulted from textural analysis into a small aliquot of at least 300 grains. These samples were carefully scattered onto a micropaleontological picking tray which was divided into numbered grids. This approach to point counts is particularly easy to perform for uncon solidated sediment as grains can be readily reoriented using a camel's hair brush or picking off the tray and placed on glass slides for examination under transmitted light with or without refractive index oils. The counting of at least 300 grains allows one to determine mineral or biogenic constituents at the 95% confidence level (Krumbein and Pettijohn, 1938). Once the sample has been placed on the picking tray, all grains must be counted, because larger grains tend to fall out of the splitting pans first, ! | followed by progressively smaller grains. When gross j ! overestimates were made of the split fraction, and too many1 grains were placed on the tray, every other grid was | counted, using the area method suggested for point counts of thin sections (Galehouse, 1971a, p. 391-392). 77 Textural Analysis Textural analysis of the fine-grained sediments from this study area was performed using pipette analysis for silt and clay-sized fractions, and, if necessary, settling tube analysis for the sand-sized fraction. Textural analysis was performed for 78 surface (0-2 cm) samples from box cores, 150 samples from 8 selected box cores for down-core analysis, and for 521 samples for selected intervals from 14 piston cores. Pipette analysis followed the method of Galehouse (1971b). Each sample was first treated with a mixture of 50% distilled, demineralized water and 50% acetone for two 24 hour periods to remove organics, and the supernatant fluid was decanted. Then each sample was washed with distilled, demineralized water for two 24 hour periods to remove salts. Presence of either salt or organics would induce flocculation and render the results of pipette analysis meaningless (Galehouse, 1971b). After removal of organics and salt was complete, each sample was dispersed with 10 ml of sodium hexametaphosphate (Calgon) solution (65 g/1) in a malt mixer by agitating for five minutes, wet-sieved through a 63 micron sieve and placed in a 1000 ml graduated cylinder. This procedure for sample preparation prevented flocculation in all but a very few samples, which were subsequently re-analyzed. The cylinder was then filled to 500 ml with demineralized water, and pipette analysis begun using a 50 ml pipette with enlarged opening following procedures for depth of removal and time sequence of removal in Galehouse (1971b) for analysis at whole-phi size intervals. Using a Monroe model 1860 programmable caculator, measures of central tendency were computed by the method of moments (McBride, 1971). It should be emphasized here that moment measures were calculated using the complete size spectrum data, rather than by using less accurate graphical techniques. If the percentage of sand retained on the 63 micron sieve exceeded 10% of the total sample weight, the sand fraction was analyzed using a settling tube (Cook, 1969; Felix, 1969) and analysis was, likewise, performed at whole-phi intervals. Settling tube data were then combined with pipette data and measures of central tendency were computed by the method of moments. Calcium Carbonate and Organic Carbon Determination of calcium carbonate and organic carbon contents was performed for all box core tops (0-2 cm) at 68 sample localities, for a downslope transect of six piston cores, and for one additional piston core west of the Hueneme Sill. Total samples analyzed for the piston cores was 3^6. Samples analyzed in piston cores were chosen at even 10 cm depth in core plus sufficiently thick gray layers, where present. The goal of analysis of calcium carbonate content across the basin floor was to ascertain the role of terrigenous dilution caused by current trans port of river-derived sediment from the Santa Clara and Ventura Rivers to the slopes and deep basin floor. The goal of analysis of organic carbon content across the basin floor was to assess varying organic carbon preservation on the slopes and deep basin floor as a response to water column oxygen content and biologic utilization. Down-core analysis of these two factors in piston cores allows interpretation of variations with time for the last few thousands of years. Analytical determination of calcium carbonate content and organic carbon content was accomplished using the technique of Kolpack and Bell (1963), which utilizes a Leco combustion furnace and gasometric determination apparatus. First, all samples were oven-dried at 90 degrees C and powdered prior to analysis. Calcium carbonate is deter mined in this method as a weight percent by volumetric determination of absorbed carbon dioxide which results from acid digestion. Total carbon is determined by combustion of all carbon and measurement of the volume of evolved carbon dioxide. Organic carbon is then derived from the difference between total carbon content and carbonate carbon content by the formula: organic carbon = total carbon - (calcium carbonate/8.33) Analysis of replicate samples yielded a precision of between 1-2% of the value. During analysis, accuracy is calibrated frequently by using reagent-grade calcium carbonate or standard carbon rings. ' Piston Core Extrusion and Sampling All piston cores were stored vertically in a refriger ated walk-in cold room until sufficient desiccation occurred to allow undistorted core extrusion. Piston cores were extruded in approximately 50 cm sections, which seemed to hold compression of the sediment sample to a minimum. The sediment was extruded by placing a long rod fitted with a rubber cork of the same size as the internal diameter of the core liner into the downcore end of the core liner. Then the core liner was evenly pulled down past the cork as the sediment was extruded onto a half-cylindrical extruding i apparatus. When this extrusion technique is performed steadily and firmly on adequately dried samples, very little distortion of the sediment occurs. Breaks in the sample were noted, as well as areas of compression or flow, which were nearly always restricted to the bottommost 1-2 cm, if found at all. Core length was measured before and after extrusion, and most foreshortening resulted from the squeezing out of void space in the sediment left by degassing or dewatering. Foreshortening was the exception, rather than the rule, perhaps because the period in which the samples desiccated in the cold room allowed sufficient settling of the sediment to remove void space. Distortion of the sediment sample between the top of the sample and the extruding cork was surprisingly rare. Where present, it could be seen through the clear core liner or after extrusion. In both cases, the exact interval distorted was noted to prevent misinterpretation of sedimentary struc tures by subsequent radiography. Once extruded, each cylindrical core sediment sample was sliced longitudinally using a fine metal guitar string strung on a coping saw frame (hereafter: core cutter). After slicing, the sliced sample was cut every 25 cm with the core cutter. Each 25 cm segment was then transferred to a 25 cm long half-cylindrical core liner using a long spatula to free the sample. Then the longitudinal cut was smoothed carefully with a wet spatula to remove core cutter marks. After this half-cylinder of sediment was inverted onto a rectangular plastic piece, it was once again sliced longitudinally and smoothed to produce a 1 cm thick rectan gular sample for radiographic analysis of sediment struc tures. The remaining portion of the core sample was set aside until radiography was completed, to he recombined with the rectangular slab for sample cutting. Core descriptions were made and recorded for each core from the core surface downward. A Munsell color chart was used to differentiate sediment color. All sedimentary structures, bedding, lamination and other sedimentary features were measured and recorded, to facilitate analysis of radiographs. Slides or prints of each core were made using a camera stand. Either the uncut half-cylindrical core or the individual segments were photographed, depending on which best represented the core sample. After cutting, the unused half-cylindrical sediment sample was covered with plastic wrap, placed in a d-shaped plastic tube with a wet paper towel to reduce desiccation and stored in the cold room horizontally as an archive. Box Core Extrusion and Sampling The purpose of extruding a separate sample for the box core was to accurately sub-sample down-core for texture, 83 calcium-carbonate content, organic carbon content and geochemical properties (used for a separate study). The 10 cm by 10 cm plastic sampling tube 60 cm long taken on board ship was the source for these samples (hereafter: square core liner). After being brought back to the laboratory, these square core liners were stored vertically and allowed to dry slowly in order to be sufficiently consolidated for core extrusion, as with the piston core samples. Depending on original water content of the sediment sampled, 4-6 weeks of drying proved adequate. In no case were the samples allowed to desiccate completely, as this would render the samples worthless for textural analysis and would destroy the sedimentary structures. An unavoidable | side-effect of the drying process was settling of the i sediment sample and core shrinkage. For samples from the i most watery portion of the Santa Barbara Basin floor, core shrinkage can be considerable. Typically, the core samples foreshortened 1-10 cm during desiccation, depending upon original sample thickness and original water content. Average foreshortening was aproximately 4-5 cm for a 55-60 cm long sample. Shipboard records of original core length allow comparison with the desiccated core length used in sub-sampling. Sometimes cracks developed during desicca tion, but these were readily discernible from radiography. After sufficient desiccation, extrusion was quite simple. The core liner was laid horizontally on a labora tory bench. Then a 10 cm by 10 cm wooden block 60 cm long fitted with a flat 10 cm by 10 cm aluminum plate at the top was slowly inserted into the core liner until the entire sample was extruded onto a flat plastic piece. Once extruded one or two plastic strips 10 cm wide, 2 cm deep and 30 cm long were placed on each side of the rectangular solid of sediment. The wire cutter was then drawn along the plastic pieces, and the slabs of sediment removed and the cut surface smoothed. These slab samples were then used for radiography to document the sediment structures from this sample for comparison with radiographs taken from the 2 by 18 cm slabs to ascertain that samples were taken from the same respective layers or units equivalent to those in the large radiography sample. After the radio graph sample was taken, the remaining sediment was cut at 2 cm intervals with the wire cutter for analysis of texture, calcium carbonate content, organic carbon content and geochemical samples. All samples were bagged, labelled and sealed for future use. 85 Radiography The goal of analysis by x-radiography is to determine primary and secondary sedimentary structures in unconsoli dated marine sediments for this study. Because sedimentary structures are one of the most important criteria in determining depositional processes and post- depositional alteration, this technique was essential to this study. X-radiography as a technique for studying sedimentary structures was introduced into sedimentology by Hamblin (1962) in his study of seemingly homogeneous sandstones. The technique was then applied to the study of unconsoli dated marine sediments (Calvert and Veevers, 1962; Bouma, 1964). It is particularly valuable in studying apparently homogeneous, unconsolidated sediments to demonstrate sedimentary structures. In addition, this technique enhances sedimentary structures which can be visually observed, giving contrast and clarity undectable by the eye . The theory, technique and interpreted examples of * x-radiography are thoroughly covered by Bouma (1964, 1969). j ! This technique is based on differential passage of x-radia- i tion through a thin sample of unconsolidated sediment onto j j a special film. Differences in bulk density are caused by i differences in water content, porosity, texture and miner alogy. These differences in bulk density cause differences in absorption of the x-ray radiation which, in turn, causes variation in the amount of radiation that reaches the film. This produces different contrasts of light and dark on the exposed film. Lighter areas on the exposed film then correspond to lower bulk density, higher water content and porosity, coarser grain size, differences in mineralogy or a combination thereof. Because of the relative uniformity of mineralogy throughout the Borderland in terms of x-ray i absorbance, this factor can be neglected. Sedimentary structures are caused by changes in density, water content, porosity and grain size. Consequently, radiography records sedimentary structures. A number of preparation errors can cause signatures on the final prints including desiccation cracks and ’ ’chatter marks" from sample cutting, writing or otherwise putting pressure on the undeveloped film and developing errors (Bouma, 1969). Desiccation cracks and "chatter marks" are easily detected and one can prevent misinterpretation by taking notes on sample description and i cutting as was done in this study. The latter two errors can be avoided by being careful. Two types of samples were examined with the use of ; x-radiography: piston core samples and box core samples. The box core samples (both 10 cm and 18 cm wide) were 87 brimmed bo a bhickness of 2 cm, which yields reasonable image clariby and conbrasb, while allowing one bo visualize relabively bhick sbrucbures (Edwards, 1979). The radio graph conbrasb is sbrongly affecbed by sample bhickness (Bouma, 1969). Consequenbly, bo geb bhe sharpesb images wibh bhe besb conbrasb bhe pisbon core samples were brimmed bo 1 cm bhickness. Thicknesses of 2 cm were abbempbed early in bhe sbudy, bub decreased clariby and conbrasb resulbed relabive bo bhe 1 cm samples. A bhickness of 2 cm on bhe box core samples was rebained merely because of mebhodological uniformiby. All obher box core samples had been done in bhe same manner, and bhe waber conbenb of bhe sedimenb for bhese samples makes bhe secbioning of only 1 cm bhickness impossible, in any case. An addibional, and equally imporbanb reason for brimming bhe pisbon core samples bo a 1 cm bhickness was bo remove bhe curvabure ab bhe side of bhe formerly half-cylindrical sample. This curvabure resulbs in a lighber radiographic prinb, because x-ray absorpbion is inversely relabed bo sample bhickness (Bouma, 1969). Ib is very imporbanb bhab samples nob be allowed bo dry before being subjecbed bo x-ray radiabion. Pisbon core samples can dry over nighb when lefb exposed ab room bemperabure on a warm day. To prevenb desiccabion and bhe cracks that result, piston core samples were x-rayed immediately after cutting. Box core 18 cm wide samples, which were wrapped in plastic wrap aboard ship, were subjected to x-ray radiation as soon as brought back to the laboratory. Box core 10 cm wide samples were subjected to x-ray radiation very soon after extrusion. Both types of box core samples were wrapped in plastic wrap and stored refrigerated until the radiographs were developed, in case mistakes were made in either exposure or development. Such radiographs were retaken. Care must be taken to remove the plastic wrap before exposure, because the resolution of the system is such that folds in the wrap as well as scratches in the plexiglas used for holding the samples during irradiation will show in the radiograph. X-radiographs were taken with an Hewlett-Packard Faxitron Series model 43805N x-ray system. Settings used were 45 kv and 3 mA for all samples. This x-ray system is particularly well-suited for radiography of unconsolidated sediments as it has a film selector for using various film types, a fine exposure control for adjusting contrast an automatic exposure meter, and two different heights at which the sample can be placed depending on sample size. Once placed in the "oven” over the film with lead identifi- cation pieces for sample station number and core depth, the machine automatically exposes the sample for the correct time. For piston core samples, the automatic exposure time varies between 1.2 and 2.6 timer units and averages 1.9 timer units. For the box core samples, the exposure time was much more variable. In nearly all cases, added exposure was given to all the box core radiographs using the manual setting (usually 0.5 timer units), as too much |exposure can be corrected for in the printing process, but not too little. Piston core radiographs were subjected to supplemental radiation only when the automatic exposure seemed suspiciously short, based on the operator1s experi ence and the x-ray log book of all exposures. In these cases, usually 0.2-0.4 timer units were added using the manual exposure setting to correct for an apparently inadequte sample exposure. Because gaps or differences in sediment bulk density will be detected using x-radiography (Bouma, 1969), it is ( imperative that the sample be situated in the Faxitron "oven” such that a typical portion of the sample directly underlies the automatic exposure meter. Piston core radiographs were typically with three 25 cm long core j samples on one film. Consequently, the sample in the middle, underneath the exposure meter was chosen to be either typical of the other two or denser than the other two if differences in density were observed. Many mistakes in radiographs result from incorrect positioning under the exposure meter. Positioning is more difficult for the box core 18 cm-wide samples, as difference in density are often not visualy discernible due to the wetness of the sample. Kodak Industrex AA-2 and Kodak Industrex M-2 were the film types used in this study. Film size was 25.4 by 30.5 cm in both cases. Kodak M-2, a special film designed to give maximum contrast, produced superior radiographs. One must merely be careful to change the film selector on the x-ray system when changing film types. Film development was performed using a six minute immersion in the devel oping tank to maximize contrast. Most mistakes on radio graphs resulted from developing errors. Streaks and lines sometimes appeared, caused by not rinsing the fixer off completely or not rinsing the developer off completely before immersing in the fixer. The most common source of error, however, was old developing chemicals, which result in very little contrast. Developer and fixer must be replaced every three months for best results. Once devel oped and fixed, radiographs were examined and those with mistakes re-taken. The radiographs were printed as positive prints onto photographic paper using an exposure guide for length of exposure. Each radiograph had to be printed onto two 8 by 10 inch pieces of print paper because of the radiograph size. After printing, prints were examined and, when necessary, re-printed at different exposure times to produce better contrast. Water Content, Saturated Bulk Density and Porosity These three basic physical properties were surveyed in the eastern half of the basin on the uppermost sediment collected in box cores at the sediment surface and bottom of box core. The goal of analysis of these properties was to establish their range and their areal distribution. The western half of the basin was sampled before the sampling technique for these physical properties was developed. It was surmised that there might be a general relationship between these properties and zones of mass movement. If not , values of these properties in areas of mass movement would have been documented. The weight-volume or "tube” method used for determina tion of these properties was suggested by E.L. Hamilton (pers. communication). The basic sampling pieces were a 25 by 25 cm hollow stainless steel cylinder cut from plumbing pipe. Each cylinder was engraved with a number, weighed and measured with a vernier calipers. Thus, weight and internal volume were established before sampling. After 92 sampling aboard ship (previous section), these cylindrical samples were brought back to the laboratory immersed in sea water to prevent desiccation and maintain saturation and were stored in a refrigerator. Essentially, all three tests are done at the same time, but use different formulas and the same data. The cylinder, with wet sediment is weighed first in a Petri dish, dried at 105 degrees C and weighed again. No correc tion was made for amounts of dried salts in the porosity determination, because the difference only amounts to 1% (additional) porosity at porosities near 30% (Hamilton, 1970) and this study was not concerned with fine distinc tions in porosity. In the formula for saturated bulk density, 2.60 and 1.02 were assumed for the specific gravities of sediment and pore sea water, respectively. Thus, the determination of these physical properties was performed by the following formulas. A. Wet and dry weight. Wet weight = wet weight sediment-(Petri dish+cylinder weight) Dry weight = dry weight sediment-(Petri dish+cylinder weight) B. Water Content (weight percent) Wet % = ((wet weight-dry weight)/wet weight)x100 Dry % = ((wet weight-dry weight)/dry weight)x100 C. Saturated Bulk Density Saturated Bulk Density = (wet sediment weight)/((dry sediment weight/2.60) + ((weight water(wet-dry)/1,02)weight water) D. Porosity Water volume = weight water/1.02 Porosity = water volume/cylinder volume Water contents referred to in this study used water content determined as wet percentage. This gives water contents less than 100%. The reproducibility of the "tube" I ! method is quite good, and has been reported to by 1% of the absolute value for bulk density and porosity (Bennett and others, 1970). IAtterberg Limits Determination For fine-grained soils and marine sediments, most engineering interest focuses on their mechanical behavior at different water contents (Scott and Schoustra, 1963). A convenient way to measure the qualitative mechanical I behavior of fine-grained soils is by means of simple, empirical mechanical tests called the Atterberg limit ! tests. The tests are named after the Swedish chemist who devised them for use in the clay and pottery industries (Scott and Schoustra, 1968). Atterberg observed that, over a range of water contents, a clay exhibits a characteristic variation of behavior from that of essentially a liquid at 94 | extremely highwater contents to that of a brittle solid when most of the water is absent. Then he devised tests to distinguish when olay behavior changed from that of a viscous liquid to that of a plastic or moldable solid, and to determine when the clay behavior changed from plastic to brittle as the clay dried out (Scott and Schoustra, 1968). The Atterberg limit tests, then, consist of the liquid limit test and the plastic limit test. The liquid limit test determines the liquid limit of soil. By convention, it is defined as: "the water content, expressed as a percentage of the weight of the oven dried soil, at the boundary between the liquid and plastic states. The water content at this boundary is arbitrarily defined as the water content at which two halves of a soil cake will flow together for a distance of 1/2 inch (12.7 mm) along the bottom of the groove separating the two halves, when the cup is dropped 25 times for a distance of 1 cm (0.5937 in) at the rate of 2 drops/s." (Am. Assoc. Testing Materials, 1979)- Procedures used followed A.S.T.M. standards (Am. Assoc. Testing Materials, 1979, p- 125-126) with the following exceptions: 1) three separate tests were taken and averaged; 2) the sample was not sieved to remove coarse sand as no coarse sand was present in any of these samples; 5) samples were much smaller than the suggested 100 g sample size, because of limited sample availability. For 95 xhe liquid limit test, the sample is reworked with water and placed in the cup of a simple device called the Atter berg limit device and a standard groove is cut in the sample. Then, the cup is rapped on a hard surface, which imposes small shearing stresses on the clay and tends to close the sides of the groove (Scott and Schoustra, 1968). Two equations are used to determine the liquid limit (Am. Assoc. Testing Materials, 1979): water content = (mass water/mass dried soil) x 100 and liquid limit = w (N/25)^*^ where: w = water content; N = number of drops of the cup required to closed the groove at the water content, w. The second Atterberg limit test, the plastic limit test, determines the lower limit of plastic behavior for a soil (Scott and Schoustra, 1968). A soil is considered to behave plastically when it can be molded or worked and will retain a new shape without returning to its original shape or fracturing, much like modeling clay (Scott and Schoustra, 1968). By definition: 1 1 The plastic limit of a soil is the water content expressed as a percentage of the mass of oven-dried soil, at the boundary between the plastic and semisolid states. The water content at this boundary is arbitrarily defined as the lowest water content 96 at which the soil can be rolled into threads breaking into pieces” (Am Assoc. Testing Materials, 1979). After this test is completed, the plastic limit is: plastic limit = (mass of water/mass of oven-dried soil) x 100 at which the sample can be rolled into a 1/4 inch thread and just barely break. From the knowledge of these two Atterberg limit tests, a third simple mechanical property can be calculated, the plasticity index. The plasticity index is the range of water contents over which the clay behaves plastically, and is represented by the difference between the liquid limit and plastic limit (Am. Assoc. Testing Materials, 1979): plasticity index = liquid limit - plastic limit Atterberg limits are, admittedly, empirical tests and subject to large amounts of operator error. For the liquid limit test, the number 25 is arbitrary and the way the groove is carved as well as the rapping technique are selected variables (Scott and Schoustra, 1968). If any one of these variables were changed, the results would change. The results depend subjectively on the manner in which the operator carries out the test, the preparation of the soil and many other variables connected with the actual opera tion of the equipment. Comparable results must be produced by the same operator, so only one operator was used for all the Atterberg limit tests reported in this study. After having performed the tests numerous times, the operator reduced the tests to a standard, consistent routine. The determination of the plastic limit is a simpler test and, with a bit of practice, becomes quite simple to determine the stage of behavior consistently. If these Atterberg limit tests are performed consistently, they yield a valuable indication of the type of soil tested and its jmechanical properties (Scott and Schoustraa, 1968). Computer-assisted Contouring and Data Base To improve the objectivity of contouring sediment parameters, a contouring program was utilized called SYMAP (Synagraphic Mapping System; Dougenick and Sheehan, 1977). The program was originally developed by Howard T. Fisher at Northwestern University with seed money supplied by William C. Krumbein. A map is generated in the program to very closely aproximate the shoreline and islands at 1:250,000 scale. All data points are inserted into the program and the program automatically interpolates between values to produce a contour map at contour intervals selected by the user. For this study, the contour lines were excluded, because the output consists of number values covering the entire map surface which can be hand contoured. The advantage of hand contouring the map after the output has directed where the contours must go is that some smoothing of the contours is desirable. Smoothing of contours is necessary because the program tends to produce orthogonal patterns due to the nature of the interpolation algorithm. Several computer-assisted contour maps were previously hand-contoured and compared later with the computer-as sisted version. While the hand-contoured maps were very similar, computer-assisted contouring is easier, quicker and lends objectivity and credibility to the results. A number of electives were desirable to use in the program (Dougenick and Sheehan, 1977). A value range was specified within which all values of interest were included. For several maps, two different value ranges were specified in order to see which value range best demonstrated patterns within the deep basin. A value class interval for contour values was specified at 10 for all contour maps. Specification of minimum and maximum valid data values was made so that any invalid data points were printed as such. Usually, invalid data points resulted from errors of keypunching of the original data, so correc tion could be made before repeating the program. An extrapolation minimum and maximum was specified to prevent meaningless extrapolation beyond expectable value limits. A maximum search radius was specified so that unwarranted interpolation would not be made in significantly large areas lacking control. Data points which are too close together to be separated at this map scale are printed as superimposed data points, most of which were located in the Hueneme Sill or offshore from Gaviota. To supplement sample control collected for this study, additional data sets were utilized so that the entire shelf, slope and basin floor of the Santa Barbara Channel could be included. For the insular shelves of the Northern Channel Islands 344 data points from box core topes were included from Day (1979), aproximately one-third of which were located within the map area designated. These data points had information on mean phi grain size, sorting (standard deviation), skewness, percent fine fraction, sand/mud ratio and percent calcium carbonate. For the shelf and slope area off Gaviota, 24 data points from box core tops were included from Gatto (1970), which had information on percent fine fraction, sand/mud ratio, percent total organic matter, and percent calcium carbo nate. In the Hueneme Sill area, 43 data points from box core tops were used from Edwards and Gorsline (1978), which I included information on all sediment paramenters, except for 26 data points which had no information on percent total organic matter or calcium carbonate percent. In the area of the Ventura-Santa Barbara Shelf, 16 data points were included from box core tops from Kolpack (unpublished data) , which included information on all sediment parame ters, except for percent total organic matter and percent calcium carbonate. In the western portion of the Hueneme Sill and a slope transect from off Santa Cruz Island across Montalvo Ridge, 10 data points from box core tops were included from Hein (unpublished data). These additional data points supplemented the 68 box cores collected in the slope and basin floor of Santa Barbara Basin by covering mostly shelf areas. Only box core tops were used, to | insure that all sample material was essentially the same. Chapter III SURFICIAL SEDIMENT CHARACTERISTICS RESULTS Moment Measures i , ■ . . , . , , ! j Analysis of characteristics of the surficial sediment within a basin allows inference of ongoing sedimentary i processes. In this study, analysis of samples representing the uppermost 2 cm was performed with good areal coverage for textural characteristics as well as organic carbon and calcium carbonate contents. This interval represents about 5-10 years of sedimentation, based on a sedimentation rate determined later in this study. The results of this analysis are shown in the following eleven figures (Figure 18 to Figure 28). ; i The average grain size within the basin (Figure 18) does! not vary a great deal within the deep basin, where the meanj I grain size is between 3-9-7.8 microns (7-9 phi units), near' | I the silt/clay boundary. Mean grain size decreases from the; shelves lying to the north and south of the central basin floor towards the central basin floor. Mean grain sizes on | j 102| the northern and southern shelves are within the sand and coarse silt size classes. A distinct deflection of mean size (phi) isopleths is evident in the Hueneme Sill area between Ventura and Anacapa Island, where an elongate region of medium to very fine sand (1-4 phi) is present. Skewness (phi) within the basin (Figure 19) shows little variation within the deep basin, where sediments are unskewed, or slightly positively or negatively skewed. An area outlined by the -0.2 isopleth (Figure 19) lies just to the west of the basin center indicating slight skewness toward the fine end of the size spectrum. Elsewhere, throughout the basin, slopes and shelves the skewness is positive, or towards the fine end of the size spectrum. The Hueneme Sill area exhibits a great deal of variability in skewness as does the Northern Channel Islands shelf. Sorting, expressed here as phi standard deviation (Figure 20) lies within the poorly sorted to very poorly sorted categories using the conventions of Folk (197*0. Other than portions of the northern and southern shelves, the best-sorted area in the basin lies on the Hueneme Sill where the isopleth of 1.5 phi standard deviation units encircles the area. A second area, roughly halfway between Santa Barbara and Santa Cruz Island, and outlined by the 1.6 isopleth, lies on the crest of the Montalvo Ridge. ! Figure 18: SYMAP computer contour map of mean phi diameter! ! of surface (0-2 cm) sediments. Stippled lines denote isobaths. 104 •• Point Conception MEAN PHI 20km CONTOUR INTERVAL • I PHI UNIT J W e n tu ra \ . * Santa V C I a r a V R iver Santo Crui I jpigure 19: SYMAP computer contour map of phi skewness I surface sediments (0-2 cm). Stippled lines denote isobaths. 107 Point onctption SKEWNESS I CONTOUR INTERVAL J*. 0.3 3 4 *2 0 'N Vtnturo Sonto C* Clara VA ivir iFigure 20: SYMAP computer contour of phi sorting of ! surface sediments (0-2 cm) expressed as | standard deviation, i Stippled lines denote isobaths. 108 : . Point • I20*W * . Coal Oil •Point •.•sqnta Barboro. • . . * • . • • • * Conception SORTING 0- 2.0 POORLY SORTED >2.0 V . POORLY STD 0 1 0 2 ( C E E IIK Z Z Z lH H i .•CONTOUR INTERVAL O.IS.D 3 4 *2 0 N . Santa Clara Rlvar u . 1.5 _ L5 % Santa The deeper portions of Santa Barbara Basin deeper than the 200 m isobath are nearly devoid of sand, with less than 10% sand (Figure 21). Exceptions to this rule are the Hueneme Sill area which contains 10-90% sand, and a single sample point on the Montalvo Ridge which contains 20% sand. Thus, the map of percentage mud (silt plus clay) merely shows the inverse of the percentage sand map, with some greater detail along the Northern Channel Islands shelf and the small sea valleys southeast of Point Conception due to greater number of sample data points (Figure 22). The basin deeper than 200 m is comprised of sediment with greater than 90% mud. By focusing on the percentage of sand for areas having less than 10% sand essentially the entire basin deeper than 200 m, it is evident that most of this portion of the basin has less than 2% sand (Figure 23). A small area within the 550 m isobath has greater than 1% sand in an arcuate pattern deriving from the southern slope. 110 Figure 21 SYMAP computer contour map of percentage sand of surface sediments (0-2 cm). Stippled lines denote isobaths. 1 1 1 1 12 ! - .- .V . .......................... Il20*w i - > ‘ • t ‘ • Cool Oil / • Santa Barbara IQ- 3 Point jConception %SAND 20 km * q a r —■ • CONTOUR INTERVAL 3 4 ° 2 0 N 550m C 3 San M Santa Santo Cruz I. * m Anocopo 10% v*Ventura River JiVentura Santa m Clara P i. V River v y Figure 22: SYMAP computer contour map of percentage mud (silt plus clay) of surface sediments (0-2 cm). Stippled lines denote isobaths. tr L I • Point jConceptioi V • • %MUD Santo Barbara CONTOUR INTERVAL : • • 10% ■Vantura V ' Santa V C Ia ra Y R ivtr 550m Santa Rosa Santo Cruz I. Figure 23: SYMAP computer contour map of percentage sand of surface sediments (0-2 cm) for the areas having less than 10% sand. All areas landward of 10% isopleth have greater than 10% sand, including the Hueneme Sill area. Note that contours are at selected intervals to best demonstrate patterns. Stipple lines denote isobaths. P oint ll20W c o n c e p t i o n s T — 1 ■ * > . . * a. \ . # £ 0 f l | q h T ; •Santo Barbara. . . %SAND<10% • ^ 1 . * — r • B . . » ■ . . — - ■ “ 20km 200m 34*20'- •CONTOURS AT 1,2,3,5,10 Ventura River © * 0^ j: San Mi Santa Rosa I. Anocooo Santa ^■Ventura Santa Clara River As most of the basin deeper than 200 m consists of mud (silt plus clay), it would be worthwhile to examine each of these size fractions. The distribution of percentage silt (Figure 24) is probably the most interesting and signifi cant textural map of the basin floor. Several salient features are evident. First of all, the 60$ silt isopleth offshore from Point Conception defines an area of dominance of the silt size class. Secondly, the 50$ silt isopleth forming an arcuate area on the west portion of the 550 m isobath is striking. The remainder of the eastern portion of the basin deeper than 200 m has greater than 60$ silt, with the exception of the Hueneme Sill area, which is low in silt content because it is predominantly sandy (Figure 21). One can trace the 60$ silt isopleth from offshore of the Ventura River, along the north slope, down to the deep basin floor, where the isopleth mimics the 550 m isobath, and then east, where the isopleth circumscribes the silt- j poor Hueneme Sill. More specifically, the 70$ silt isopleth defines a smaller region as it spreads to the west1 from north of the mouth of the Ventura River and returns to ! just south of the mouth of the Santa Clara River. Thus, j the silt pattern at the 70$ isopleth appears clearly related to the mouths of the Ventura and Santa Clara j Figure 24: SYMAP computer contour map of percentage silt of surface sediments (0-2 cm). Stippled lines denote isobaths. 118 Point . . . . li?o*w ^nnrontiftn V Z 0 W c . * Coal Oil 'S * * • Point . 70 ° f i \ v v6u y X S ' V*« J * J I J 1 • V .__ ( J • *\ • Santa B arbara. • %SILT 20km 50 • c/ •' CONTOUR INTERVAL 10% 34°20' Ventura River 60 i 16CLC70 Ventura Santa Clara River San Miguel I. Santa Rosa I. n; Santa Cruz r ' A Anocopo ■ / > / 0 i - 1 0 40 » m "\ 50 The remainder of the mud fraction, which is clay, exhibits a simpler pattern than percentage silt (Figure 25). A broad area from Anacapa Island to the mouths of the Ventura and Santa Clara Rivers contains less than 10% clay. The 30% clay isopleth very nearly mimics the 250 m isobath for the eastern two-thirds of the deep basin. Once again, a 50% isopleth forms an arcuate pattern at the west end of the 550 m isobath, essentially identical to the same pattern for silt (compare Figure 24 and Figure 25). The shelves to the north and south have clay contents usually less than 20%. Sand/mud ratios indicate the relative importance of these two size fractions within the basin (Figure 26). A sand/mud ratio of 1.0, meaning equal portions of sand and mud, is mostly confined to the shelf for the northern shelf, with a slight excursion down-slope at the submarine valleys near Point Conception. On the Northern Channel Islands shelf, however, the sand/mud ratio of 1.0 mostly lies on the slope with one incursion onto the shelf off Santa Rosa Island and another at the western extreme of the shelf (Figure 26). 120 Figure 25: SYMAP computer contour map of percentage clay for surface sediments (0-2 cm). Stippled pattern denotes isobaths. 121 Point I . I i g h f i nii • # •••••••#■ 120* W ................... V - • . Cool Oil / -------------- O s * * - P o i n t . e 4 B . Y ^ 'S c V . . . • Sonto Borboro • . %CLAY 2 0 km coition <0* 3 4 *2 0 'N 550m 250m VST-> . Son Mlauol I. Sonto Roto I. , 0 ^ r r W I Sonto C \. CONTOUR INTERVAL 10% Ventura Rivor Ventura Santa Clara • River 'S2-io . ' o A n o cw o l> ( N Figure 26: SYMAP computer contour map of the ratio of sand to mud of surface sediments (0-2 cm). i Stippled lines denote isobaths. The shelf break is delineated as the 100 m isobath. 123 124 550m, r n-j Son . liguel L * Sonto Roto 'Sonto Cruz I C alcium Carbonate and Organic Carbon Calcium carbonate contents (Figure 27) for the basin and shelves show considerable variability, but is limited to less than 10% in the deep basin, reflecting the diluting effect of terrigenous imput to the basin. The area of highest calcium carbonate contents on the Northern Channel | Islands shelf and slope (greater than 10% and up to 98%) reflects the dominance of shell fragments and other carbo nate material on this shelf as noted by Day (1979). An area of low calcium carbonate content exists on. the Hueneme Sill, with values of less than 3%. Off Coal Oil Point, deflection of isopleths of 2-7% intrudes onto the basin floor deeper than 550 m (Figure 27). Maximum values of calcium carbonate up to 9.4% on the deep basin floor are displaced to the west end of the basin area deeper than 550 i m . i Organic carbon contents reach a maximum of 3.6% in an area on the east side of the basin floor at 550 m, as part of a broad elongate area greater than 3.4% organic carbon (Figure 28). A large portion of the basin area is greater than 2.6% organic carbon, and isopleths mimic isobaths at the east end of the basin north of Santa Cruz Island (Figure 28). A u-shaped pattern centered on the western | i 125 L figure 27: SYMAP computer contour map of percentage calcium carbonate of surface sediments (0-2 cm) . Stippled pattern represents the 550 m isobath. 126 127 ; %CaCC>3 ^smadEzzsdi* San Migutl I. edge of the 550 m isobath is defined by the 2.6% organic icarbon isopleth. 128 Figure 28: SYMAP computer contour map of percentage organic carbon of surface sediments (0-2 cm). Stippled patterns represent isobaths. 129 ; ; Point .[; • .....ji20*w V:or.c«Pt i o , % ORGANIC CARBON :. cmi oil c o n to u r m tc rv a l Rivtr ^■Vmhiro V * M o 200m ^ — - Son Miguel r s r ♦ • | 4 • • • • • Sonto Roto L uo o Sand Grain Constituents Constituents counted in point counts of the sand fraction included many biogenic and detrital elements, but mineral grains and foraminifera appeared to be the most indicative of depositional environments as well as the most abundant. Percentage mineral grains includes mostly i !quartz, and some feldspar, mica, opaque minerals, and j |traces of other minerals including some glauconite for i shallow stations. With the exception of glauconite, which is a relatively minor constituent, all the minerals are terrigenous. Percentage mineral grains varies from 21% on i the western part of the central basin to 99% in the eastern basin (Figure 29). A well-defined minima exists in the west basin center, with values increasing to the northwest and east. Most of the basin has greater than 80% mineral grains, especially the eastern two-thirds. Mica content is highest in the eastern part of the basin and off Point Conception (Figure 30), reaching a maximum of 18% off Point Conception. Foraminifera are highest in concentration in the central basin, with the maxima on the west side of the central basin, composing as much as 62% of the sand !fraction (Figure 31). Planktic foraminifera constitute the majority of foraminifera, with a maximum of 58% of the sand fraction in the west basin center (Figure 32). Radiolaria are most abundant on the west basin slope just east of the sill, reaching a maximum of 12% of the sand fraction j(Figure 33). Figure 29: Percentage mineral grains in the sand fraction Contour interval = 10%. Dashed lines indicate isobaths. 133 Pt. Conception %MINERAL GRAINS Figure 30: Percentage mica in the sand fraction. Contour interval = Dashed lines indicate isobaths. 1 36 Pt. Conception %MICA F ig u r e 31: P e rc e n ta g e f o r a m i n i f e r a i n th e sand f r a c t i o n . Contour interval = 10$. Dashed lines indicate i s o b a t h s . 8£ I Pt. Conception ■ — \ ✓ 90m — % FORAMINIFERA Figure 32: Percentage planktic foraminifera in the sand fraction. Contour interval = 10%. Dashed lines indicate i sobaths. 139 Pt. Concepti 34°20' % PLAIMKTIC FORAMINIFERA Figure 33: Percentage radiolaria in the sand fraction. Contours at 6 and 10%. Dashed lines indicate i sobaths. 141 Pt. Conception - ^ ^ ^ / 34°20' — . • • 550m "-90m— — * %RADIOL ARIA DISCUSSION Textural trends reflect the importance of sources of sediment in Santa Barbara Basin, the dispersal patterns of this sediment under the influence of currents and reworking by currents of already-deposited sediment. The influence of down-canyon transport in this basin is minimal, in marked contrast to other basins of the California Border land (Gorsline and Emery, 1 959; Haner, 1971; Malouta and others, 1981). This is largely because no large submarine canyons enter the basin, only two small canyons, Alegria and Sacate Canyons, enter the basin southeast of Point Conception (see Figure 2). These two canyons are small, having only a maximum of 46 m of relief as they cross the shelf break (Gatto, 1970). The importance of these two small canyons is only evident in the distribution of percentage mud (Figure 22), which is deflected basinward about 8 km from the shelf break because of higher sand contents in canyon axis and intercanyon areas (G-atto, 1970). While the evidence of sand transport across the shelf into the axes of these canyons is good (Gatto, 1970), it appears that their influence is largely restricted to a small portion of the upper slope. These canyons are not presently major contributors of sediment to the basin. 143 The broader area termed the Conception Submarine Fan is presently starved in terms of sediment supply because of Pleistocene to Holocene folding and uplift in the area, which has displaced sediment transport to the west of Santa Barbara Basin into the Arguello Fan (Fischer, 1972). Deflections of isopleths of mean phi diameter and percentage silt (Figure 18 and Figure 24) are evident in this area, and the pattern for 60% and 70% silt isopleths is particularly striking (Figure 24). It seems likely that the trends in silt percentage could reflect: 1) relict silt deposited on the Conception Fan when it was active; or 2) transport of silt around Point Conception during the Holocene. Fine-grained sediment is presently being trans- s ported around Point Conception in surface waters as turbid plumes of suspended sediment driven by winds and surface currrents. This is evident both from measurements of light transmission (Drake, 1972) and LANDSAT images (Figure 5). Measured suspended sediment concentrations in this plume were greater than 1.0 mg/1 in the nearshore, and measured flood deposition after the 1969 floods was 1 cm on the ! Conception Fan (Drake, 1972). Thus, although the fan morphology is a relict from the Pleistocene, the fan is i ! presently aggrading (Drake, 1972). The silt contents shown in Figure 24 likely reflect suspended sediment delivery around Point Conception, with Holocene deposition occurring on a relict fan surface. In the Pleistocene, this fan was I 144 the locus of multichanneled sand and mud deposition of material derived from rivers north of Point Arguello (Fischer, 1972). With the sand redirected to the west into the Arguello Fan by rising sea level and uplift of the Conception Fan, the fan area has now become a silt machine,” with its effect probably limited to the area northwest of the 60% silt isopleth (Figure 24). Silt is the most indicative size class of sediment in | terms of textural trends and sources of sediment in the s absence of significant concentrations of sand in the Santa Barbara Basin. Particularly noteworthy are the associa- ! tions between silt percentage and the mouths of the Ventura i .and Santa Clara rivers, the predominant source of sediment :for the entire basin (Drake, 1972; Drake and others, 1972; i Fleischer, 1972). While the 60% isopleth covers most of the eastern two-thirds of the slope and basin floor, the pattern outlined by the 70% silt isopleth is very inter esting (Figure 24). It appears to be an average pathway for river-derived silt from the Ventura River-Santa Clara River point source, and is located in the same position as the turbid plume derived from these two rivers for January 20, 1978 (compare Figure 5 and Figure 24). The only actual [measurements of flood layers were performed by Drake (1972). The portion of the measured 1969 flood layer is iquite similar to that of the outline of the 7 0% silt 145 isopleth at the contoured flood layer thickness of 2 cm, except that the 1969 flood layer was more extensive (Drake, 1972, p. 118). Is it possible that the 70$ silt isopleth reflects reworked 1969 flood layer sediment? Significant precipitation occurred in January to March, 1978, and a flood layer was observed in box cores from the shelf taken that summer measuring up to 8 cm thick off Pitas Point. It is therefore likely that deposition has occurred on top of the 1969 flood layer. However, because the samples used for this study represent the uppermost 2 cm, some 1969 flood layer material may have been incorporated. The 70$ silt isopleth may reflect an average trajectory of river- derived silt that is persistent over time. Surface currents of the California Current, especially the Anacapa Current are likely agents for transporting the silt from the riverine point source. It is thus no coincidence that the 70$ silt isopleth, the plume outline from LANDSAT for January 20, 1978 and the course of the Anacapa Current determined from drift cards coincide very closely (compare Figure 5, Figure 9 and Figure 24)- One other sedimentary parameter displays evidence of terrigenous dispersal patterns, the contour map for calcium! carbonate content. Deflection of isopleths basinward off Coal Oil Point (Figure 27) would seem to imply increased terrigenous dilution. The trajectory of this dilution does not coincide with other textural indicators. Perhaps, the nature of terrigenous dilution is not dependent upon grain size hut, rather, a downslope transport by mass movement. The Hueneme Sill area is unique in Santa Barbara Basin, because it is the only area with a significant sand fraction (Figure 21). That fact is reflected in a number of other textural parameters, including mean phi diameter, skewness, sorting, percentage mud, percentage silt, percentage clay and sand/mud ratio (Figures 18, 19, 20, 22, 24 and 25) as well as very low percentages of organic carbon and calcium carbonate. The textural evidence, as well as lineations on bottom photographs, suggests that this portion of the sill is presently being winnowed of fine sediment by the Anacapa Current (Edwards and Gorsline, 1978)- This current winnowing removes the clay fraction (Figure 25) and results in the best-sorted area of the basin, outlined by the 1.5 isopleth of phi standard devia tion (Figure 20). Once deposited, the sand-silt-clay admixture is winnowed by the Anacapa Current as it crosses the Hueneme Sill. The area subjected to this current winnowing lies not in the central axis of the sill, but is displaced onto the slope to the north (compare area outline by 10$ sand isopleth on Figure 21 and Figure 9)- Because of proximity to the Ventura River-Santa Clara River point source, it is probable that the sediment is derived from the rivers, and merely represents the portion of the sand fraction which was not transported down Hueneme Canyon, the main sink for sand from the littoral cell which extends from Point Conception to Point Mugu, the Santa Barbara Cell (Inman and Brush, 1973). i On the west side of the 550 m contour of the central i basin floor, a crescentic area exhibits several unique characteristics. It is the finest-grained area in the basin (Figure 18), is slightly positively-skewed (Figure 20) and has nearly equal proportions of silt and clay i j (Figures 24 and 25). This area seems to be the most i removed from silt influx from the Ventura River-Santa Clara River point source and Conception Fan silt influx. It might be termed the "hemipelagic core” of the basin at present, because it is the area within the basin where terrigenous and biogenous input is most balanced. This is i further supported by the fact that it also has the highest I percentage calcium carbonate within the basin, caused by j high concentrations of planktic and benthic foraminifera and unidentifiable calcium carbonate material identified in jpoint counts. Point counts support this notion of a "hemipelagic core’ 1, as minima in mineral grains which are I jderived from terrigenous sources occur in the same area ! (Figure 29) and maxima in planktic foraminifera which have I ! j 148 _ _ _ .. ... . . . I settled out of the the water column are located in the same area (Figure 32). Maxima in percentage radiolaria (Figure 33) are located to the west of the "hemipelagic core” perhaps in response to water column nutrient levels more favorable to radiolaria growth or due to a differential transport phenomenon for these small tests. Mica has proven useful in indicating areas of fine-grained deposi tion because its platy habit gives it hydraulic equivalence to finer-grained sediment (Doyle and others, 1968). Mica is found in the entire Santa Barbara Basin, with maxima off Point Conception and the eastern basin (Figure 30) further supporting evidence from silt content that indicates the importance of terrigenous tranport on the Conception Fan and from the Ventura and Santa Clara Rivers. A summary of the influences of the various sedimentary processes and sources of sediment inferred from textural characteristics at the surface is shown (Figure 34). Calcium carbonate contents in surface sediments of the basin reflect the interacting influences of terrigenous input and organic productivity of carbonate-secreting organisms. The effect of terrigenous dilution is most j pronounced in the same two areas where silt influx is i greatest: the Conception Fan and the eastern two-thirds of the basin where river-derived silt dominates. This can be Figure 3^• Sources and areas of similar sedimentary processes in surficial sediments of Santa Barbara Basin and adjacent shelves. Line encircling "best sorted" area represents the 1.5 isopleth of standard deviation. Stippled lines denote bathymetry. 150 _i 151 I2 0 W Coal Oil Point SOURCES Santa Barbara a '00* 250m - — 20Km Ventura Ventura Santo d e r w §2 HEMIPELAGIC CORE 50%Silt \J T J i 50% ClayV^ 550m " v : ,\ W " - 2 5 0 m - ^ "-- " h J 100m-- CURRENT-WINNOWED SAND San Mi 4 ^ Sand Best-sorted'~ 7 ' • ^ Santa Rosa I Santa Cruz I. Anocopo seen for the Conception Fan by the deflection of the 3-5% isopleths of calcium carbonate (Figure 27). The effect of terrigenous dilution by material transported from the Ventura River-Santa Clara River point source results in displacement of maximal values to the west of the central basin, in the area of the "hemipelagic core” (Figure 27). Input of calcium carbonate from the Northern Channel jlslands shelf, which is rich in skeletal debris (Day, ! 1970), is limited to the southern slope, largely to water 'depths shallower than 300 m (Figure 27). Organic carbon contents are fairly high on the basin floor (Figure 28). In general, organic carbon increases from the shelf to the basin floor. Why are the highest values in the basin, outlined by the 3.0% organic carbon isopleth (Figure 27) located on the east side of the central basin floor? Although no analysis of types of organic carbon were conducted, presumably the carbon is mostly derived from terrigenous sources by way of transport i from the two rivers. In addition, one might expect the ! highest values of organic carbon to coincide with the | finest-grained area of the basin floor, as there is a i long-recognized inverse relationship between grain size and i I organic carbon content, all other factors being equal | (Trask, 1932). This is not the case in this instance, as ; the finest grain size lies to the west of the high organic carbon zone in the area of the "hemipelagic core” (Figure 18). One might further expect that maximum values of organic carbon would coincide with the area of the basin floor underlying waters of lowest oxygen content of about 1 ml/1, which are located at 550 m and deeper (Hulsemann and Emery, 1961). However, the high organic carbon zone lies half in and half shallower than the 550 m isobath (Figure 28). Perhaps, there is a relationship between bottom nepheloid transport and organic carbon maxima on the basin floor. If this area coincided with the maximum bottom j ;nepheloid transport of organic-rich silty clay, the maximum in organic carbon might result from the finest size fraction transported during successive flows. Having been | transported along the bottom by such flows, the organic- rich sediment would have been subjected to less oxidation ! in the water column than surface-transported material. Once deposited on the basin floor, low oxygen content of j ; near-bottom waters would minimize oxidation. | I I ! I Stanley and Wear (1978) have introduced the term ! "mud-line" as a line on the upper continental slope below j which the percentage of mud no longer increases signifi cantly with depth. They conclude that such a line is | primarily dependent on seasonally variable water mass flow j j ; patterns and suspended sediment concentrations at and below j the shelf break. In Santa Barbara Basin, the "mud-line" would correspond to the 0.1 contour of sand/mud ratio (Figure 26). Below that line, no significant increase in mud percentage takes place (also demonstrated by the 90% mud isopleth, Figure 22). While the significance of the "mud-line" may be evident for the study area of Stanley and Wear (1978) on the eastern U.S. margin, it is not apparent in Santa Barbara Basin. The "mud-line" shows no apparent relationship to patterns of suspended sediment transport (Drake, 1972) and may, instead, reflect small-scale mass movement phenomena displacing sand on the shelf basinward across the shelf-slope break. 154 Chapter IV SEISMIC, STRATI GRAPHIC AND GEOTECHNICAL RESULTS SEISMIC REFLECTION DATA The stratigraphy and structure of the entire Santa Barbara Basin is beyond the scope of this study. Rather, the emphasis here will be on interpreted mass movement features and Holocene structural deformation of the upper most 10-40 m of the stratigraphic record. Seismic sections referred to in this section are located in Figure 35 The most structurally complicated area surveyed in this study was the area of the Montalvo Trough, which is flanked to the south by the Montalvo Ridge (Figure 2), a west- trending anticlinorium (Fischer, 1972). Starting on the eastern limit of the surveyed area, several normal and reverse faults are evident (Figure 36) which cut across Pleistocene and Holocene sediments to the surface. Assuming a sound velocity of 1530 m/sec for clayey silts and silty clays of shelf and slope environments (Hamilton, 1970), each 10 msec time line would correspond to 7.7 m of sediment penetration. The block of parallel reflectors in Figure 35: Location of Seismic Lines. o W RTP u \ N SEISMIC LINES 0 20 I I Km K \ \ W 7 ►» / Figure 36 appears to have been tilted upward on the north side by approximately 15 m. This set of faults, as well as several others of similar or smaller scale, can be traced laterally to the west downslope into the deep basin. In Figure 37, the same two faults which bounded the tilted block in Figure 36 have produced a zone of chaotic reflec tors, which, in part, may be caused by gas within the section that shows up as "bright spots." Further downslope, the same fault zone is present, as well as a "sag pond" caused by a splay of the southernmost fault in this fault zone from upslope (Figure 38). "Bright spots" are evident north of the fault zone, again, and may be due to downslope creep or gas in the section. An entirely different perspective of the geometry of these west- trending faults can be gained by viewing a roughly slope- normal seismic line located further to the west, and extending from the slope to the basin floor (Figure 39). Starting on the north side of the section on the middle slope, slope-parallel reflectors are interrupted at point 1 by a set of normal faults and hummocky reflectors extending down to point 2 (Figure 39). Pagoda structures (Emery, 197*0 are present between points 1 and 2 as alternating light-dark areas. Pagoda structures may be caused by gas hydrates (clathrates) or by downslope creep (Emery, 1974; Embley and Morley, 1980). While gas hydrates are theoreti cally stable in Santa Barbara Basin based on methane measurements deeper in the basin (Claypool and Kaplan, 1974), downslope creep would seem a more likely explanation for this slope area. At point 2 (Figure 39), a normal fault drops the section down to a plateau which extends to point 3- Surface-parallel reflectors are evident from point 2 to point 3- A downdrop at point 3 is caused by another normal fault forming another plateau, which extends to point 4. Between points 4 and 5, the same fault zone | which has been outlined in the previous three seismic : sections forms the escarpment at point 5, but the northern fault at point 4 is more evident on north trending seismic sections at the same location as this section. Pagoda structures are evident from just north of point 3 to point 4. Downslope creep would seem the most likely explanation . for these structures, although the larger scarps are ! fault-controlled. Deeper hyperbolic reflectors in the | section from north of point 3 to point 5 (Figure 39) may be indicative of the vertical extent of downslope creep, the deepest of which lie at about 40 m. From point 5 to deeper water depths, the central basin floor is devoid of fault- related or slope-failure-caused seismic signatures. i Instead, very even, bottom-parallel reflectors extend across the area down to the limit of penetration of 30-40 m. The entire area surveyed in the Montalvo Trough area will be heretofore referred to as "Slope Area G.M The area 159 extent of Slope Area G typified by Holocene faulting and downslope creep is 200 square kilometers. Gradients are highly variable from 0.06-1.8 degrees, or 0.13-4.0%. The other features on the slopes of Santa Barbara Basin are less structurally complex than Slope Area G, and are also simpler to interpret. On an otherwise regular slope off the Montalvo Ridge on the east side of the basin, the slope is interrupted by a compound mass movement feature, | Flow I (Figure 40). On the upslope side of the feature, a steep scarp about 15 m high lies adjacent to a downslope block of sediment 8 m high which appears to be a rotated slump block. Downslope from the slump block, a scarp 2-5 m | deep extends a short distance down the slope to a thin pile of acoustically homogenous sediment with a mound at the upslope end, which thins downslope. It appears that the slumping initiated a small mud flow basinward from the slump, transporting material from the scarp to the mud flow deposit. While not apparent from the seismic profile, it ! appears that more material must have been transported ! beyond the mud flow deposit, perhaps as a turbidity i current, in order to explain the displaced volume of sediment from the scarp. Seismic coverage was not in a slope-normal orientation, and only two seismic lines define I : the feature. Consequently, more of the mud flow deposit may exist between the two lines. Approximate areal extent Figure 36: 3.5 kHz High-resolution Seismic Profile Line A- B. 161 152 Figure 37: 3.5 kHz High-resolution Seismic Profile Line C- D. 163 rs r'- ‘T V 7 K i l l Figure 38: 3.5 kHz High-resolution Seismic Profile Line E- F. 165 riOms Figure 39: 3-5 kHz High-resolution Seismic Profile Line G- H. 167 C10 ms > 7 0 of the entire feature is 4 square kilometers, assuming a linear to arcuate scarp and convex-downslope deposit geometry. The gradient is 1.10 degrees or 2.4/6. A feature similar to Flow I is located to the west, Flow E (Figure 41). It possesses the same upslope scarp, with two hyperbolic reflectors downslope probably indicative of rotational slump blocks. A scarp 3-4 m deep is followed downslope by an area of hummocky topography that terminates in a toe, much like the longitudinal profile of subaerial debris flows (Johnson, 1970). The amount of deformation increases towards the toe, much like a rug which has been pushed from one side, and no internal structure is evident from the acoustic record. It would thus appear that the deposit represents a mud flow generated by the slumping upslope and removal of the scarp area below the slump. This feature has been previously mapped (Crissman and Ploessel, 1979), was re-surveyed for this study, is well- defined by several seismic lines and covers a total area of 5 square kilometers. Gradient is 1.44 degrees or 3«2$. A larger feature, Flow D, is located on the southern basin slope (Figure 42), first mapped by Crissman and Ploessel (1979), and was re-surveyed for this study. The feature is quite large, covering 26 square kilometers, and is typified by large-wavelength hyperbolic reflectors with no apparent internal structure. A problematic point about 169 Figure HO: 3*5 kHz Seismic Line I-J. 170 SLUMP SCARP MUD FLOW DEPOSIT . 10ms Figure 41: 3.b kHz Seismic Line K- L . 172 K FLOW E Rotational ing Mud Flow eposit Two-way Travel Time L Slump Scarp 0 1.4km the feature is that there is no apparent scarp upslope. Probably, the scarp has been subsequently obscurred by sedimentation, as this feature is not well-defined in comparison to the other features, and appears to be "old.” The head and the toe of the feature are definable, however, and the echo character of the surface is anomalous compared to the adjacent slopes which are uninterrupted by such hyperbolic reflectors, but, rather, display even, bottom- parallel acoustic reflectors. The origin of this mass movement feature is problematical, but, given the muddy character of the slope sediment in the area, lack of internal structure, and hummocky surface echo character, it may be a single mud flow. Areal extent of Flow D is slightly modified from that first mapped by Crissman and Ploessel (1979). The gradient immediately upslope from the feature is 2.00 degrees or 4.4%. To the west of Flow D, another mass movement feature, Flow C (Figure 43) is better-defined. It is elongate in a slope-parallel orientation, with a rounded scarp truncating a single, shallow upslope reflector. The deposit consists of two broadly mounded areas, the shallower portion of which displays some vaguely-defined bedding intermixed with l small hyperbolic reflectors. The downslope mound is more chaotic in acoustic signature, with numerous small-wavel- ength hyperbolic reflectors which terminate abruptly at the Figure 42: 3*5 kHz Seismic Line M - N . 175 175 IFLOW D ^ D fp o tit Two-way Travel Time 1.3km toe of the deposit where a shallow, broad anticline exhibits internal acoustic relfectors . The feature is well-defined by seismic lines (Crissman and Ploessel, 1979), covers an area of 15 square kilometers and is thin and elongate parallel to slope. The maximum thickness of the deposit is about 2-3 m and the scarp has 1-2 m of relief. The echo character and geometry seems to indicate a mud flow origin for this feature. The gradient immedi ately upslope is 2.34 degrees or 5.2%. On the western slope, on the old Conception Fan surface, a feature quite unique in the basin compared to other mass movement features, Flow A, lies on a slope gradient of 0.56 degrees or 1.2 % (Figure 44). In an area typified by an even, uniform slope surface with regular, bottom-parallel acoustic reflectors an extremely hummocky scarp area leads downslope to a smooth deposit (Figure 44). The scarp is typified by numerous discrete hyperbolic reflectors, some of which dip slightly downslope and the scarp is somewhat |curved in map view, curving in and out of section on an !adjacent profile (Figure 45). The relief on this rugged 'scarp is highly variable, between 1-4 m. The deposit can be traced in thickness to 8 m below the sediment surface at the downslope end, were upfolding of a faint reflector i indicates that the moving flow dragged underlying material along with it at the base to cause this distortion on the Figure 43: 3-5 kHz High-resolution Seismic Line 0-P. 178 0 IFLOW C Scarp Depos Two-way Travel Time r o 1.5km downslope side. The deposit is acoustically transparent, quite smooth in section along its surface and thicker at the downslope end. The morphology of the deposit is identical to longitudinal profiles of subaerial debris flow deposits (Johnson, 1970). The morphology of the scarp and deposit is quite similar to mud flows on the Mississippi Delta slope (Prior and Coleman, 1979; Prior and others, 1979). On the Mississippi Delta, these mud flows originate in distinct channels and "ooze" downslope, to coalesce and I form a single, larger deposit. It would appear that the Mississippi mud flows would be a good analogue for Flow A. In Flow A, the irregular scarp may represent individual channels of mobilized sediment which coalesce downslope to form the mud flow deposit. Flow A was first mapped by Crissman and Ploessel ( 1979),and was re-surveyed and mapped for this study. While the mapped extent based on this study is slightly larger, the essential shape is the same. Flow A covers 18 square kilometers. i On the northern slope of the basin west of Slope Area G, I I Flow H has been defined (Figure 46). On the upslope jportion of the feature, a normal fault is found which was l [traced from the east for a distance of 13 km. To the east, I ithe trace this normal fault is parallel to 2-5 other normal [faults. On this seismic section, the fault can be traced i 1 Figure 44: 3.5 kHz High-resolution Seismic Line Q-R 181 ms 1km Figure 45: 3.5 kHz High-resolution Seismic Line S-T. 183 ClOms 1km i * Mud Flow Deposit 4 1 * v j ift ■ f FLOW A vertically 24 m and penetrates deeper into the subsurface than the mass movement feature found downslope. The acoustic character of Flow H consists of overlapping low-high amplitude hyperbolic reflectors, with no evidence of internal stratification. The acoustic character of the flow stands in marked contrast to that of upslope, downslope and underlying sediments typified by numerous, regular, continuous acoustic reflects. The thickness of the feature varies from 3-27 m, increasing in the downslope direction. This feature was first mapped and detailed by Duncan and others (1971; cited in U.S. Geological Survey, 1974) who defined a detachment scarp up to 6 m high at the upslope end, followed by an exposed slip surface 460-850 m long in a slope-normal orientation and, finally, a "slide bulge" as the deposit from this mass transport. A notch, or scarp can be found upslope of Flow H (Figure 46), which has 1.6 m of relief, located 700 m upslope from the deposit. A second scarp, with about 1.2 m of relief is located 2,080 m upslope from the deposit. The overall ; shape of the surface from the uppermost scarp to the | | deposit is convex downward, compared to the normal slope i | profile, indicating that material was derived from the entire area. It is difficult to envision, as Duncan and i I others state (1971; cited in U.S. Geological Survey, 1974) i | that all of the slide material was derived from the area j between the lower scarp and the slide. Rather, it seems I 185 likely that the material was derived from the larger area of both scarps, in addition to underlying material apparently incorporated by the traction of the transport over it. Perhaps, the mass movement occurred in two phases. First, the larger area was mobilized from the uppermost scarp, then a second smaller flow was mobilized from the lower scarp. The location of the normal fault, which crosscuts the surface, and the area from which the slide was derived may well be no mere coincidence. Movement along the fault, with an associated earthquake could have generated the slide. "Slide" may not be the best term for this feature, but disorganized slide (Field, 1981) or possibly mud flow would be appropriate. The chaotic nature of the deposit, with no stratification evident argues against a simple translational glide or rotational slump origin. The nature of the acoustic reflectors, with numerous internal hyperbolic overlapping reflectors probably reflects a great deal of internal deformation on a scale of meters to tens of meters, unlike any of the other inferred mud flows. Also, the surface of the feature is considerably more rugged than the other flows, with relief up to 2 meters on the deposit's surface. Disorganized slide zone (Field, 1981) may thus be the best term to use to infer its origin and morphology. The slope gradient on the undisturbed slope above the scarp is 1.45 degrees or 3*2%. The area covered by the slide, assuming the mapped area from Duncan and others (1971; cited in U.S. Geological Survey, 1974) is approximately 7 square kilome ters . Another different feature is found on the north slope of the Hueneme Sill. This feature (Figure 47) is different from any mass movement features in the basin. The surface reflectors consist of wave-like forms up to 20 m in ampli tude, quite evenly spaced up the slope. Slope-parallel acoustic reflectors can be traced beneath the waves down- I slope. The lack of a scarp, regular spacing, wave-like morphology and that fact that textural evidence and hydro- graphic data support active current winnowing on this same portion of the Hueneme Sill all point to a sand wave origin for these features. The fact that they are located on the north slope of the sill is not accidental, as current shear from the Anacapa Current as it passes in a northerly direction across this portion of the sill would be greatest where the flow thins going into shallow water. Decreasing flow depth would produce higher flow regime and produce the | ! bedforms. The sand wave field covers 29 square kilometers. The areal outlines of mass movement features, the 1 outline of the sand wave field on the Hueneme Sill area, and the traces of faults correlated from adjacent seismic lines are shown on Figure 48. Where definable, upslope scarp or scar areas from which the mass flow deposits were J 187| Figure 46: 3.5 kHz High-resolution Seismic Line U-V 138 189 L x F a u l t N \ _______ Pgl^arps j FLOw H ' Slide . H Figure 47: 3*5 kHz High-resolution Seismic Line W - X . 190 derived have been traced. All faults are those breaking the Holocene sediment surface, and thus show evidence of Holocene activity. In Slope Area G, a great deal of ; downslope creep is evident between fault scarps, mostly ! located on the northern two-thirds of the area. The northernmost limit of the creep and consequent deformation i ! is shown where it can be defined. The area, gradient and ! | average thickness of the mass movement deposits are given ; in Table 1. 192 j Figure 48: Mapped faults, mass movement areas and sand wave field in Santa Barbara Basin. Where mass movement features have two areas mapped within them, the upslope area is the scar area and the downslope area is the deposit. Limit of deformation in Slope Area G- demarks shallowest extent of creep or normal faulting. Hatchures indicate downthrown $ide of normal faults, sawteeth on upthrown side of low angle reverse faults, with dip where it could be measured. 193 Capiton J20 Pt Conception FAULTS, MASS MOVEMENT Naples SANTA BARBARA Coal Oil Pt. 20 90 EDGE OF SCALE (Km) ^DEFORMED MASS Ventura s tiv e r V ’ 4 - \ 3 4 Ventura iS.C.R. SAND WAVE FIELD' - t r AVERAGE DEPOSIT FLOW AREA (km ) GRADIENT THICKNESS TYPE G 200 highly variable 0.06-1.8 degrees 0.13-4.0% 20 m sediment creep, grav ity faulting, reverse faul ting D 26 2.00 degrees, 4.4% 5 m? mud flow A 18 0.56 degrees, 1.2% 5.8 m mud flow C 15 2.34 degrees, 5.2% 3 m? mud flow F 11 1.5 degrees, 3.3% 5 m? mud flow H 10 1.45 degrees, 3.2% 19 m disorganized slide zone E 5 1.44 degrees, 3.2% 3 m slump and mud flow I 4 1.10 degrees, 2.4% 1 m slump and mud flow B 3 1.2 degrees, 2.6% 2 m? mud flow? TABLE 1 Characteristics of Mass Movement Zones and Slope Area G. 195 HOLOCENE STRATIGRAPHY The Laminated Zone and Gray Layers Two basic sampling methods allow inference of fine-scale stratigraphy in surficial sediments of Santa Barbara Basin. X-radiographs of box core samples provide a wide (18 cm) cross-section of sedimentary structures down to a maximum depth of 60 cm into the sediment. X-radiographs of piston cores provide a narrow (5 cm) cross-section of sedimentary structures down a subsurface depth of about 6 m at the maximum for piston cores collected in this study. The most singular characteristic of the Holocene stratigraphy of Santa Barbara Basin is a zone on the basin floor in which laminated sediments are produced in response to very low oxygen levels in the water column, and the seasonal nature of terrigenous sediment delivery to the basin (Emery and Hulsemann, 1962; Hulsemann and Emery, 1961; Soutar and Crill, 1977). This is well exemplified in an x-radiograph contact print of a core in the laminated zone (Figure 49). In addition to the laminated sediments, there are two other types of strata: gray layers and homogeneous olive gray layers (Figure 50). Laminated sections of the core are usually visually detectable and are colored olive gray (5Y 4/1) with the dark-colored, finer-grained laminae consisting of terrigenous sediment and the lighter-colored, coarser-grained laminae consisting of a larger biogenous fraction (diatoms and other microfos sils) with some terrigenous sediment (Hulsemann and Emery, 1961). Homogenous layers are the same color as the laminated sections of the stratigraphic column, but possess no laminations, which is clearly confirmed by x-radiography I (Figure 50). Gray layers are medium gray (N5) in color. !In this particular core, there are 37 gray layers with a I total thickness of 91.1 cm, a mean thickness of 2.46 cm/layer (standard deviation of 2.42 cm) and the individual layers vary in thickness from 0.1 cm to 8.4 cm. There are 1 four homogeneous olive gray layers with a total thickness | of 14.7 cm, with a mean of 3-67 cm (standard deviation of i i 0.87 cm), and these layers vary from 2.4-4.3 cm in thick ness. The differentiation of these three types of strata in this core in, in terms of % core thickness are: 4% homogeneous olive gray layers; 26% gray layers and 70% laminated, varved sediments. This is quite different from the calculations of Hulsemann and Emery (1961), who found much larger percentages of homogeneous strata (39% homoge- jneous, 25% laminated, 22% disturbed laminated; 24% gray layers for four cores), probably due to the fact that x-radiography was not used to determine sedimentary struc tures. In addition, they added a fourth category to , describe the stratigraphy, ’’disturbed laminae," which is not really necessary. There are several parts of the core which are laminated, but show evidence of varying degrees of disturbance by filamentous trace fossils, possibly produced by low-oxygen tolerant nematodes (Kristin Fauchald, pers. communication). Many laminated sections which appear homogeneous visually are in fact laminated when viewed in x-radiographs. There are other areas of laminated strata, however, where the character of the lamination is faint, but there is no evidence of bioturba- tion. While the cause of both types of "disturbed laminae" may either be a cessation of anaerobic conditions due to basin water overturn (Sholkovitz and Gieskes, 1971) or possibly mass movement, the evidence of past times of higher oxygen conditions resulting in homogeneous strata asserted by Hulsemann and Emery (1961) is unfounded when x-radiography is employed. Areas of disturbed laminae, rather than the homogeneous olive gray layers, show evidence of basin water overturns. By examining the x-radiographs of sections of this core more closely, added insight as to the origin of these strata can be gained from their sedimentary structures (Figure 50). Eighty percent of the gray layers thicker than 1.5 cm have multiple laminae within them, whereas the others are visually and radiographicaly homogeneous. Most gray layers seem, visually, to be normally graded (fining Figure 49: X-radiograph of piston core AHF 27875* Numbers refer to depth in core in centime ters. Dots denote gray layers, with thicker gray layers outlined by lines and arrows in addition to a dot. Letter H denotes homo geneous olive gray layers. Remainder of core is laminated. White marks are gaps in core resulting from drying. 199 2 7 8 7 5 99 0 49 • X . . . 4 9 9 9_! 204 254 287 333 152 204 254 200 Figure 50: X-radiograph of 57-96 cm portion of piston core AHF 27375. Depths in core in cm to right of core section. Letter H on right denotes homogeneous olive I gray layer. Dot denotes gray layer. Remainder I of core is laminated. 201 --— — - 'TST- — 202 upward), and those with multiple layers appear normally graded within each layer. The laminae within the gray layers are not to be confused with the thicker laminae of the laminated sections (Figure 50) for which the darker laminae average 0.65-0.68 mm and the lighter laminae average 0.50-0.51 mm for the uppermost 350 cm in four cores, while, below 350 cm the dark average 0.46 mm while the light average 0.395 (Hulsemann and Emery, 1961). By contrast, almost all laminae within the gray layers are thicker than 1.0 mm and many are thicker than 4.0 mm. In terms of Bouma turbidite intervals, most gray layers possess D (laminated) and E (pelagic, homogeneous) units, although usually only the E unit is reserved for muddy intervals (Bouma, 1962; Piper, 1973). Some gray layers even have two or more sets of D and E, probably indicative of two separate turbidite depositional events. For example, Figure 50 shows 7 laminae of Unit D from the base upward, overlain by a homogeneous lamina of Unit E. Then two faint laminae of Unit D are overlain by a homogeneous lamina of Unit E. The filamentous trace fossils referred to earlier are evident in the upper half of this gray layer, and have been found previously in the Gulf of Mexico (Bouma, 1968; cited and illustrated in Reineck and Singh (1975). These nematode trace fossils evidence transport of the worms from shallower depths and their low oxygen tolerance allows them to survive and burrow the E units for organic matter. It thus seems clear that most gray layers thicker than 1.5 cm are of turbidite origin, as they possess the sedimentary structures of the D and E Bouma units of the Bouma Sequence, are laterally continuous and are correlative over the basin floor as turbidites should be (Hulsemann and Emery, 1961). Pine grain size and lack of a sand fraction does not preclude turbidity current origin, as numerous examples of muddy turbidites have now jbeen documented (e.g. Piper, 1978). What then of the other 20$ thick gray layers, some of which are quite thick (up to 4*0 cm)? Most are visually graded, which is usually a characteristic of Bouma A unit sand turbidites. In this basin, there is no sand, so there is no size fraction available for the A-unit. Conse quently, turbidite units, modified from Bouma (1962) which have been used by Hesse (1975) and modified by Piper (1978) i for muds (clay plus silt) and silts seem especially useful and significant here. Piper (1978) modifies the Bouma i sequence to match observations on turbidite muds and silts on deepsea fans and abyssal plains. He subdivides the E j unit of Bouma into (from bottom to top): E1, laminated j mud; E2, graded mud; and E3, ungraded mud. Then, overlying| the E-unit, the P unit consists of (hemi-) pelagic j sediment. Some of the graded unlaminated gray layers are | probably E2 units where E1 and E3 are simply missing and the turbidite sequence is incomplete as it is many times in abyssal environments (e.g. Rupke, 1975) and other turbidit environments. The Bouma sequence was initially only a idealized sequence, so these departures are not surprising. Other ungraded E3 type gray layers are structureless with the exception of small floating sand clasts (often benthic | forams) , as has been noted before in ungraded massive silts (Piper, 1978). Many of these incomplete E unit gray layers jmay be merely more distal turbidites than other gray layers I which have two or three of the E units present of Piper ( 1978) . The homogeneous olive gray layers (Figure 50) are Jproblematic in origin. They are clearly not merely biotur- I bated laminated sections. On the x-radiograph, they appear i I identical to ungraded mud, E3 gray layer turbidites and 'also have some small floating clasts like ungraded massive silts (Piper, 1978), as in the example in the piston core (Figure 50). Two of the other homogeneous layers in this [particular core have a single lamination at the base, I similar in ’’optical1' character to the E1 laminations in Jturbidite gray layers in the core with apparently ungraded |E3 layers overlying the lamina. The fourth homogeneous ilayer shows little structure, with the exception of a few nematode mycelia trace fossils. It seems probable that these homogeneous layers are also turbidites, but they must be derived from already-deposited olive gray hemipelagic sediment on the slope, rather than from flood-derived sediment of the shelf and slope. This could also explain the color difference, as the lower organic carbon content of the gray layers (Mulhern, 1976) may in part explain their gray color, as organic carbon content is an important factor in determining hemipelagic sediment color in addition to mineralogic differences (Emery, 1960; Reineck I and Singh, 1 975) . The nature of laminations in the laminated strata can also be seen in this core (Figure 50). Because the yearly nature of the laminations in Santa Barbara Basin is fairly well established by dating and sedimentologic considera tions (Emery, 1960; Hulsemann and Emery, 1961; Soutar and Crill, 1978), a couplet of one dark and one light-colored lamina constitues a postglacial varve, a varve due to seasonal input of river-derived sediment (Morner, 1973). The light-colored diatom-rich dry season lamine in each I varve is slightly coarser-grained (Hulsemann and Emery, 1961) and thus results in a darker x-radiograph signature. The finer-grained terrigenous, rainy season lamina results kn a 1ighter-colored x-radiograph signature (Figure 50). 206 The gray layers less than 1.5 cm thick (Figure 49), which constitute 20% of this core are all structurely homogeneous, without any floating sand grains or visible size grading, and were too thin to dissect for textural analysis. It seems likely that these gray layers represent flood layers as suggested by Fleischer (1972). After the 1969 floods, Drake and others (1972) demonstrated formation of a gray layer which extended all the way to the basin floor, covering much of the laminated zone (Drake, 1972). These flood-derived gray layers were up to 2-3 cm thick in the central basin floor of the laminated zone (Drake, 1972). Assuming an initial water content of 80-35% typical of those measured in this study for the 0-2 cm interval for the same area, and a water content of 50% at 2 m sediment depth, a 2 cm thick flood layer would compress under subsequent sedimentation, dewatering and consolidation to 1.12 cm thick, well within the range of measurements of this type of gray layer in the piston cores. The distribution of all gray layers and the laminated zone is shown (Figure 51). The near-coincidence of the edge of the laminated zone and the 550 m isobath may reflect critically low oxygen levels, which were found to be 0.1 ml/1 close to the 550 m isobath (Hulsemann and Emery, 1961). This value of 0.1 ml/1 has been shown to be a crucial limiting oxygen content for organisms in Santa Cruz Basin to the southeast (Edwards, 1979). The gray layers (all types) clearly originate from the north slope as originally asserted by Hulsemann and Emery (1961). This is consistent with the pathway of suspended sediment delivery (see Figure B) which results in higher sedimenta tion rates of river-derived material to the northeast of the central basin floor and laminated zone. Gradients on | the north slope are also quite steep, which would be an | 'important factor in turbidty current generation. The outline of the gray layers is also consistent with the 1969 flood layer distribution, which angles in from the north east to the deep basin floor (Drake, 1972). The gray flood layers and gray turbidite layers both have the same miner alogy (Fleischer, 1972), which should come as no surprise, as the source of sediment for both is the same--the Santa Clara River-Ventura River point source (D.E. Drake, pers. communication). 2 0 8 Figure 51: Distribution of laminated zone of varved sediments and gray layers of the slope and basin floor. Distribution of both zones is based on box | cores and piston cores, but piston core j locations are shown here as squares, as they provide the maximum penetration into the stratigraphic column. Box cores are located elsewhere (Figure 14). The outline of the laminated zone follows the 550 m isobath quite closely, being slightly shallower on the west than the east. 209 .Santa Barbara. n.mi Pt. Conception 20 km Laminated ^ y e n tu ra R Santa Clara R. N . 34 Slope Area C r The area of downslope creep and Holocene normal and reverse faulting extends from the northeast section of the laminated zone up into the Montalvo Trough (see Figure 48). Some evidence of deformation can he seen in box cores of the laminated zone at the southeast end of Slope Area G- (Figure 52), but most of the evidence of deformation is gained from piston cores (Figure 55)- One of these cores, AHF 17640, was taken directly on a fault scarp, and the deformed laminae and minor faults thus probably were due to Holocene fault movement and associated small-scale mass movement following breaks along the fault accompanied by earthquake shocks. This core is located to the south of the downslope creep area which does not appear in sedimen tary structures of box cores. Most of the other evidence of deformation in Slope Area Gr is less spectacular, as it occurs outside the laminated zone where there are very few marker beds to record the deformation. In addition to numerous trace fossils caused by burrowing organisms, approximately one-third of the cores in this area have minor faults, small folds, rounded soft sediment clasts and other deformational elements. River-derived laminations, not like those of the laminated zone, are present in many cores from shallower than 400 m water depth. Figure 52: X-radiograph of box core AHF 28282. Note convoluted laminae near top of core. Depth in centimeters. 21 2 Figure 53: X-radiograph of piston core AHF 17640 from Slope Area G. Note minor faults, convolute laminae and other deformed laminae at arrows, and between arrows for core depth of 234-294 cm. 21 5 0 40 122 17640 200 240 280 320 360 400 440 480 m 40 80 122 162 200 240 280 320 360 400 440 480 516 Mass Movement S c arp s and D e p o s it s Analysis of sedimentary structures from all the mass movement areas which were sampled (all except Flows B, H, and I) provides important evidence of the nature of sediment deformation and mobilization from mass movement, which supplements the larger-scale evidence from seismic profiles. Sedimentary structures in Flow F are typified by I angular deformational elements from 10-105 cm and matrix- supported, rounded, soft sediment clasts from 45-63 cm (Figure 54). Below 105 cm, laminations are even to lensoidal probably derived from seasonal river-derived plumes or resuspension of flood-deposited shelf sediment. The matrix-supported clasts from 45-63 cm show evidence of j interclast matrix movement and the clasts appear to be floating in the matrix, a characteristic of debris flow deposits in the stratigraphic record (Crowell, 1957). Because this deposit is muddy in texture, the term mud flow would be most appropriate for this mass movement deposit. The other core from this mud flow, Flow F, AHF 18450 (not shown) has matix-supported floating clasts and the same angular deformational elements down to 5 meters sediment depth, which would give an upper limit on the thickness of the mud flow. 216 Figure 54: X-radiograph of piston core AHF 27866 from Flow F. See Figure 48 and Figure 15 for flow location and core location, respectively. Scale is in centimeters. 2 7 8 6 6 5 0 100 « r 129 This core, one of two from from Flow F contains rra- trlx-supported clasts and angular deform ationai features overlying river- derived lam inated clayey silts. 100 2 18 Sedimentary structures in the compound slump-mud flow feature, Flow E, which are found in a piston core taken from the mud flow deposit, AHF 27867 (Figure 55) are less spectacular than those from Flow F. However, deformational elements including lensoid structures, boudinage of lamina tions and "swirled” x-radiograph signatures are present. This pervasive deformation, coupled with the seismic evidence supports a mud flow origin for the deposit which was apparently generated by a slump upslope within the feature, which resulted in transport of the mud flow downslope a short distance. Flow A on the western slope of the Conception Fan (Figure 48) is one of the best seismically-defined mass movement features in the basin. Sedimentary structures from the scarp or scar area from which the deposit was derived (Figure 56) contain numerous rounded clasts, lensoid structures and other deformational sedimentary structures. This attests to the fact that the sediment mobilization within this scarp resulted in mud flow movement within the scarp to about 350 cm (Figure 56). Thus, some of the mobilized sediment was probably trans ported only a short distance, whereas other parts of the sediment pile continued to flow downslope to form the deposit, well-defined seismically (Figure 44 and Figure 45). The sedimentary structures from the deposit shown in Figure 55: X-radiograph of piston core AHF 27867 from mud flow deposit of Flow E. See Figure 48 and Figure 15 for location of core and mass movement feature, respectively. Scale is in centimeters. Dots to left of sections denote deformational sedimentary structures. 2 2 0 1 0 0 1 5 0 ■ssatem 5 0 100 221 piston core AHF 23082 (Figure 57) are similar to other mud flow deposits on the basin slopes (Figure 54 and Figure 55) in that rounded soft sediment clasts are evident in the upper part of the core. In particular, fairly large rounded clasts are evident at 42-45, 51-57, 66-69 and 101-104 centimeters, supported by matrix between the clasts. It is particularly significant that these large clasts are found near the top of the deposit stratigraphy, as this might be evidence of inverse grading of these ! deposits, characterized by an increase in clast size or clast percentage which has been found in debris flows in the stratigraphic record (Fisher, 1971). This inverse grading can be explained by the buoyancy of the mud flow j which holds the clasts floating in matrix (Hampton, 1979) and can be explained by any of a number of proposed mecha nisms for producing this inverse grading (Bagnold, 1954; Middleton, 1970; Naylor, 1981). Sedimentary structures within the uppermost skin of Flow A recorded in box cores also provide evidence of mass flow, surprisingly enough. This may attest to the recency of the mass movement and supplement the observation from high-res- olution seismic evidence that Flow A "looks" young and fresh. Box core AHF 27144 shows a large flow structure and: numerous lensoid structures, possibly caused by dewatering Figure 56 X-radiograph of piston core AHF 28081 from scarp area of mud flow, Flow A. See Figure 48 and Figure 15 for flow location and piston core location, respectively. Dots to left of section denote floating rounded clasts, high-angle deformation structures, minor folds and other deformational structures within the core. Scale is in centimeters. 223 28081 0 44 137 187 235 285 235 ■w ’ ■*!< 285 224’ Figure 57: X-radiograph of piston core AHF 28082 deposit of mud flow, Flow A. See Figure 48 and Figure 15 for flow and piston core location, respectivel denote floating, rounded clasts and o deformational structures. Scale is i centimeters. from location y. Dots ther n 225 28082 195 244 292 338 389 438 489 and consolidation during mobilization of the scar area (Figure 53). This flow structure must have accompanied the mobilization of the scar area, and possibly represents fluid escape caused by chaotic mixing and movement of the scarp sediment during generation of the mud flow. Box core AHF 27386 in the deposit of Flow A has a generally "swirled” x-radiograph signature indicative of sediment deformation and mobilization which has been observed elsewhere in the California Borderland in mass flow deposits (Edwards, 1979; Nardin and others, 1979a). In addition, there is a zone of lensoid structures at 25-28 cm which look like dish structures observed in silty and sandy sequences and interpreted as dewatering features formed in rapidly-deposited underconsolidated or quick beds as they consolidate (Lowe and LoPiccolo, 1974), such as in the A unit of the Bouma turbidite sequence (Wentworth, 1967). While dish structures have been recognized in siltite beds (e.g. Nilsen and others, 1977), they have not been commonly reported before from mud flow deposits. The very small dish structures in this mud flow deposit (Figure 58) laverage 2 cm wide and 0.5 cm high. The "swirled" radio- I Igraph signature of this box core from the mud flow deposit, i as well as the dish structures provide evidence of rapid deposition of a pervasively-deforming mass of mud to form the mud flow deposit. 227 Figure 58: X-radiographs of box cores AHF 27144, 27886 and; 27891 from mud flows. AHF 27144 is from the scarp of Flow A, AHf j 27886 is from the scarp of Flow A, and AHF ! 27391 is from the deposit of Flow C. See | Figure 48 and Figure 14 for flow location and box core location, respectively. Scale is in centimeters. Arrow in 27144 points to base of large flow strucutre and arrow in 27886 points I to layer of dish structures. I 223 FLOW A 27144 27886 ^ t p i y i i i r ■£ ■ v ; - - • ' , N o t* flow structures in 27144, dlth(7) structures in 27886 a apparently disturbed and broken worm tubes in 27891. In Flow C to the east along the south slope, worm tubes in a box core are broken up and randomly oriented down to 21 cm (Figure 58). This is particularly significant because another box core from the deposit has perfectly vertical worm tubes down to 20 cm (AHF 27890, not shown). Perhaps, this represents a smal1er-scale flow deposited on top of a portion of the older, elongate Flow C. Bottom Photographs: the Depositional Surface. Bottom photography collected over the entire basin floor accompanied nearly all box core stations. Mostly, the representation of the present depositional surface documents the nature of burrowing activities of organisms, rather than the nature of the mass movement forms or faulted areas because the photographs cover such a small area (1.2 by 1.8 m). All of the mass flow deposits and scar areas photographed showed varying intensities of bioturbation and benthic organism activity. This is typified at the west end of the basin by deep burrows apparently caused by starfish at AHF 27143 (Figure 59). The entire box core section for this area shows no sedimen tary structures other than complete bioturbation of the uppermost 25 cm. This type of bioturbation by starfish is typical of the entire western row of box cores in the basin as well as a few of the box cores in the second row to the east, but not the scar area of Flow A (see Figure 14). 230! Coincident with the outline of the laminated zone, benthic activity shown in bottom photographs disappears, typified by a lifeless depositional surface shown at AHF 27158 (Figure 59). The surface of the laminated zone has a flat I surface with very small mounds of unknown origin which might represent dewatering of the sediment during consoli dation. This is thus the surface upon which the most recent lamina of the annual varve is being formed. Repeti tion of this depositional surface countless times i : |throughout the Holocene then results in the typically laminated box and piston cores of the laminated zone. The transition from the lifeless laminated zone depositional surface to actively bioturbated surfaces is remarkably abrupt, although there is a qualitative decrease in benthic activity of organisms from the slopes to the laminated zone ! I : iboundary on all sides of the basin based on analysis of I ' bottom photographs. Evidence of current winnowing on the Hueneme Sill can be seen from lineations and bottom ripples j (Edwards and Gorsline, 1978). AHF 25421 is at the eastern ] ; lextreme of the Hueneme Sill and also shows grazing sea | I urchins. Thus, where the Anacapa Current enters the basin I at its east end, the intial restiction of the flow to 250 m j |water depth causes ripple marks and winnowing of the fine ! | sediment which is certainly present there (all box cores have at least 5% mud). AHF 25417 is just 2 km southeast of j 'the sand wave field on the Hueneme Sill and provides evidence that the bottom currents are sufficiently strong to produce small current ripples with lag material on the downcurrent side. Unfortunately no bottom photographs were taken on the sand wave field. Figure 59: Bottom photographs of various features on the basin floor, the present depositional surface. See Figure 14 for box core locations where these photographs were taken. Long dimension of photographs is approximately 1.8 m, except for AHF 27157, where long dimension is 0.9 m. 233 234 27157 25417 27143 25421 DOWN-CORE TEXTURAL DATA Down-core textural data was examined at two scales: in box cores at 2 cm intervals and in piston cores at 10 cm intervals. Cores were selected on mass flow deposits, scarps, in the laminated zone, in Slope Area G and on the Conception Fan slope downslope from Flow A. Crucial goals of the down-core textural analyses were: 1) to test if mud I flow deposits exhibited inverse grading; 2) to test the i origin of the gray layers and homogeneous olive gray layers within the laminated zone to see if they exhibited normal grading (fining upward) cycles characteristic of turbi- dites; and 3) to see the overall grain size trend of the basin. j I The Montalvo Trough area as it leads down to the central I basin floor and laminated zone coupled with the north slope constitute the present main depocenter in Santa Barbara | Basin. This is apparent based on considerations of suspended sediment transport, the location of gray flood i ! layers and turbidites, surface texture and surface calcium carbonate contour maps. Thus, the general down-core ! i textural trends in this area should reflect the overall ( grain size trends in the whole basin as most terrigenous i sediment reaching the basin floor must pass through the north slope-Montalvo Trough corridor. Piston core AHF ! 235 27370 in 180 m water depth on the upper slope of this corridor exhibits an overall coarsening upward (inverse grading) trend in texture (Figure 60). Percentage silt, in the absence of significant sand in the basin, demonstrates textural patterns effectively because it is the "indicator” size class indicative of terrigenous pathways from the Santa Clara River-Ventura River point source (see Surficial Sediment Characteristies chapter). In this core, percentage silt increases from less than 60% at the base of the core to nearly 80% at the top (Figure 60). Kurtosis, skewness and sorting are fairly uniform from the base of the core to 50 cm, at which point kurtosis, skewness and sorting all increase (decrease in numerical value of standard deviation indicates an increase in degree of sorting; Folk, 1974). In fact, most of the variation in texture and moment measures occurs within the uppermost 50 cm, although the coarsening-upward pattern for mean phi grain size and percentage silt still holds for the entire core. It thus appears that a slight coarsening-upward pattern is typical of the north slope-Montalvo Trough corridor. On the south slope of the Montalvo Trough, box core AHF 28039 at approximately 480 m water depth shows some fine structure in the textural trends which do not correlate with any discernible sedimentary structures in the radiograph of the sample (Figure 61). Two narrow zones of fining-upward and coarsening upward are present near the surface, underlain by a uniform hemipelagic zone and fining-upward sequence at the base, possibly a turbidite. Piston core AHF 27866 from mud flow Plow P on the south slope of the Montalvo Trough shows a dramatic coarsening- upward trend throughout the core, exemplified by mean grain size and % silt (Figure 62). This coarsening-upward trend is much greater in magnitude (7*6-6.2 phi from bottom up) than the more gradual coarsening-upward pattern for the control core on the north slope-Montalvo Trough corridor (see Figure 60). In addition, kurtosis and skewness also increase markedly upward, while sorting is uniform throughout the core. Clearly, this coarsening-upward pattern represents a different phenomenon than the basin- wide trend in the slope control core. Possibly, it reflect mechanisms in mud flows which cause inverse grading even on the scale of clay or silt-sized particles. This is signif icant, as previously-reported inverse grading observations in debris flows were limited to the clast size inverse grading. Another core from another mud flow deposit on the south slope off the Montalvo Ridge, Plow E, shows less conclusive results (Figure 63)• There is a subtle coarsen ing-upward sequence from 1.1-0.2 m, but is of the same magnitude as the basin-wide coarsening-upward trend shown in Figure 60 (compare with Figure 63)* Figure 60: Down-core texture and moment measures for piston core AHF 27870 in the north slope- Montalvo Trough corridor. Core depth is in meters. K-3 represents kurtosis-3. Sorting is standard deviation. Location is shown elsewhere (Figure 15). 238 lAHF 2 7670 MeonO %Silt K-3 Skewness Sorting 80 -I 7 8 60 I — 2 — 4— Figure 61: Down-core texture and moment measures in box core AHF 28089 on south slope of Montalvo Trough. Abscissa represents, from left to right: mean phi grain size, % silt, kurtosis-3, skewness and standard deviation (sorting). Arrows indicate direction of fining. CU = coarsening upward; FU = fining upward; HM = homogeneous hemipelagic mud; and TUR = possible turbidite. See Figure 14 for location. Depth is in centimeters. 240 AHF 28089 X0 %Si It K-3 Sk SD o- CU 20- 4 0 - FU 241 I Figure 62: Down-core texture and moment measures for | piston core AHF 27866 in the mud flow deposit of Flow F. Core depth is in meters. K-3 represents kurtosis-3. See Figure 15 for core location. Sorting is standard deviation. CU = coarsening ! upward cycle. 242 AHF 276861 MeonO % Silt 6 7 6 0 7 0 I 1 _l S k w m ttt Sorting 0 I z 4 . 243 Figure 63: Down-core texture and moment measures for piston core AHF 27867 in mud flow deposit of Flow E. Core depth is in meters. K-3 represents kurtosis-3. Sorting is standard deviation. See Figure 15 for location. 244 AHF 27867 M e a n 0 %Silt K-3 Skewness 7.2 7.7 5 6 66-1 0 I I__________I _________J Sorting 2 245 Textural and moment measures in the laminated zone provide another control for testing the coarsening-upward trend noted for the north Montalvo Trough corridor (see Figure 60). Indeed, there is an overall coarsening-upward trend in the laminated sections from the base of the cores upward from 7.7-7.2 phi units (Figure 64). It is best exemplified in the section from 2.1 m to the top. This coar sening-upward trend is not as marked as the same trend from the north slope-Montalvo Trough corridor, where the trend is from 7.7-6.8 phi from the base upward. Perhaps, the trend is more subtle in the laminated zone because it is further from the source and less coarse silt reaches this depositional setting. Gray layers which were thick enough to subsample in two or more parts are all normally graded (fining upward), which supports the turbidite origin hypothesis. In general, gray layer turbidites which are thicker than 1.5 cm are normally graded, whereas thinner gray layers are finer-grained or coarser-grained than adjacent laminated sediments, or of the same grain size. Thicker gray layers are always coarser at the base than underlying laminated sediments. Homogeneous olive gray layers are of roughly the same grain size as laminated sediments (Figure 64). Unfortunately, gray turbidite layers could not be subsampled in small enough intervals to test the different subunits of the E-unit of Piper (1978) model. However, there is an overall fining-upward pattern throughout an entire gray layer turbidite. The visual and radiographically apparent grading within individual E1 laminae or the E2 unit is overprinted by the overall normal size grading within the complete gray layer sequence. In addition, the E3 units, while ungraded within, are finer- grained than underlying E2 and/or E3 units. In general, the E3 units analyzed have slightly poorer sorting (higher standard deviation) which reflects the presence of floating | sand grains which are apparent in E3 units in x-radio- | ! 'graphs. The grain size analyses thus support the turbidite origin of thicker gray layers and provide information on j ! their fine structure and overall normal grading. The grain i ' size data of the thinner (less than 1.5 cm) gray layers is, at least, little different than the contiguous laminated sections which lends some support for their origin as flood layers. However, one might have expected that they should also have been finer-grained than the laminated sections as! ! j ;they would not possess sand-sized microfossils. The fact |that the two homogeneous olive gray layers are very close 1 ■to the texture of the contiguous laminated sections j i I (provides support for their inferred origin from lower or j i i middle slope failures of hemipelagic sediments, rather than |coarser-grained river-derived silt as it true for the gray (layers. Thus, the homogeneous olive gray layer turbidites jare derived from texturally similar slope sediments. ! 247 | Figure 64: Down-core texture and moment measures for piston core AHF 27875 in the laminated zone of the central basin floor. Depth in core is in meters. K-3 and Sk represent kurtosis-3 and skewness, respectively. Sorting is standard deviation. G = gray layer, shown by solid horizontal lines with thicker gray layers denoted by two lines. H = homogeneous olive gray layer, shown by dashed line. Remainder of core is laminated. Arrows indicte graded turbidite gray layers, where gray layer was thick enough for two or more subsamples. See Figure 15 for core location. 248 IAHF 27875 K-3 Sk Sorting % Si It 60 Mean 0 0 — 8 2 -o- 3 Two box cores from the mud flow deposit, Flow C, on the south slope both possess coarsening-upward sequences in mean phi grain size and ^ silt (Figure 65)* The coarsen ing-upward sequence in AHF 27891 could well extend to 20 cm. It is difficult to say whether this inverse grading for Flow C is due to a smaller flow superimposed on the other flow (see stratigraphy) or typical of a longer trend, of which this only represents the uppermost skin. Core samples from the mud flow scar and deposit of Flow A (Figure 48) provide information on two scales: every 2 cm in box cores and ever 10 cm for piston cores. Box cores from the scar and deposits show small cycles of normal and inverse grading (Figure 66 and Figure 67, respectively). These grading cycles may merely represent the fine-scale nature of mud flow movement and internal turbulence within the flow. The turbidites inferred solely from textural evidence in AHF 27886 are not reflected by turbidite sequence sedimentary structures within the x-radiograph (see Figure 58). Rather, it is interesting to note that j the inferred dish structures, or lensoid structures in this core (Figure 58) coincide with the top of the upper |normally graded (FU) cycle (Figure 66). This fact may help |to explain their occurrence as they might reflect the top i I jof some type of layer which is being sorted by the mechan- 250 Figure 65: Down-core texture and moment measures for box cores AHF 27891 and 27893 from the mud flow deposit, Flow C. On abscissa, from left to right, symbols represent mean phi grain size, % silt, kurtosis-3, skewness and standard deviation (sorting). Depth is in centimeters. CU = coarsening upward. See Figure 14 for core location. 251 252 AHF 27891 10 - 20- AHF 27893 %Sllt K-3 6 . 5 70 IQ - 20- ical nature of the mud flow. The dewatering which these dish structures reflect may be fastest at the finest- grained top of the unit and results in the dish structures. Longer sections of the mud flow stratigraphy in Flow A demonstrate that a distinct coarsening-upward (inversely graded) trend is present throughout the core (Figure 68). The magnitude of this inverse grading, a full phi unit, from 7.9 phi at the base to 6.9 phi near the top (Figure 68) as well as the fact that the inverse grading is accom panied by increases in kurtosis and skewness is more similar to the pattern in another mud flow core (Figure 62) rather than the basin-wide more subtle coarsening-upward pattern (Figure 60 and Figure 64). This inverse grading thus seems characteristic of mud flow deposits on the basin slopes. In the mud flow deposit of Flow A, characterized by matrix-supported clasts in the top of the core, a distinct coarsening-upward cycle of inverse grading 1 overlies a lower cycle of slight normal grading from ! i 5.2-2.8 m (Figure 69). The mud flow deposit appears to be , more than 5 m thick in this location, based on seismic ! interpretation , so the fining-upwards cycle must be j i I explained by the mechanics of the flow movement. Perhaps, i this zone represents the ’ ’rigid plug” of debris flows i I proposed by Hampton (1972) where a non-deforming plug is carried along in tact where shear stress is less than the ] Figure 66: Down-core texture and moment measures of box core AHF 27866 from the scarp of mud flow deposit, Flow A. On abscissa, from left to right, the symbols represent: mean phi grain size, % silt, kurtosis-3, skewness and standard deviation (sorting). Arrows indicate direction of fining. FU = fining upward (normally graded, HM = homogeneous hemipelagic mud, and TUR = turbidite(?). Core depth is in centimeters. See Figure 14 for location. AHF 27886 Xq %Silt K-3 Sk SD 6 5 7 6 0 80 - I 0 I 2 O-i 10- HM 20 - 3 0 - TUR 4 0 — FU 255 Figure 67: Down-core texture and moment measures of box core AHF 27390 from deposit of mud flow, Flow A. On abscissa, from left to right, the symbols represent: mean phi grain size, % silt, kurtosis-3, skewness and standard deviation (sorting). Arrows indicate fining direction. CU = coarsening upward, FU = fining upward, HM = homogeneous hemipelagic mud. Core depth is in centimeters. See Figure 14 for location. 256 257 AHF 27890 X0 % Silt K-3 Sk SD 7 7.5 65 70 -1 0 I 2 cu 20- FU HM 3 0 - 4 0 - shear strength of the material. This section may represent a rigid plug of normal grading reflecting the basin-wide trend of inverse grading. To test whether a turbid layer was generated downslope from this mud flow, Flow A, because experimental results of Hampton (1972) suggested turbid cloud generation during debris flow movement, a core downslope from the mud flow deposit was analyzed in detail for the uppermost 0.4 m (Figure 70). No normal grading characteristic of a turbi- dite was detected, which might be expected from such a turbid cloud. Instead, the top 1.6 m is essentially quite uniform in mean grain size and other moment measures. Below this zone, the remainder of the core is inversely graded from 7.8 phi at the base to 7.1 phi at the top. This is of the magnitude of the basin-wide inverse grading, so, perhaps, it represents the basin-wide trend which has a superimposed homogeneous layer subsequently deposited on | i it. The upper homogeneous layer may be due to silt deposi tion from plume deposition on top of the abandoned Concep tion Fan, which has been of uniform texture for the time period over which it was deposited. Prior to this silt j i deposition, the basin-wide trend of inverse grading dominated, which may mean that the eastern source of | terrigenous sedimentation dominated even this western area until the time period of homogeneous silt deposition on the Figure 68: Down-core texture and moment measures for piston core AHF 28081 from the scar of mud flow, Flow A. Core depth is in meters. Sorting is standard deviation. CU = coarsening upward and arrow indicates direction of fining. See Figure 14 for location. IAHF 280811 Mean 0 %Silt K -3 Skewness Sorting 7.0 7.5 60 70 “I 0 I 2 igure 69: Down-core texture and piston core AHF 28082 of Flow A. Core depth is in meters. Sorting is standard deviation. CU = coarsening upward, and arrow indicates direction of fining. HM = homogeneous hemipelagic mud. See Figure 14 for location. 261 moment measures for from the mud flow deposit AHF 28082 Mean %Silt K-3 Skewness Sorting 7.5a0S060 H 0 I 2 26 2 Conception Fan surface. The fact that a normally graded sequence was not found on the top of this piston core does not preclude that one was present for two reasons: 1) it is a piston core, and the top 10-30 cm have been lost as a result of the sampling device elsewhere in the basin. Hence, the top where evidence of normal grading might have been found has been blown away; 2) perhaps the turbid cloud did not extend this far downslope, but evidence of it could be found closer to the mud flow deposit. Figure 70: Down-core texture and moment measures for piston core AHF 28077 downslope from mud flow deposit of Flow A. j Core depth is in meters. Note expanded j vertical scale for uppermost 0.4 m. Horizontal line separates upper zone of texturally homogeneous hemipelagic clayey silt (HM) from zone of inverse grading (CU = coarsening upward). Arrow indicates direction of fining, j K-3 is kurtosis-3 and SD is standard deviation (sorting). See Figure 15 for core location. L 264 ... j lAHF 280771 MEAN 0 %SILT K-3 SKEWNESS SO 7 8 50 70 -I 0 I 2 HM C U 2 65 G-EQTECHNICAL PROPERTIES Water content, porosity and saturated bulk density are all determined with the same analytical test and are all interrelated. Water content is quite high in the central basin, where it reaches a maximum of 31%, and a large portion of the basin surface sediment has greater than 70% water content (Figure 71). It varies from 30-31%. Isopleths of water content mimic isobaths, which can be seen clearly in the Montalvo Trough area near the 200 m isobath. Water contents decrease markedly down-core in box cores, where water contents commonly decrease 20% from the surface to the base of the box core (variable, from 30-60 cm) for cores where surface water contents are greater than 60%. For samples where the surface water content is less than 60%, water contents at the base of the box core are typically 5-10% less. Porosity in surface sediments reflects the same pattern as water content (Figure 72). Most of the basin sediment at depths below 200 m has porosities greater than 70%, and a large area of the central basin sediments have porosities greater than 80%. Once again, isopleths mimic isobaths, most notably in the Montalvo Trough. Porosities vary from 51-85% across the basin (Figure 72). Saturated bulk density varies from 0.99-1.73 across the basin (Figure 73), with most of the basin sediment at depths deeper than 300 m having bulk densities of 1.3 or less. Isopleths mimic isobaths, especially in the Montalvo Trough area (Figure 73). All three properties are interrelated, and it is apparent that the finest-grained deep basin sediments have the highest water contents, highest porosities and lowest bulk densi ties. This is in concert with shipboard observations that laminated zone sediments and those from slightly shallower | depths have almost a slurry-like consistency, as they are so watery. It is easy to envision that downslope creep, as I was interpreted from seismic lines from the north slope, could easily take place given such high water contents. J Such downslope creep may well take place on a very small j scale, perhaps tens of centimeters thick, given such high water contents, and may not be detectable with the seismic- reflection profiling. Atterberg limit tests within cores from the mass movement forms provide some empirical mechanical insight into the nature of these sediments. Samples analyzed from mud Flow F (Figure 74) vary from 50-80% liquid limit, 1 30-50% plastic limit and have plasticity indices of from 10-40%. There seems to be a relationship in this core ;between sedimentary structures and the Atterberg limits, with the deformational zone from 10-100 cm corresponding to a zone of increasing and then decreasing (from top down) Figure 71: Percentage water content for 0-2 cm box core samples. Contour interval is 10%. Water content calculated on basis of % wet. Dotted lines indicate isobaths, with unlabelled contour = 550 m . 269 % WATER CONTENT Pt. Conception „ * 200m-.. ^ 34°20 Figure 72: Porosity in 0-2 cm box core samples. Contour interval =- 10^. Dotted lines cate isobaths, with unlabelled isobath 550 m. ind i- 270 • . • • • • - '120° Pt. Conception •.......-200m.. 34° 20' % POROSITY Figure 73: Saturated bulk density of 0-2 cm box core samples. Contour interval =0.1 g/cc. Dotted lines indicate isobaths, with unlabelled isobath 550 m. Pt. Conception \ • • • * * * • . 200m 34°20 1 .2 / 200m • • BULK DENSITY liquid and plastic limits. This may reflect the mechanical nature of the deformation within this zone. In a core from Slope Area G which is near a fault scarp, but not in an area of downslope creep, the radiograph shows a variety of deformational structures, of which only the hard layers correlate well with Atterberg limits, as they have the lowest liquid limit, plastic limit and plasticity index in I the core (Figure 75). These hard layers thus have a narrow range over which they deform plastically, which is consis- | : tent with observations during core cutting and description that these layers were quite indurated and consolidated, essentially brittle. It is possible that these hard layers I may represent "crust” zones caused by the shearing associ ated with sediment movement. This sediment movement may be associated with or caused by the adjacent fault. These "crust” zones have been found on the Mississippi prodelta (Bohlke and Bennett, 1980). Whether the similar features found in Santa Barbara Basin represent the same phenomenon could only be proven with clay fabric analysis and, I perhaps, shear stength profiles as Bohlke and Bennett (1980) did for the Mississippi crusts. Of the Atterberg i limits graphed for these two cores, the plasticity index is| probably the most significant in terms of inferring j sediment shear strengths, as shear strength increases more : j rapidly with depth for soils with higher plasticity index : values (Fisk and McClelland, 1959; Skempton, 1953). Figure 74: Atterberg limits from piston core AHF 27866 from mud Flow F. See Figure 15 for core location. Sedimentary structures on right were determined from analysis of x-radiographs. 275 276 MUD FLOW F AHF 27866 L IQ U ID L IM IT P L A S T IC P L A S T IC IT Y LIQUID LIMIT l|M |T |N 0E X 50 70 , 30 50 20 40 0 I !■ I -i D E P T H I N M E T E R S ^ Horn oqtneous_ _ Angular Clasts Round, Matrix- supported Clasts Matrix Flow High-angle Deformational Elements Laminated, Some Lensoid Layers Figure 75: Atterberg limits of piston core AHF 27868 from Slope Area G, 700 m south of fault. See Figure 15 for location. Sedimentary structures on right were determined from analysis of x-radiographs. 277 278 SLOPE G AHF 27868 LIQUID LIMIT PLASTIC LIMIT 90 40 50 70 PLASTICITY INDEX 30 40 D E P T H I N M E T E R S < C T Horizontal m \ • s. Laminae • ■ m / Inclined / \ y Layers / ^N^Horizontal Laminae m j ( y Angular and w / J m Rounded Fabric _ ( _________________ ______________ _ /JHor!zpntal_Lgyers_ A / Inclined m Layers m S^22HarcTLayeTsZ Z ) Inclined / \ Layers — Atterberg limits for the scar area of Flow A are quite uniform, with liquid limits of 82-108, plastic limits of 45-65 and plasticity indices of 30-52 (Figure 76). There appears to be a slight increase in liquid limit, plastic limit and plasticity index from 1 m to the base of the core. An additional geotechnical factor has been added to the graph: activity. Activity, defined as the ratio of plasticity index to % clay is related to the mineralogy of the clay fraction and to the proportion of their shear strength contributed by true cohesion (Skempton, 1953). Activity represents the surface activity of the clay fraction, such as the increased ion exchange capacity and absorption of water with decreasing grain size (Kogler, 1967). The relationship between activity and mineralogy has been well-established for continental shelf sediments off the Mississippi River (Fisk and McClelland, 1959). Activities in Red Sea, Gulf of Aden and Nile Delta sediments vary from 0.6-7.8 (Einsele, 1967). Sediments from the Arabian and Baltic Seas have activities of 0.1-6.9 (Kogler, 1967). The activities of clays have been classi fied by Skempton (1953) into three categories: 1) inactive clays, with activities of less than 0.75; 2) normal clays, with activities between 0.75-1.25; and 3) active clays, with activities greater than 1.25. The activities in the core from the scar of Flow A vary from normal to active, and there is a fair correlation of decreasing activity with depth (Figure 76). It appears this relationship is largely caused by grain size variations, as this core shows inverse grading throughout its length (Figure 68). For the core from the deposit of Flow A, plasticity index decreases slightly with depth, and activity decreases with depth to 4.3 m (Figure 77). This relationship is not so easily explained solely by textural evidence, as only the upper most 2.8 m is inversely graded (Figure 69). Thus, this change in activity is real, and would reflect decreasing cohesion with depth. Part of the decrease in cohesion can be explained by the decrease in the plasticity index. The decrease in cohesion with depth would suggest that this mud flow deposit likely "froze" from the top down after deposi tion, as the most cohesive clays would "freeze" first. To test whether the decrease in activity with depth is characteristic for mud flow deposits, two other profiles of activity are shown for Flows E and F, as well as a "control" core not from a debris flow deposit (Figure 73). Clearly, activity decreases with depth for these other two mud flow deposits and the correlation (R) is greater than that for Flow A. For the control core, the correlations are very poor, but if any relationship exists, it is one of i two segments of increasing activity, rather than decreasing activity. This activity versus depth relationship for AHF !Figure 76: Atterberg limits and activity of piston core AHF 28081 from scarp area of Flow A. Depth in core is in meters on ordinate. See Figure 15 and Figure 48 for core and flow | location, respectively. 281 AHF 28081: FLOW A SCARP LIQUID PLASTIC PLASTICITY LIMIT LIMIT INOEX ACTIVITY 80 100 9 0 6 0 SO 90 100 170 I - 2— 3- i^LIN E A R ' REGRESSION R » -0.56 Figure 77: Atterberg limits and activity for piston core | AHF 28082 on mud flow deposit of Flow A. Depth is in meters on ordinate. See Figure 15 and Figure 48 for core and flow location, respectively. R = (-0.50). 283 FLOW A DEPOSIT AHF 2 8 0 8 2 LIQUID PLASTIC PLASTICITY LIMIT LIMIT INDEX ACTIVITY 150 (LINEAR 'REGRESSION -— 0 - 4 . 3 m 284 27870 exists despite a subtle coarsening-upward trend in texture (Figure 60). The decrease in activity with depth for core AHF 27866 must be, in part, due to the coarsening- upward textural trend (Figure 62), while the same relation ship in AHF 27867 is not explainable by texture as no significant coarsening-upward occurs (Figure 63). The decreasing activity is not explained by trends in plasticity index with depth, either, as the plasticity index increases in AHF 27866 (Figure 74) and so would tend to produce higher activities if texture were uniform. The decrease in activity with depth thus appears to be charac teristic of the mud flow deposits analyzed in the basin. Although activity varies with mineralogy (Fisk and McClel land, 1959; Skempton, 1953) and there are down-core varia tions in clay mineralogy in laminated zone piston cores (Fischer, 1972) they are not marked enough to explain the large differences in activities. For the Louisiana conti nental shelf, abrupt changes in mineralogy explained changes in activities (Fisk and McClelland, 1959), not I subtle changes as occur in Santa Barbara Basin. I The importance of organic matter in slope stability has been recently stressed, as organic matter absorbs water and aggregates clay-size particles to form an open fabric resulting in exceptionally high water contents, porosities, plasticities and low bulk densities (Keller, 1981). In Activities of piston cores AHF 27866, 27867 and 27870. Depth in core is in meters on ordinate. See Figure 15 and Figure 48 for core and flow location, respectively. Best fit of linear regression for AHF 27870 was found by separating field into two segments, as shown. 286 AHF 2 7 8 6 6 30 Activity «o 0 rJ 11 I t LINEAR I — AHF 27870 30 Activity lso 1 1 1 1 ■ » * REGRESSION R --0.S2 AHF 27867 too Activity uo i i i tiii LINEAR / REGRESSION R » -0.73 [FLOW El UNEAR — i DEGRESSION 0-L 5 m _ R-O.IS UNEAR REGRESSION I.S-4.4«t R>0.22 \ 1 NOW-FLOW 1 287 addition, some form of bonding of sediment particles is apparent from undrained shear strength and sensitivity (Keller, 1981). A direct relationship between organic carbon content and plasticity index was found for upwelling margin slopes off Peru and Oregon (Keller, 1981). A similar relationship exists in Santa Barbara Basin for 0-2 cm box core samples between plasticity index and organic carbon content (Figure 79)* The correlation is quite good (R = 0.72) and the relationship between plasticity and organic carbon content is achieved from linear regression, which yields: Plasticity index = 36.9(organic carbon) - 37»9 ‘ Higher organic carbon contents result in higher plasticity j indices. The same is probably also true for porosity and water content, as the areas highest in these values tend to j i be in the deeper basin where organic carbon is highest. | The same areas coincide with lowest bulk densities, so j Keller’s (1981) observation probably also holds here. i I Relationships between plasticity index and liquid limit j I provide differentiation of soils based on plasticity ! i characteristics as originally proposed by Gasagrande \ (1948). Plasticity characteristics can be separated into several categories (Terzaghi and Peck, 1967; Scott and : 288 S Figure 79: Organic carbon versus plasticity index for 0-2 cm box core samples across Santa Barbara Basin and northern shelf. Formula represents the relationship between plasticity index and organic carbon content derived from linear regression. R represents correlation coefficient. 289 6- 4 - 2- 0 0 ORGANIC CARBON (O.C.) PI=36.9(0.C.)-37.9 • / / • : . , . • ; I • • • » ^ — - • * • • • / ' Linear Regression R=0.72 n = 45 > N ro O “ i— i — i — i — i — i — i — i — i — r 20 40 60 80 100 300 PLASTICITY INDEXCRI.) Schoustra, 1968). This approach has proved useful in differentiating plasticity characteristics of eastern U.S. slope sediments (Keller and others, 1979) and for differen tiating different mass flow types (Hein and G-orsline, 1981). For this basin, clear differentiation exists among the mass movement types analyzed (Figure 80). All areas analyzed have medium to high plasticities, with the excep tion of some of the samples from Flow F, which have low plasticities. On this chart, all samples are plotted below the A-line, and thus consist of organic clays and inorganic silts. The separation of the fields of plasticity charac teristics may reflect any or all of a number of influences on plasticity which differ from one mass movement area to another: texture (clay content), and sedimentation rate. The scarp and deposit of Mud Flow A show clear separation, with the scarp showing lower plasticity indices. There are thus clear differences between the deposit and the scarp from which it was derived in terms of plasticity character istics. Nearly adjacent flows show distinct separation, asj i with Flow E and Flow F (Figure 80). The separation of mass movement areas in terms of plasticity may reflect basic i mechanical differences better delineated by more sophisti cated geotechnical tests. Figure 80: Plastici mass mov slopes. A-line i clays, a inorgani represen each mas 292 ty chart for piston cores from various ement features of Santa Barbara Basin s empirical boundary between inorganic bove line, and organic clays along with c silts below line. Letter , f n, f ts number of Atterberg limits tests for s movement feature. p 80- L _ A S T 60- 1 _ C 1 T 40- Y - 1 N 20- D E X o - Mud Flow A Scarp (ns35).... Mud Flow (n*65) Mud Flow E * (n«82) X / ^ T i * . Mud Flow A Deposit > (n *4 9 )— Active Slope 6 (n*!25) — o 20 40 60 80 100 LIQUID LIMIT (%Dry B a s i s ) 120 i \ : Chapter V SUMMARY AND DISCUSSION SURFICIAL SEDIMENT CHARACTERISTICS AND SEDIMENTARY PROCESSES Santa Barbara Basin is presently the depositional site tof a mixture of terrigenous and biogenous sediments to form i hemipelagic sediments. The dominance of the terrigenous fraction over the biogenic is apparent for the entire basin based on several criteria. Sand-sized constituents are predominantly terrigenous mineral grains, except in the area of the T,hemipelagic core" on the west side of the laminated zone, where planktic and benthic foraminifera predominate over terrigenous components (Figure 29 and Figure 31). Mica, as an indicator of terrigenous influx (Doyle and others, 1968) attests to the dominance of terrigenous transport to the eastern half of the basin from the Ventura River-Santa Clara River point source, influx of ! terrigenous sediment from suspended sediment transport on i | the Conception Fan surface (Drake, 1972), as well as minor i influx from the Northern Channel Islands platform (Day, i 1979; Figure 30). 294 Textural data on surficial sediments allow definition of sources and transport trajectories of terrigenous sediment (Figure 34). The ' ’hemipelagic core1 ’ is charac ter i zed by equal portions of clay and silt, is most removed sedimento- logically from sources of terrigenous influx, and, based on constituent sand counts, is the site of the most pelagic influx of biogenous constituents, mostly planktic foramin- ifera in the sand fraction. Percentage silt appears to be |the most diagnostic size fraction in surficial sediments, -because of the absence of sand in the vast majority of the basin floor (Figure 24). Most of the basin is typified by silt percentages greater than 60% and nearly the entire I basin floor has greater than 50% silt. Influx of terrige nous silt from the river point source is well-defined by I the 70% silt isopleth, which leads directly from the river mouths to the Montalvo Trough-north slope corridor, the | area of highest sedimentation rates and the main depocenter in the basin at present (Figure 24 and Figure 34). This : transport path is also corroborated by lower calcium ! carbonate percentages on the north slope trending southwes- ! | terly into the deep basin (Figure 27). Consideration of j the mode of transport of basinal silt requires an important j role for suspended sediment transport into the basin from ' the rivers. Suspended sediment is transported in surface waters by the Anacapa Current, which carries the load in a west-northwesterly direction to the deep basin (Figures 5 to 10). The coincidence of the 70% silt isopleth (Figure 34) with surface suspended sediment patterns (Thornton, 1981; Figure 5) and the location of the 1969 flood layer (Drake, 1972) points to the long-term importance of this transport pathway for basin infilling (Drake and others, 1972). In sum, terrigenous transport from this source dominates the sediment budget of the entire basin and must also require suspended sediment transport within the water column, especially along the bottom as a more concentrated nepheloid layer (Drake, 1971; Drake, 1972; Drake and others, 1972). It deposits a blanket of poorly-sorted, silty clay and clayey silt low in calcium carbonate content which covers the entire basin floor, except in the Hueneme Sill. A lesser influx of terrigenous sediment is apparent from the northwest over the relict Conception Fan (Fischer, 1972; Figure 34), which presently receives a thin blanket of terrigenous sediment deposited from a suspended sediment plume entering the basin around Pt. Conception (Figure 5; Drake, 1972; Thornton, 1981). The present influx of sediment to the basin is centered in the water column (water column-centered system of Gorsline, 1973). The two minor submarine canyons on the northwest slope exert no textural or moment measure control on contoured p a t t e r n s , e x c e p t w i t h i n t h e i r heads and s h a llo w ends o f the c h a n n e ls ( F i g u r e 22) and th u s are i n s i g n i f i c a n t o r n e g l i g i b l e pathways o f se d im e n t t r a n s p o r t . The l a c k o f im p o r t a n t c a n y o n -c e n te re d se d im e n t t r a n s p o r t i n Santa B a rb ara B a sin i s u n iq u e among the in n e rm o s t row o f B o rd e rla n d b a s in s . I t s ta n d s in marked c o n t r a s t to th e s e d im e n ta ry s e t t i n g in Santa Monica B asin to the s o u t h e a s t , where I down-canyon t r a n s p o r t d o m in a te s p ro c e s s s e ts ( G o r s l i n e , 1978; G o r s lin e and Emery, 1959; M a lo u ta and o t h e r s , 1981). An anomalous area i n te rm s o f s u r fa c e s e d im e n ta ry c h a r a c t e r i s t i c s e x i s t s on th e Hueneme S i l l . I t i s th e o n ly p o r t i o n o f the b a s in w i t h a s i g n i f i c a n t sand f r a c t i o n ( F i g u r e 2 1 ). The p a t t e r n s o f sand p e rc e n ta g e , skewness, i mean p h i g r a i n s iz e in th e v e r y f i n e to medium sand s iz e ( F i g u r e 1 8 ), and th e b e s t s o r t i n g in th e b a s in p o i n t to the im p o rta n c e o f c u r r e n t w in no w in g where th e Anacapa C u r r e n t e n t e r s th e b a s in o v e r th e s i l l ( F i g u r e 7 and F ig u r e 9 ) . T h is c u r r e n t causes w in n o w in g and u n i d i r e c t i o n a l bed loa d t r a n s p o r t o f r i v e r - d e r i v e d s e d im e n t, r e s u l t i n g in sandy, r e l a t i v e l y w e l l - s o r t e d se d im e n ts ( F i g u r e 3 * 0 . Bottom r i p p l e s i n th e Hueneme S i l l area ( F i g u r e 59) as w e l l as 1 l i n e a t i o n s and c u r r e n t shadows (Edwards and G o r s l in e , 1978) and l a r g e sand waves ( F i g u r e 47 and F ig u re 48) f u r t h e r s u p p o r t th e i n f e r e n c e o f b o tto m t r a c t i o n t r a n s p o r t i n the Hueneme S i l l . The f a c t t h a t th e sand waves a re o r i e n t e d r o u g h l y e a s t - w e s t , p lu s th e f a c t t h a t th e c u rre n t-w in n o w e d area i s d i s p l a c e d to the n o r t h s id e o f th e Hueneme S i l l is due to th e t r a j e c t o r y o f the Anacapa C u r r e n t as i t e n t e r s th e b a s in and v e e rs to the n o r t h w e s t ( F i g u r e 7 and F ig u re 9 ) . Shear i s th u s s e t up where the w a te r s h o a ls to the n o r t h s id e o f th e s i l l , th e d e p th o f f l o w d e c re a s e s , and a h ig h e r f l o w re g im e r e s u l t s in p r o d u c t i o n o f th e se l a r g e sand waves. In summary, th e s u r fa c e s e d im e n ts o f Santa B arbara Basin c o n s i s t o f a b l a n k e t o f h e m ip e la g ic s e d im e n ts , w i t h the t e r r i g e n o u s f r a c t i o n c l e a r l y d o m in a t i n g . T r a n s p o r t o f t h i s t e r r i g e n o u s f r a c t i o n i s th ro u g h suspended se d im e n t plumes a t th e s u r f a c e ( T h o r n to n , 1981), i n m id w a te r p a r t i c l e maxima (D ra k e , 1 9 7 1 ), and a lo n g th e b o tto m as a n e p h e lo id l a y e r (D ra k e , 1972; Drake and o t h e r s , 1 9 7 2 ). T h is suspended se d im e n t t r a n s p o r t i s r e s p o n s i b l e f o r s u p p ly in g the f i n e - g r a i n e d b l a n k e t o f mud. P e l l e t i n g a c t i v i t i e s o f o rg a n is m s in th e s u r fa c e mixed l a y e r (Dunbar and B e rg e r, 1981) enhance s e t t l i n g o f t h i s suspended s e d im e n t. Only where s u r f a c e c u r r e n t s f e e l th e b o tto m , i n th e s h a llo w Hueneme S i l l , i s bed lo a d t r a n s p o r t s i g n i f i c a n t deeper than th e s h e l f b re a k . tectonic deformation and sediment creep in slope area g Most o f th e d e f o r m a t io n i n t h i s area ap p e a rs to be f a u l t - r e l a t e d . T h is i s p a r t i c u l a r l y e v i d e n t i n th e u p slo p e p o r t i o n o f the area s u rv e y e d , where a p p a re n t f a u l t d is p la c e m e n ts are in a r e v e r s e sense on b o th s id e s o f a t i l t e d se d im e n t wedge ( F i g u r e 3 6 ) . Along the f a u l t near A i n t h i s f i g u r e , two a c o u s t i c r e f l e c t o r s on th e h a n g in g w a l l are 10-12 m h ig h e r th an th e same r e f l e c t o r s on th e o p p o s it e s i d e . Towards p o i n t B, th e sense o f m o tio n i s more d i f f i c u l t to i n t e r p r e t in th e s e s h a llo w p e n e t r a t i o n s e is m ic r e c o r d s . The d e p r e s s io n ne ar B c o u ld r e p r e s e n t one o f two p o s s i b i l i t i e s : 1) a zone o f norm al f a u l t i n g caused by e x t e n s io n and downdrop, o r 2) a sag pond formed by r e v e rs e f a u l t i n g ( F i g u r e 3 6 ) . A lth o u g h i t i s d i f f i c u l t to t r a c e r e f l e c t o r s i n t h i s s e c t i o n w h ich would c l a r i f y th e sense o f movement, th e c o m p re ssio n e v i d e n t in the f a u l t n e ar p o i n t A w ould r e c o n c i l e more e a s i l y w i t h co m p re ssio n and r e v e rs e f a u l t i n g le s s than 2 km away. The c o m p re s s io n a l n a t u r e o f t h i s f a u l t zone i s more a p p a re n t f u r t h e r down s l o p e . Here i t i s m a n if e s t e d as a zone o f d e f o r m a t io n 1.8 km w ide near p o i n t D ( F i g u r e 3 6 ) . The c h a o t i c n a t u r e o f th e i n t e r n a l a c o u s t ic r e f l e c t o r s c o n t r a s t s w i t h a p p a r e n t l y u n d is t u r b e d s e d im e n ts to the n o r t h and s o u th o f th e f a u l t zone w i t h t h i c k n e s s e s o f 15-30 m. The sense of motion on the two faults bordering this fault zone is virtually impossible to establish because of the lack of marker acoustic horizons within the zone. However, the fault towards D has a raised sediment mound on the north and parallel reflectors on the south which appear to be the foot wall of the fault. Further downslope, the compressional nature of the fault zone continues to be apparent (Figure 38) and reverse faulting along the southern fault at point 5 in Figure 39 explains the scarp separating the relatively flat central basin floor from Slope Area G-. The thickening of acoustic reflectors on the central basin floor (south) side of the fault at point 5 in Figure 39 is problematical. This thickening may be explai nable by mass movement of material from the fault with continued Holocene fault displacement in a reverse sense or t>y growth faulting in a normal sense on this fault. These two possibilities are contradictory as the apparent sense of offset is reverse on the fault to the north at point 4, which is better seen in upslope seismic sections (Figure 36) . The compressional nature of this fault zone is but part of the deformational picture in Slope Area G. Sediment creep is apparent as pagoda structures along the north slope, which are best exemplified between points 1 and 2 and north of point 3 on Figure 39 and south of point F on 300 Figure 38 in an otherwise acoustically transparent sedimen tary section. These pagoda structures have been inter preted elsewhere as indicative of sediment creep (Embley and Morley, 1980). Considering the high water contents in sediments of most of the deeper basin (Figure 71), sediment creep would seem quite possible on even very gentle slopes. Why, then, is this the only area where sediment creep has been detected from seismic data? Perhaps the scale of downslope creep is the key to the answer. Along seismic section G—H (Figure 39) creep appears evident in the uppermost 8-10 meters of the section where it is well-de fined . Creep on a smaller scale of a meter or two is essentially near the limits of seismic resolution (about 0.8 m for this system), and, thus would not likely be easily resolvable or detectable seismically. Normal faulting is evident in Slope Area G and results in the step-like bottom topography evident in Figure 39 which results in a "terraced" or "plateaued" bottom topog raphy. It seems likely that this faulting may be, at least in part, gravity-driven and triggered by the continued seismicity in the area (Yerkes and others, 1980). Indeed, gravity-driven normal faulting has been interpreted to the west in the subsurface by Duncan and others (1971; cited in U.S. Geological Survey, 1974). If the normal faulting is gravity-driven, its concentration along the north slope may be e x p la in e d by th e h ig h e r s e d im e n t a t io n r a t e s caused by suspended s e d im e n t d e l i v e r y as w e ll as th e r e l a t i v e l y s te e p g r a d i e n t s o f th e n o r t h s lo p e . L i t t l e d e t a i l e d f a u l t c o r r e l a t i o n has been p e rform ed w i t h i n Santa B a rbara B a s in . R a th e r, o n l y r e g i o n a l - s c a l e s e is m ic co ve rag e has been a v a i l a b l e ( F i s h c e r , 1972; Y erkes and o t h e r s , 1 9 80). Most o f th e Slope Area G where r e v e rs e f a u l t i n g , n o rm a l f a u l t i n g and se d im e n t c re e p have been i n t e r p r e t e d in t h i s s tu d y has been termed " l a n d s l i d e to p o g ra p h y " by Yerkes and o t h e r s ( 1 9 8 0 ) . The p resence o f e v e n ly -s p a c e d b o t t o m - p a r a l 1ed a c o u s t ic r e f l e c t o r s between i n t e r p r e t e d normal f a u l t s ( F i g u r e 39) su g g e s ts t h a t the area i s n o t h i g h l y d i s t o r t e d by th e f a u l t i n g , e x c e p t alo n g th e f a u l t p la n e s . The o n ly e v id e n c e o f mass movement i s t h a t o f se d im e n t c r e e p , w h ich appears r e l a t e d to th e normal f a u l t s . The norm al f a u l t s e s t a b l i s h r e l i e f w hich may, i n p a r t , c o n t r o l se d im e n t c r e e p . Thus, th e area i s b e t t e r - e x p la i n e d as an area o f se d im e n t c re e p and normal f a u l t i n g w hich may be g r a v i t y - c o n t r o l l e d . The l a r g e - s c a l e t e c t o n i c p i c t u r e o f th e M o n ta lv o Trough a r e a , o f w hich Slope Area G i s th e most w e s te rn and b a s in a l p a r t , i s one o f o n g o in g co m p re ssio n th r o u g h o u t th e P l e i s t o cene to the p r e s e n t ( F i s c h e r , 1972; Y e a ts , 1981; Y e rkes and o t h e r s , 1 9 80). T h is co m p re ssio n i s m a n i f e s t as th e M o n ta lv o A n t i c l i n o r i u m ( F i s c h e r , 1972) as w e l l as n o r t h - d i p p i n g , w e s t - t r e n d i n g r e v e rs e f a u l t s and s e i s m i c i t y i n d i c a t i v e o f r e v e r s e - l e f t - o b l i q u e s l i p on th e se f a u l t s (Lee and o t h e r s , 1973, 1979; Y erkes and o t h e r s , 19 80). I t a p pea rs t h a t o n l y a p a i r o f f a u l t s c u t t i n g the H olocene s t r a t i g r a p h y w hich have been mapped in t h i s s tu d y ( F i g u r e 36; s o u th e a s te r n m o s t p a i r o f f a u l t s i n 48) show r e v e rs e m o t io n . The re m a in d e r show n o rm al d is p la c e m e n t more e a s i l y e x p la in e d by g r a v i t y c o n t r o l than t e c t o n i c s . The g r a v i t y - c o n t r o l l e d normal f a u l t i n g may m e re ly r e p r e s e n t a ve n eer on deeper s t u c t u r e c o n s i s t e n t w i t h th e c o m p r e s s i o n a l , r e v e r s e f a u l t e d t e c t o n i c s e t t i n g (Y e rk e s and o t h e r s , 1 980). The p a i r o f r e v e r s e f a u l t s may be c o r r e l a t i v e w i t h th e Oak Ridge r e v e r s e f a u l t o r t h r u s t ( F i s c h e r , 1972; Yerkes and o t h e r s , 1 9 8 0 ). The Oak Ridge f a u l t has n o t been mapped in t h i s r e g io n and th u s th e t r a c e o f i t i s s i g n i f i c a n t , e s p e c i a l l y because i t tr e n d s more e a s t - n o r t h e a s t i n Slope Area G than i t does in th e r e s t o f th e Santa B a rbara Channel (Y e rk e s and o t h e r s , 1 9 80 ). Perhaps t h i s i s i n d i c a t i v e o f a change in th e o r i e n t a t i o n o f co m p re ssio n to w a rd s the c e n t e r o f Santa B a rb ara B a s in . V e ry l i t t l e i s known a b out th e f a u l t t r a c e s to the w est o f th e b a s in c e n t e r w hich c o u ld c o r r e l a t e w i t h t h i s n e w l y - e s t a b l i s h e d e x te n s io n o f th e Oak Ridge f a u l t . From s e is m ic s e c t i o n s i n t e r p r e t e d in t h i s s tu d y i t appears t h a t t h i s Oak R i d g e - c o r r e l a t i v e r e v e rs e f a u l t zone d ip s deeper i n t o th e s e c t i o n and th u s i s n o t d e t e c t a b l e in s h a l l o w - p e n e t r a t i o n , h i g h - r e s o l u t i o n p r o f i l e s . The few f a u l t s w hich have been mapped to the w est o f Slope Area G are shown as w e s t - t r e n d i n g (Y e rk e s and o t h e r s , 1980) e x c e p t f o r a Q u a te rn a ry f a u l t i n th e area mapped by Yerkes and Lee ( 1 9 7 9 ) . SEISMICITY AND MASS MOVEMENT In o r d e r to t r i g g e r mass f a i l u r e on a s lo p e , some c y c l i c o r e p i s o d i c f o r c e must o p e r a te on th e sea f l o o r ( F i e l d , 1 981 ). The most o b v io u s t r i g g e r i n g e v e n t in th e Santa B a rb a ra B a sin i s e a rth q u a k e - in d u c e d h o r i z o n t a l and v e r t i c a l a c c e l e r a t i o n . E p i c e n t e r s o f e a rth q u a k e s in th e Santa B a rb a ra Channel r e g io n from 1934-1975 (H a m ilto n and o t h e r s , 1969; Lee and o t h e r s , 1979) c o v e r most o f the Channel a re a , b u t a re most c l u s t e r e d a lo n g th e M o n ta lvo Trough and Ridge e a s t o f 119 degrees 40 m in u te s l o n g i t u d e ( F ig u r e 8 1 ) . There i s a cru d e w e s t - t r e n d i n g o r i e n t a t i o n to th e mapped e p i c e n t e r s , e s p e c i a l l y n e a r th e o f f s h o r e e x te n s io n o f th e Oak Ridge f a u l t ( S y l v e s t e r and o t h e r s , 1970; F ig u r e 8 1 ) , w h ich e x te n d to the d e e p e s t p a r t o f the b a s in . Thus, b o th r e c e n t s e i s m i c i t y and s e is m ic s e c t i o n s w i t h e v id e n c e o f H olocene d is p la c e m e n t i n t h i s s tu d y c o n f i r m the c o n tin u e d movement a lo n g t h i s r e v e r s e f a u l t i n re spon se to n o r t h - s o u th c o m p re s s io n . F i g u r e 81: E p i c e n t e r s o f e a rth q u a k e s i n th e Santa B arbara Channel from 1934-1975. From H a m ilto n and o t h e r s (1969) and Yerkes and o t h e r s ( 1 9 8 0 ) . Open c i r c l e s r e p r e s e n t o l d e r e p i c e n t r a l l o c a t i o n s ( H a m ilto n and o t h e r s , 1969) w h ich may be le s s p r e c is e in l o c a t i o n th a n th e s o l i d d o ts r e p r e s e n t i n g e p i c e n t r a l l o c a t i o n s (Y e rk e s and o t h e r s , 1 9 8 0 ). Open sq u are s r e p r e s e n t two o f th e h i s t o r i c a l r e c o n s t r u c t i o n s o f the 1812 M is s io n e a r th q u a k e , th e w e s te rn m o s t o f w hich i s from R ic h t e r (1958; mapped by Yerkes and o t h e r s , 1 9 8 0 ). A t h i r d l o c a t i o n i s o f f th e map to the so u th between Santa Cruz and Santa Rosa I s l a n d s a t the l o n g i t u d e m ark. 1972 c l u s t e r o f s m a ll e a rth q u a k e s shown. 305j □ JO (0 -Nl O t o o o v- y ° • o P ^ j f c " o ° o ° o ° • ® 'o b -O ® & ° '^Soo ° °? OO ° • • o 8 r O O Q The i n f l u e n c e o f e a rth q u a k e - in d u c e d shear in p ro d u c in g s lo p e f a i l u r e can be more i m p o r t a n t tha n g r a v i t y as a cause o f s lo p e f a i l u r e a c o n s id e r a b l e d i s t a n c e from the e p i c e n t e r (Lee and o t h e r s , 1981; c i t e d in F i e l d , 1 9 81). For one case example o f f n o r t h e r n C a l i f o r n i a , on a s lo p e o f 5 degrees a t a d i s t a n c e o f 30 km from a m a g n itu d e 7 e a rth q u a k e , th e e a r t h q u a k e 's i n f l u e n c e was found to be 3-4 tim e s g r e a t e r than t h a t o f g r a v i t y on s lo p e s t a b i l i t y (Lee and o t h e r s , 1981; c i t e d in F i e l d , 1 981 ). In Santa B arbara B a s in , most mass movement f e a t u r e s are on l e s s e r s lo p e s where th e g r a v i t a t i o n a l f o r c e would be l e s s i m p o r t a n t . In a d d i t i o n , a l l mass movement f e a t u r e s are w i t h i n 8 km o f re c o rd e d e p i c e n t e r s from 1934-1975 ( H a m ilto n and o t h e r s , 1969; Lee and o t h e r s , 1979). W h ile some o f th e o l d e r e p i c e n t e r l o c a t i o n s may be in c o n s i d e r a b l e e r r o r (Lee and o t h e r s , 1 9 7 9 ), i t i s s a fe to say t h a t numerous t r i g g e r i n g e v e n ts ( e a rt h q u a k e s ) have e x i s t e d in th e p a s t 40 y e a rs to cause s lo p e f a i l u r e s . As some o r a l l o f th e mass movement f e a t u r e s mapped in t h i s s tu d y are p r o b a b ly o l d e r than t h i s , th e r e c e n t s e i s m i c i t y m e re ly t y p i f i e s t h a t p i c t u r e f o r t h i s area f o r th e Holocene and P le is t o c e n e o ve r w h ich t e c t o n is m has c o n tin u e d ( F i s c h e r , 1972). The i n t e r e s t i n g p o i n t ab ou t the l o c a t e d s e i s m i c i t y i s t h a t th e r e seems to be a c lo s e c o rre s p o n d e n c e between some mass movement fo rm s and e p i c e n t e r s . In p a r t i c u l a r , Flow A ( F i g u r e 48) i s m e re ly 2 km from a c l u s t e r o f s e v e r a l s m a ll e a rth q u a k e s ( F i g u r e 3 1 ) , one o f w hich was m a g n i t i d e 2 .7 a t 6.1 km f o c a l d e p th (Lee and o t h e r s , 1 9 79). The a p p a re n t " y o u t h f u l " appearance o f Flow A in th e s e is m ic p r o f i l e , as w e l l as d e f o r m a t iv e s t r u c t u r e s in box c o re s b o th su g g e s t t h i s i s a f a i r l y r e c e n t mass movement f e a t u r e . Perhaps i t was caused by th e 1972 e a rth q u a k e and a s s o c ia te d l i q u e f a c t i o n o f th e s u r f i c i a l s e d im e n t mass w hich g e n e ra te d th e mud f l o w . L a c k in g any c h r o n o l o g i c d a ta on th e p i s t o n c o r e s , t h i s c o n c lu s i o n must be viewed w it h c a u t i o n . Flow C o c c u rs r i g h t on th e l o c a t i o n o f C h a rle s R i c h t e r ?s b e s t guess o f th e 1812 M is s io n e a rth q u a k e e p i c e n t e r (mapped by Yerkes and o t h e r s , 1980; F ig u r e 3 1 ) . T h is mass movement f e a t u r e i s o n l y p o o r l y - r e s o l v e d in te rm s o f the s u r fa c e echo c h a r a c t e r on s e is m ic l i n e s . I t i s th u s te m p tin g to s p e c u la te t h a t i t was caused by th e 1812 e a r th q u a k e , p e rh a p s , a lo n g w i t h th e a d ja c e n t Flow D a t th e same tim e ( F i g u r e 4 8 ) . The l o c a t i o n o f th e 1812 e a rth q u a k e c o u ld be c o n s i d e r a b l y i n e r r o r , because no i n s t r u m e n t a t i o n was a v a i l a b l e a t t h a t tim e and o t h e r , q u i t e d i s t a n t l o c a t i o n s have been proposed w i t h i n th e Santa B a rbara Channel (mapped by Yerkes and o t h e r s , 19 80). I t seems l i k e l y , h o w ever, t h a t an e s tim a te d m a g n itu d e 7 - 7 1/2 e a rth q u a k e such as th e 1812 e v e n t (Y e rk e s and o t h e r s , 1980) would i n i t i a t e s lo p e f a i l u r e somewhere on Santa B a rb ara Basin s lo p e s , even te n s o f k i l o m e t e r s from the e p i c e n t e r . Flow F i s l o c a t e d d i r e c t l y on th e main c l u s t e r o f the 1969 e a rth q u a k e swarm and l a t e r e a rth q u a k e s ( F i g u r e 48; F ig u r e 81 S y l v e s t e r and o t h e r s , 1970; Y erkes and Lee, 1 9 79). I t th u s seems l i k e l y t h a t s e i s m i c i t y a t one tim e or a n o th e r has g e n e ra te d th e m a t r i x - s u p p o r t e d c l a s t f a b r i c . Perhaps t h e r e i s a f i n e d i s t i n c t i o n between e a r t h q u a k e - i n - |duced l i q u e f a c t i o n and mud f l o w movement. An area o f l i q u e f a c t i o n o v e r an e a rth q u a k e swarm would be e xp e cte d to have some downslope movement a s s o c ia te d w it h i t even on th e r e l a t i v e l y g e n t l e s lo p e o f t h i s b a s in g iv e n th e h ig h w a te r c o n t e n t s w hich e x i s t in th e a r e a . MASS MOVEMENT SCARS AND DEPOSITS: STRUCTURE, STRATIGRAPHY AND GEOTECHNICAL PROPERTIES I f t h e r e i s one word w hich c h a r a c t e r i z e s th e mass movement f e a t u r e s in t h i s b a s in , i t i s " d i v e r s i t y . 1 1 Sedim ent c r e e p , a d is o r g a n iz e d s l i d e zone, two slumps and s e v e r a l mud f lo w s are p r e s e n t on th e s lo p e s o f th e b a s in . 1 | j The m o r p h o lo g ie s , i n c r o s s - s e c t i o n and p la n , a re q u i t e ! v a r i e d . The s lo p e s on w h ich th e mass movement f e a t u r e s are ! i found are v a r i e d , b u t q u i t e g e n t l e . A l l o c c u r on s lo p e s o f l e s s th a n 2 .5 d e g re e s . Two are on v e r y lo w g r a d i e n t t s lo p e s , Flow A on a s lo p e o f 0 .6 d e gre es and th e area o f 309| s e d im e n t creep in Slope Area G on g r a d i e n t s o f 0 . 0 6 - 0 . 5 d e gree s (T a b le 1 ). T h is i s s l i g h t l y h ig h e r than the i n c r e d i b l y low g r a d i e n t s lo p e s o f th e M i s s i s s i p p i D e lta where mass movement has been documented a t g r a d i e n t s o f 0 . 0 1 - 0 . 4 5 degrees ( P r i o r and Coleman, 1973) and th e v e r y s l i g h t r e g i o n a l g r a d i e n t s o f 0 . 3 - 0 . 6 degrees o v e r w hich d e b r i s flo w s have t r a v e l e d on th e Amazon Cone (Damuth and Embley, 19 81). In Santa B a rbara B a s in , s e d im e n t cre e p o c c u rs a t the lo w e s t l i m i t on g r a d i e n t in an area w i t h p r o b a b ly th e h i g h e s t s e d im e n t a t io n r a t e based on c o n s id e r a - i t i o n s o f p r e s e n t s e d im e n t t r a n s p o r t pathways (70% s i l t on F ig u r e 3 4 ) . Thus, s e d im e n t a t i o n r a t e p r o b a b ly a ls o c o n t r o l s s e d im e n t c r e e p . In term s o f s iz e c l a s s i f i c a t i o n f o r mass movement d e p o s i t s , a l l the f e a t u r e s in t h i s area a re o f th e l a r g e s c a le ( s i z e 4) a c c o r d in g to th e c l a s s i f i c a t i o n proposed by F i e l d ( 1 9 8 1 ) . The volumes i n v o lv e d in th e d e p o s i t s v a r y 6 ' z 8 5 fro m 4 x 10 - 1 .3 x 10 so a c o n s id e r a b l e volume o f s e d im e n t i s i n v o l v e d . A l l mass movement f e a t u r e s are q u i t e t h i n compared to t h e i r l a t e r a l e x t e n t . i The d o m in a n t ty p e o f mass movement i n th e b a s in appears to be th e mud f l o w , th e muddy form o f d e b r i s f l o w (N a rd in |and o t h e r s , 1980). The f a c t t h a t th e se mass movement f e a t u r e s are m o s t ly mud flo w s i s based on th e m a t r i x - s u p - p o r te d f l o a t i n g c l a s t f a b r i c b e s t e x a m p l i f i e d in Flow A, Plow E and Flow F ( F ig u r e s 56, 57, 55 and 54, r e s p e c t i v e l y ) as w e l l as m o r p h o l o g i c a l s i m i l a r i t i e s between these and p r e v i o u s l y - i n t e r p e t e d d e b r i s f lo w s (see Damuth and Embley, 1981) e v id e n ce d in h i g h - r e s o l u t i o n s e is m ic p r o f i l e s . The f l o a t i n g c l a s t s are most abundant to w a rd s th e to p o f th e cored s e c t i o n s f o r a l l these f l o w s , w h ic h r e f l e c t s the in v e r s e g r a d in g in term s o f c l a s t abundance r e p o r t e d from the s t r a t i g r a p h i c r e c o rd by F is h e r ( 1 9 7 1 ) . On a s m a lle r s c a le , i n v e r s e g r a d in g i s p r e s e n t o v e r a l l o r a p a r t o f th e cored s e c t i o n s a t th e s i l t to c l a y s iz e f o r Flow A and Flow F ( F ig u r e s 62, 68 and 6 9 ) . W h ile t h e r e i s a b a s in - w id e tr e n d in c o a rs e n in g -u p w a rd (see F ig u r e s 60 and 6 4 ) , th e m a g n itu d e o f th e in v e r s e g r a d in g i s u s u a l l y g r e a t e r than t h a t o f th e b a s in - w id e tre n d in th e s e mud f l o w d e p o s i t s and sca r a re a s . T h is phenomenon has n o t been r e p o r t e d e ls e w h e re , and i s a b i t s u r p r i s i n g because o f the f i n e g r a i n s iz e sp e ctru m i n v o l v e d . I f t h i s i n v e r s e g r a d in g i s , in d e e d , caused by th e n a tu r e o f t r a n s p o r t in mud f l o w s , th e n the v a r io u s mechanisms proposed f o r i n v e r s e g r a d in g in d e b r i s f lo w s (B a g n o ld , 1954; M i d d l e t o n , 1970; and N a y lo r , 1981) must be o p e r a t i v e even on a s i n g l e c l a y o r s i l t p a r t i c l e , i n a d d i t i o n to c o h e r e n t c l a s t s o f m a t e r i a l . Because much f i n e - g r a i n e d se dim e n t re a c h e s the b a s in f l o o r th ro u g h f e c a l p e l l e t i n g in c o n t i n e n t a l m a rg in s and s p e c i f i c a l l y i n t h i s b a s in (Dunbar and B e rg e r, 1 9 8 1 ), one would e x p e c t th e p e l l e t s to be th e f i n e s t g r a i n s iz e p r e s e n t . Perhaps th e m echanics o f sh e ar w i t h i n th e moving mud f l o w e a s i l y b re a k s th e p e l l e t s i n t o t h e i r c o n s t i t u e n t i n d i v i d u a l p a r t i c l e s to b e t t e r e x p l a i n t h a t the m echanics o f f l o w a c t a t such an i n c r e d i b l y f i n e s c a l e . T h e o r e t i c a l c o n s id e r a t i o n s s u p p o r t the c o n t e n t i o n t h a t i n v e r s e g r a d in g s h o uld be p ro d u c e d , b u t the d i s t i n c t n e s s o f th e g r a d in g sh o uld be le s s f o r f i n e - g r a i n e d d e b r i s f l o w d e p o s i t s (mud f lo w s ) because th e competence d e c re a s e w i t h shear would be le s s (Hampton, 1 9 75). There i s , h o w e ve r, a case f o r are as o f norm al g r a d in g w i t h i n mud flo w s depe n din g on the d i s t r i b u t i o n o f shear (Hampton, 1 9 7 5 ), and t h i s may e x p l a i n the zone o f norm al g r a d in g a t the base o f the Flow A d e p o s i t ( F ig u r e 6 9 ) . A c t i v i t i e s o f mud f l o w d e p o s i t s in c r e a s e to w a rd s the to p o f th e d e p o s i t in a l l c a s e s , w h ich c o n t r a s t s w i t h an example n o t from a mass movement f e a t u r e ( F i g u r e s 76, 77 and 7 8 ) . In p a r t , t h i s in c r e a s e in a c t i v i t y i s due to a d e c re a s e in c l a y c o n t e n t ( i n v e r s e g r a d in g ) p r e s e n t in mud f l o w d e p o s i t s . However, i n c r e a s i n g a c t i v i t y to w a rd s the to p even o c c u rs f o r th e mud f l o w d e p o s i t o f Flow E where no in v e r s e g r a d in g i s p r e s e n t (compare F ig u re 63 and F ig u r e 78) and a ls o o c c u rs f o r the e n t i r e l e n g t h o f c o re AHF 28082 ( F i g u r e 69 and F ig u re 77) even th o u g h o n ly th e u pperm ost p a r t o f th e core i s i n v e r s e l y graded and the bo ttom m ost p a r t i s , in f a c t , n o r m a l l y g ra d e d . Thus, th e in c r e a s e in a c t i v i t y to w a rd s th e to p o f mud f l o w d e p o s ts i s r e a l , n o t s o l e l y e x p l a i n a b l e by t e x t u r a l changes, and may r e f l e c t a de crea se in c o h e s io n w i t h d e p th p r o b a b ly caused by th e f a c t th e th e lo w e r p o r t i o n s o f a mud f l o w would be exp ecte d to be more h i g h l y sheared (Hampton, 1975; N a y l o r , 1981). T h e r e f o r e , i t seems l i k e l y t h a t th e se mud f l o w d e p o s i t s " f r o z e " from th e to p down as th e most c o h e s iv e c la y s would " f r e e z e ” f i r s t . T h is c o n c lu s i o n s h o u ld be viewed w it h i ■great c a u t i o n . A c t i v i t y i s r e l a t e d to th e p r o p o r t i o n o f shear s t r e n g t h c o n t r i b u t e d by t r u e c o h e s io n (Skempton, 1954), b u t sh e ar s t r e n g t h i s n o t known f o r th e se s t r a t i - g r a p h ic s e c t i o n s . The v e r t i c a l d i s t r i b u t i o n o f w a te r c o n t e n t caused by th e mud f l o w t r a n s p o r t p ro ce ss may a ls o be q u i t e i m p o r t a n t in term s o f w hich p a r t o f th e f l o w " f r e e z e s " f i r s t . Based on t h e o r e t i c a l and e x p e r im e n ta l r e s u l t s , Hampton (19 75 ) has p o s t u la t e d t h a t d e b r i s f l o w d e p o s i t i o n o c c u r s when th e lo w e r b o un d a ry o f r i g i d p lu g m ig r a te s tow a rd th e b o tto m o f th e f l o w . Thus, th e d e b r i s f l o w " f r e e z e s " from th e to p downward. D i f f e r e n t i a t i o n o f the d i f f e r e n t mass movement f e a t u r e s i s p o s s i b l e when p l a s t i c i t y in d e x i s p l o t t e d v e rs u s l i q u i d l i m i t ( F i g u r e 8 0 ) . Because t h e r e i s a d i r e c t r e l a t i o n s h i p between o r g a n ic ca rb on c o n t e n t and p l a s t i c i t y in d e x ( F i g u r e 7 9 ) , d i f f e r e n c e s in o r g a n ic carbon c o n t e n t as w e l l as d i f f e r e n c e s in t e x t u r e ( c l a y c o n t e n t ) or s e d im e n ta tio n r a t e (w a te r c o n t e n t ) may i n d i v i d u a l l y o r c o l l e c t i v e l y e x p l a i n th e d i f f e r e n t f i e l d on th e p l a s t i c i t y c h a r t r e p re s e n te d by d i f f e r e n t mass f l o w d e p o s i t s . The s c a r and d e p o s i t o f Flow A show c l e a r s e p a r a t io n on th e p l a s t i c i t y c h a r t . There i s th u s c l e a r s e p a r a t io n between th e tw o , w hich makes sense i n t u i t i v e l y i f one c o n s id e r s t h a t th e d e p o s i t r e p r e s e n t s t h a t p o r t i o n o f th e f l o w w h ic h was t r a n s p o r t e d much f u r t h e r and would be exp e cte d to have a w id e r range o ver w h ich i t de fo rm s p l a s t i c a l l y in t h i s e m p i r i c a l t e s t . The h ig h e r ra n ge o f p l a s t i c i t y in th e d e p o s i t th u s r e f l e c t s the p e r v a s iv e s h e a r in g to w hich i t was s u b je c te d d u r in g t r a n s p o r t , r e l a t i v e to t h a t p a r t l e f t b e h in d in th e sca r a re a . I To d i f f e r e n t i a t e mass movement f e a t u r e s more q u a n t i t a t i v e l y and c o n f i d e n t l y , more s o p h i s t i c a t e d g e o t e c h n i c a l t e s t s a re i n e c e s s a ry . BASIN AND SLOPE STRATIGRAPHY I S y n t h e s is o f b a s in and s lo p e s t r a t i g r a p h y in Santa B a rb ara Basin i s e s s e n t i a l l y l i m i t e d to areas where i H olocene s t r a t i g r a p h y i s b e s t p re s e rv e d by lo w oxygen c o n d i t i o n s o r r a p id emplacement o f f l o o d l a y e r s , t u r b i d i t e s ; ' I o r mud f l o w s . Mud f l o w s t r a t i g r a p h y ( p r e v i o u s s e c t i o n ) p r o v id e s a g e n e r a l iz e d s t r a t i g r a p h i c column w it h m a t r i x - s u p p o rte d rounded s o f t se d im e n t c l a s t s a t th e to p o f the s e c t i o n , u n d e r l a i n by d e f o r m a t iv e s t r u c t u r e s , e l u t r i a t i o n f e a t u r e s ( a ls o p r e s e n t a t th e u pperm ost s k in o f the 314 , s e c t i o n s ) , m in o r f a u l t i n g , and, i n some cases l a m i n a t i o n s a t th e base o f th e f l o w caused by l a y e r by l a y e r shear ( F i g u r e 3 2 ) . E ls e w h e re , on th e s lo p e o u t s i d e o f th e la m in a te d zone, mud f l o w s c a rs and d e p o s i t s , and t u r b i d i t e - r i c h s t r a t i g r a p h i c s e c t i o n s , th e s t r a t i g r a p h i c column i s p e r v a s i v e l y b i o t u r b a t e d . B i o t u r b a t e d s lo p e s e d im e n ts would o n ly be c l a s s i f i a b l e by t r a c e f o s s i l t e r m i n o l o g y , w h ich was beyond the scope o f t h i s s tu d y and, i n any ca se , e a s i l y d e f i n a b l e from n a rro w p i s t o n c o r e s . W i t h i n th e la m in a te d zone and the area o f g ra y l a y e r s on th e n o r t h e a s t e r n s lo p e , some s i g n i f i c a n t c o n c lu s i o n s can be reached ( F i g u r e 5 1 ) . Most s i g n i f i c a n t l y , a pathway o f t u r b i d i t e and f l o o d - d e p o s i t e d g ra y l a y e r s i s a p p a re n t t r e n d i n g n o r t h e a s t from th e b a s in c e n te r up th e s lo p e ( F i g u r e 5 1 ) . Both t u r b i d i t e and f l o o d - d e r i v e d g ra y l a y e r s a re t r a n s p o r t e d i n t o th e b a s in from t h i s n o r t h e a s t e r n M o n ta lv o T r o u g h -N o rth Slope c o r r i d o r th ro u g h w hich th e v a s t m a j o r i t y o f the o f th e t e r r i g e n o u s m a t e r i a l seems to pass in i t s r o u t e to the deep b a s in la m in a te d zone. T h is i s c o n s i s t e n t w it h s u r f i c i a l se d im e n t c h a r a c t e r i s t i c s i n d i c a t i v e o f th e same t r a n s p o r t pathway ( F i g u r e 3^) , p r e v io u s a n a l y s i s o f suspended se d im e n t t r a n s p o r t and f l o o d l a y e r d e p o s i t i o n a f t e r th e 1969 f l o o d s (D ra k e , 1971, 1972; Drake and o t h e r s , 1972) and w i t h p r e v io u s o b s e r v a t i o n s o f a F ig u r e 82: G e n e ra liz e d v e r t i c a l sequence o f s e d im e n ta ry s t r u c t u r e s in mud f l o w d e p o s i t s o f Santa B a rb a ra B a s in . # 5 ) A r 2 ? Dewatering structures (pipes and dish? structures), lensoid deformational structures. Matrix-supported, rounded clasts, "floating" in matrix, with evidence of interclast matrix movement and shear (lineations in matrix). Convoluted bedding (if original bedding is present). High-angle deformational elements. Minor faulting with displacements of centimeters. High-angle deformational structures, with inverted "v" structures. Minor folds. Lensoid laminations, with evidence of "necking" or boudinage. Laminae pos sibly produced by layer by layer shear " n o r t h e r n " so u rce o f the g ra y l a y e r t u r b i d i t e s (Hulsemann and Emery, 1 9 61 ). T r a n s p o r t pathways o f f l o o d l a y e r s and t u r b i d i t e s i n th e b a s in c o i n c i d e , w h ic h i s e a s i l y e x p la in e d by th e f a c t t h a t b o th are sourced from r i v e r - d e r i v e d suspended se d im e n t t r a n s p o r t i n d i c a t e d by t h e i r m in e r a lo g y ( F l e i s c h e r , 1 9 72 ). The d i f f e r e n c e l i e s in the f a c t t h a t t u r b i d i t e s are p r o b a b ly g e n e ra te d from mass f a i l u r e on th e upper s lo p e and d e p o s ite d i n s t a n t a n e o u s l y , whereas f l o o d l a y e r s are aggraded o v e r a p e r io d o f a p p r o x im a t e ly 1-4 y e a rs th ro u g h c o n tin u e d r e s u s p e n s io n o f s h e l f - d e p o s i t e d s i l t and c l a y (D ra k e , 1972; Drake and o t h e r s , 1972; S o u ta r and C r i l l , 1 9 7 7 ). The absence o f s c a rs on th e upper s lo p e w hich p r o b a b ly g e n e ra te d g ra y l a y e r t u r b i d i t e s may be e x p l a i n a b l e by th e r e l a t i v e s e d im e n t - f r e e c h a r a c t e r o f th e upper s lop e s e is m ic r e c o r d . No t r a c e i s l e f t o f the sca r because e i t h e r i t has been s u b s e q u e n tly b u r ie d o r sedim en t a t i o n on th e upper s lo p e i s so m in im a l o r t r a n s i e n t . V o lu m e t r ic c o n s i d e r a t i o n s o f mass movement f e a t u r e s on the lo w e r s lo p e s are o f the same m a g nitu d e as the t h i c k e r c o r r e l a t e d g ra y l a y e r t u r b i d i t e s w h ich c o v e r the e n t i r e 0 'X la m in a te d zone (on th e o r d e r o f 1 x 1 0 m) . However, i t i s u n l i k e l y t h a t the lo w e r s lo p e mud flo w s g e n e ra te d s i g n i f i c a n t t u r b i d i t e s , u n l i k e the e x p e r im e n t a l and t h e o r e t i c a l c o n s i d e r a t i o n s c o n s id e r e d by Hampton ( 1 9 7 2 ) . Perhaps th e s lo p e s are to o g e n t l e f o r s i g n i f i c a n t t u r b i d i t e g e n e r a t i o n . E i t h e r the lo w d e n s i t y t u r b i d i t e s have been b i o t u r b a t e d beyond t e x t u r a l o r r a d i o g r a p h i c r e c o g n i t i o n , o r , more l i k e l y , th e s m a ll amount o f m a t e r i a l suspended has m e re ly been in c o r p o r a t e d i n t o th e t e r r i g e n o u s la m in a e o f the la m in a te d zone to produce a s l i g h t l y t h i c k e r la m in a w hich m ig h t e a s i l y be m i s i n t e r p r e t e d as caused by g r e a t e r r a i n f a l l . Most m a t e r i a l m o b il iz e d by mud f l o w movement 're m a in s in th e d e p o s i t and i s moved o n ly a s h o r t d i s t a n c e downslope o f about 1-5 km. The absence o f subm arine canyons i n th e area o f t u r b i d i t e g ra y l a y e r s on th e s lo p e ( F i g u r e 51) p o i n t s to an i m p o r t a n t c o n c l u s i o n : su b m arin e canyons are n o t a p r e r e q u i s i t e f o r t u r b i d i t y c u r r e n t g e n e r a t i o n o f f i n e - g r a i n e d t u r b i d i t e s o r , p e rh a p s , even sandy t u r b i d i t e s . R a th e r, I h ig h s e d im e n t s u p p ly r a t e s , s u f f i c i e n t s lo p e g r a d i e n t and a t r i g g e r i n g e v e n t (h e re e a rth q u a k e s ) are th e s e d im e n ta ry and dynamic p r e r e q u i s i t e s . Some r e v i s i o n i s n e c e s s a ry i n our m odels o f t u r b i d i t e c a n y o n - f a n - b a s in p l a i n d e p o s i t i o n a l p ro c e s s e s . Due to the abundance o f subm arine canyons a t p r e s e n t , w h ich may be a r e l i c t o f lo w sea l e v e l sta n d s d u r i n g th e P l e is t o c e n e o r f a u l t i n g ( f o r C a l i f o r n i a and o t h e r t e c t o n i c a l l y a c t i v e m a r g i n s ) , i t has been t a c i t l y and somewhat n a i v e l y assumed t h a t th e y a re a n e c e s s a ry p r e r e - i q u i s i t e f o r t u r b i d i t y c u r r e n t c h a n n e lin g d o w n slo p e . They are n o t n e c e s s a ry , b u t are o b v i o u s l y i m p o r t a n t and have been documented in numerous s t r a t i g r a p h i c and Recent 319| s e d im e n ta ry examples (see M a lo u ta and o t h e r s , 1981 f o r a n e a rb y e x a m p le ). F i n e - g r a i n e d t u r b i d i t e s need n o t be a s s o c ia te d w i t h th e t r a d i t i o n a l c a n y o n - f a n - b a s in p l a i n models ( M u t t i and R ic c i L u c c h i , 1972; W a lk e r, 1978) w hich have been exte nd ed f o r the case o f f i n e - g r a i n e d t u r b i d i t e s (H esse, 1975; P i p e r , 1 9 73). W h ile f i n e - g r a i n e d t u r b i d i t e s are p r o b a b ly a s s o c ia t e d w it h s a n d i e r , more p ro x im a l t u r b i d i t e s in proven c a n y o n - f a n - b a s in p l a i n systems ( P i p e r , 1 973), th e absence o f sand in Santa B arbara Basin removes | c o a r s e r - g r a i n e d e q u i v a l e n t s in t h i s s tu d y . The co a rse f r a c t i o n i s n o t a n e c e s s a ry p r e r e q u i s i t e f o r t u r b i d i t e s e d im e n t a t i o n , and, in i t s absence, f i n e - g r a i n e d t u r b i d i t y c u r r e n t s may be g e n e ra te d a lo n g a s lo p e p a r a l l e l l i n e so urce by s lu m p in g and mass f a i l u r e ( G o r s l i n e , 1978). The p r e s e n t s e t t i n g f o r f i n e - g r a i n e d t u r b i d i t y c u r r e n t o r i g i n i s s i m i l a r to th e model o f s e d im e n t a t io n on th e Nova S c o tia n o u t e r c o n t i n e n t a l m a rg in d u r in g th e g l a c i a t e d W is c o n s in (Stow and Bowen, 1 9 80). F i n e - g r a i n e d t u r b i d i t e s have been found in t h i s a re a , b u t th e y a re l o c a t e d on the i : L a u r e n t ia n Fan. They were p r o b a b ly formed from t h i c k , lo w j v e l o c i t y , v e r y d i l u t e t u r b i d i t y c u r r e n t s d e r iv e d from slum p in g on th e upper s lo p e (S tow and Bowen, 1973, 1980; Stow, 1 931 ). The a n a lo g y i s adequate f o r Santa Barbara B a s in , b u t no c a n y o n -fa n system e x i s t s o f any s i g n i f i c a n c e . The b e s t a nalog ue to t h i s c a n y o n - f r e e d e p o s i t i o n a l model may w e l l be th e a d ja c e n t P I i o - P l e i s t o c e n e V e n tu ra Basin where l o n g i t u d i n a l t r a n s p o r t and d e p o s i t i o n o f t u r b i d i t e sand i s e v i d e n t d e s p i t e b a s in - n o r m a l canyons (Hsu and o t h e r s , 1 9 80 ). However, f o r th e g ra y l a y e r t u r b i d i t e s a n a ly z e d in Santa B a rb ara Basin ( F i g u r e 51) th e t r a n s p o r t i s n o t l o n g i t u d i n a l b u t o b l i q u e - l o n g i t u d i n a l a lo n g the M o n ta lv o T ro u g h -N o rth Slope c o r r i d o r . A f i n e - g r a i n e d t u r b i d i t e model need n o t be proposed p i c t o r i a l l y . I t would m e re ly r e p r e s e n t a l o c a l m o d i f i c a t i o n o f a system dependent j on b a s in m o rp h o lo g y . R a th e r, th e model i s o n l y de pe n d ent on d i r e c t i o n s and r o u t e s o f se d im e n t s u p p ly , fre q u e n c y o f t r i g g e r i n g e v e n ts , s e d im e n t a t io n r a t e s and b a s in m o rp h o lo g y . A subm arine canyon i s a m o d if y in g m o rp h o lo g i c a l f e a t u r e w hich c h a n n e ls o r d i r e c t s the t u r i b i d i t y c u r r e n t f l o w and changes d i s t r i b u d t i o n f a c i e s p a t t e r n s o f j t u r b i d i t e s . I t i s n o t a p r e r e q u i s i t e nor can i t n e c e s s a r i l y e x p l a i n sand d i s t r i b u t i o n , as was re c o g n iz e d by Hsu and o t h e r s ( 1980 ) . V e r t i c a l sequence o b s e r v a t i o n s o f g ra y l a y e r t u r b i d i t e s s t u d i e d in Santa B a rb ara B asin p r o v id e i m p o r t a n t c o n f i r - jm a tin o f P i p e r ’ s (1978) d i f f e r e n t i a t i o n o f f i n e - g r a i n e d t u r b i d i t e s . Six v a r i e t i e s o f t h i s sequence have been documented from r a d io g r a p h s o f la m in a te d zone s t r a t a where r e s o l u t i o n o f the r a d io g r a p h was adequate ( F i g u r e 83 and o t h e r c o r e s ) . The E1 i n t e r v a l c o n s i s t s o f la m in a te d s i l t and c l a y w i t h l a m i n a t i o n s 1-6 mm t h i c k , sometimes c o n s t i t u t i n g the e n t i r e t u r b i d i t e ( ty p e E o f F ig u re 8 3 ) . The t h i c k n e s s o f i n d i v i d u a l la m in a e sometimes d e c re a s e s upwards in th e E1 u n i t , as has been found by Stow and Bowen (1978, 1980), b u t , as o f t e n as n o t i s u n o rd e re d and i r r e g u l a r . C o n tr a r y to P i p e r ' s (1973) a s s e r t i o n , t e x t u r a l e vid e n ce shows norm al g r a d in g o ve r the e n t i r e E1 u n i t . The E2 u n i t s are graded s i l t and c l a y and may show f a i n t h o r i z o n t a l l a m i n a t i o n s u n l i k e th e much more pronounced la m in a e o f the E1 u n i t . The E3 u n i t s are g ra d e d , m a ssive s i l t s and c l a y s , alw ays w it h some f l o a t i n g sand g r a i n s w hich are u s u a l l y p l a n k t i c f o r a m i n i f e r a were th e y can be i d e n t i f i e d . L a y e rs s i m i l a r to E3 u n i t s have a ls o been r e p o r t e d c o n t a i n i n g f l o a t i n g f o r a m i n i f e r a by Rupke ( 1 9 7 5 ) . The presence o f p l a n k t i c f o r a m i n i f e r a must c o n no te slo w s e t t l i n g o f the E3 u n i t to a l l o w i n c o r p o r a t i o n o f th e p l a n k t i c f o r a m i n i f e r a . I f th e f o r a m i n i f e r a were t r a n s p o r t e d from u p s lo p e , some s iz e g r a d in g would be e x p e c te d . However, i f th e f l o w v e l o c i t y was lo w , i t may n o t have been f a s t enough to e f f e c t the s m a ll s c a le g r a d in g o f the f o r a m i n i f e r a . The e q u i v a l e n t o f P i p e r ' s (1978) F u n i t o v e r l y i n g the E u n i t s would be th e la m in a te d co re s e c t i o n s r e f l e c t i n g h e m ip e la g ic sed i m e n t a t i o n . The e n t i r e g ra y l a y e r i s n o r m a l l y g ra d e d , where i t i s t h i c k enough to sample, and s o r t i n g d e cre a se s upwards from the E2 to the E3, r e f l e c t i n g th e presence o f sand g r a i n s in s m a ll c o n c e n t r a t i o n s . Thus, some m o d i f i c a t i o n o f P i p e r ' s (1973) model f o r v e r t i c a l sequence o f s e d im e n ta ry s t r u c t u r e s i s n e c e s s a ry . F i r s t l y , th e e n t i r e t u r b i d i t e sequence i s n o r m a l l y g ra d e d . S e c o n d ly , many v a r i a t i o n s o f incom p l e t e sequences are p r e s e n t ( F i g u r e 83) w hich may r e f l e c t !more d i s t a l p o r t i o n s o f i n d i v i d u a l t u r b i d i t e s o r v a r i a t i o n ' o f t u r b i d i t y c u r r e n t v e l o c i t y o r bed ro u g h n e s s . In c o m p le te E - u n i t sequences have been noted by P ip e r (1978) w hich p a r a l l e l s f i n d i n g o f in c o m p le te Bouma sequences f o r sandy t u r b i d i t e s in numerous s t u d i e s . These v a r i a t i o n s o f i E - u n i t s c o r r e l a t e w it h R upke's (1 975 ) ty p e s o f r h y t h m ic a l t e r a t i o n s in the M e d ite rra n e a n a b y s s a l p l a i n . F i n a l l y , a c a n y o n -fa n g e om etry i s n o t n e c e s s a ry , nor i s a sandy component u p s lo p e . O l iv e g ra y t u r b i d i t e s , w h i l e n o t as f r e q u e n t i n th e s t r a t i g r a p h i c colum n, u s u a l l y have ty p e s A o r C v e r t i c a l ' sequences ( F i g u r e 8 3 ) . The E1 lam in ae in typ e A o l i v e g ra y t u r b i d i t e s are u s u a l l y a s m a ll e r p a r t o f the sequence than ! in g r a y l a y e r t u r b i d i t e s . T h is may r e f l e c t th e f a c t t h a t th e se o l i v e g ra y t u r b i d i t e s are p r o b a b ly d e r iv e d from lo w e r s lo p e mass f a i l u r e s o f h e m ip e la g ic s e d im e n t, r a t h e r than th e upper s lo p e o r i g i n o f r i v e r - d e r i v e d f l o o d m a t e r i a l . T h is o r i g i n b e s t e x p l a i n s t h e i r s i m i l a r c o l o r , o r g a n ic carbon c o n t e n t and t e x t u r e to la m in a te d s e c t i o n s c o n tig u o u s to them. Perhaps l o w - d e n s i t y t u r b i d i t y c u r r e n t s have been i d e n t i f i e d by th e work o f S h o l k o v i t z and S o u ta r (1975) w hich c o u ld e x p la i n th e f o r m a t io n o f g ra y l a y e r s . T h is would be c o n s i s t e n t w it h th e S c o tia n m a rgin an alo g ue (Stow and Bowen, 1978, 1980; Stow, 19 81). I t i s d i f f i c u l t to t e l l from t h e i r d a t a , however , w he the r th e y c a p tu re d an i n j e c t i o n e v e n t caused by much lo w e r d e n s i t y su s p e n s a te t r a n s p o r t (D ra k e , 1971, 1972) o r an a c t u a l t u r b i d i t y c u r r e n t . The f a c t t h a t th e y found o b v i o u s l y t r a n s p o r t e d s h a llo w e r g a s tro p o d s in th e se d im e n ts w o u ld , h ow ever, s u p p o r t th e t u r b i d i t y c u r r e n t i n t e r p r e t a t i o n . An i n t e r e s t i n g and s i g n i f i c a n t c h r o n o lo g y o f g ra y l a y e r t u r b i d i t e s and f l o o d l a y e r s i s a p p a re n t from v a rv e c o u n t in g o f co re AHF 27375 where r a d i o g r a p h i c r e s o l u t i o n was adequate f o r t h i s p u rp o s e . T h is c h r o n o lo g y must r e f l e c t a p a l e o c l i m a t i c i n f l u e n c e o f f l o o d i n g in th e a re a . The tim e v a r i a t i o n o f suspended se d im e n t d e l i v e r y to v a rv e t h i c k n e s s has a lr e a d y been n i c e l y e s t a b l i h e d by S o u ta r and C r i l l | ( 1 9 7 7 ) , b u t t h a t o f th e g ra y l a y e r s has n o t been e s ta b - i : l i s h e d and i s s i g n i f i c a n t . The c h r o n o lo g y was supplem ented i by u s in g box c o re AHF 28282 from 1.2 km to the s o u th e a s t o f j th e p is t o n c o re to e s t a b l i s h th e s u r fa c e p o r t i o n l i k e l y F i g u r e 83: V e r t i c a l sequences o f s e d im e n ta ry s t r u c t u r e s in t u r b i d i t e s o f Santa B a rb ara B a s in . 325 E2 El E3 E2 E3 El El E3 F blown away by the piston coring process. In addition, where varves were too indistinct to count confidently, average thicknesses of the immediately overlying laminated sections were assumed which were very similar to those calculated by Hulsemann and Emery (1961). There is thus some imprecision in the time scale, but the absolute time scale represented by the varves is evidence seldom present for such chronologies. It is probably more accurate than radiocarbon age determinations. The chronology is given in Table 2 with absolute dates and nature of the flood layer, gray turbidite and olive gray turbidites. Some interesting frequencies are apparent from this chronology. First, gray layers (both types) occur at a frequency of 1/59 yrs. This may reflect roughly 50-yr frequency flooding in the area. Gray layer turbidites occur with a frequency of 1/120 yrs, the same as that found for thicker gray layers by Fleischer (1972), and probably indicates major 100-yr frequency floods delivering material for turbidity current generation to the upper slope. Seismicity in the basin area is so frequent as a triggering event that it can be essentially considered a "constant” and then larger floods are the variable which is reflected by this frequency, rather than being indicative of paleoseismicity. Also, most of the recorded seismicity from 1930-1975 (Hamilton and others, 1969; Lee and others, 1979) is located near the eastern half of the channel, not far from the north slope from which the turbidites radiate (Yerkes and others, 1980; Figure 81). Olive gray turbidites are probably derived from lower slope mass movement and occur at a frequency of 1/571 years. This may be indicative of the frequency of large to very large scale mass movement (Field, 1981) on the lower slope. It is not a "paleo-mass movement” indicator, as no turbidites were detected downslope from large scale mass movement features analyzed in this study. |perhaps, in some crude way, the frequency of olive gray turbidites may, in fact, represent a paleoseismicity clock of very large magnitude earthquakes. i i j Evidence of the 1925 6.1 magnitude earthquake is evident in core AHF 28082, which was also previously reported by Soutar and Crill (1977) for one of their box cores. The deformative structures in the radiograph of this core are overlain and underlain by flood layers. Perhaps the sediment failed in the less competent zone bounded by these two flood layers and was thus controlled or localized within the stratigraphic record by these layers. There are more frequent gray layers in the section representing 1000-2000 yrs B.P., which may reflect higher rainfall during that period. The sedimentation rate determined by varve counting in the box core and piston core analyzed together yields an absolute date of 2283 years for the base of piston core AHF 27875 at the 356 cm base. This yields a sedimentation rate of 173 cm/1000 years, which is slightly higher than the rate of 157 cm/1000 years apparent from Emery's (1960) radiocarbon dates of a core to the south. A lower sedimen tation rate would be expected in any case, because the major influx of terrigenous sediment is from the northeast ■ to the basin center. The two dates are thus not contradic tory. It is tempting to speculate that sedimentation rates further upslope in the northeast direction from the basin center may vary from 200-300 cm/1000 years in the main depocenter of the Montalvo Trough-North Slope corridor's deepest part, but no countable varves exist in the core stratigraphy of that area, unfortunately. Mass movement through sediment creep may make radioisotope dates meaning less in that area anyway, because of the sediment mixing which would be expected. Sedimentation rates should be lower to the west of the basin center, based on considera tions of sediment delivery pathways, and might vary from 75-150 cm/1000 years. AGE TYPE* AGE TYPE* 1925 f ? 666 f 1892 f 649 f 1795 f 616 t 1755 t? 605 f 1742 o t 603 f 1626 f 599 t 1622 t 547 t 1551 f 477 t 1539 f 397 t 1500 t 382 f 1434 o t 334 t 1273 f 309 o t 1215 t 256 f 1121 f? 136 t 1049 f 111 f 908 t 115 B .C . t 883 o t 120 B .C . t 811 t 183 B .C . t 803 f 303 B.C. core base 789 f 780 t * t y p e s : 775 t t= g ra y la y e r t u r b i d i t e 745 f f= g ra y flo o d la y e r 741 f ot== o liv e g ra y t u r b i d i t e 718 t I ! TABLE 2 Chronology of gray turbidite layers, gray flood layers and 1 olive gray turbidite layers. REFERENCES Ahnert, F., 1970, Fundamental relations between denudation, relief and uplift in large mid-latitude drainage basins: Am. Jour. Sci., v. 268, p. 243-263. American Society for Testing and Materials, 1973, Natural Building Stones; Soils and Rock; Peats, Mosses and , Humus, p. 123-128. Anderson, R.Y., 1964, Varve calibration of stratification: in Merriam, D.F., ed., Symposium on Cyclic Sedimentation: Kansas Geol. 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Creator Thornton, Scott Ellis (author)

Core Title Holocene stratigraphy and sedimentary processes in Santa Barbara Basin: Influence of tectonics, ocean circulation, climate and mass movement

Degree Doctor of Philosophy

Degree Program Geological Sciences

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Permanent Link (DOI) https://doi.org/10.25549/usctheses-c29-347257

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