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Thrust Contact Between Franciscan Group And Great Valley Sequence Northeast Of Santa Maria, California

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Thrust Contact Between Franciscan Group And Great Valley Sequence Northeast Of Santa Maria, California

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Content I 69-4517 BROWN, Jr. , James Alexander, 1941- THRUST CONTACT BETWEEN FRANCISCAN GROUP AND GREAT VALLEY SEQUENCE NORTHEAST OF SANTA MARIA, CALIFORNIA. University of Southern California, Ph.D., 1968 Geology University Microfilms, A XEROX Com pany, Ann Arbor, Michigan © 1969 James Alexander Brown, Jr. ALL RIGHTS RESERVED THRUST CONTACT BETWEEN FRANCISCAN GROUP AND GREAT VALLEY SEQUENCE NORTHEAST OF SANTA MARIA, CALIFORNIA fey James Alexander Brown, Jr. A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Geology) August 1968 UNIVERSITY OF SOUTHERN CALIFORNIA T H E G R A D U A T E S C H O O L U N IV E R S IT Y P A R K LO S A N G E L E S , C A L IF O R N IA 9 0 0 0 7 This dissertation, w ritte n by James Alexander Brown, Jr. under the direction of h X s ... Dissertation Com mittee, and approved by a ll its members, has been presented to and accepted by the Graduate School, in p a rtia l fu lfillm e n t of requirements fo r the degree of D O C T O R O F P H I L O S O P H Y Date A u g . ' . . DISSERTATION COMMITTEE . / CJ / Chairman __ ..................iVj ABSTRACT Franciscan rocks of unknown age are in thrust con- j tact with overlying Upper Jurassic and Cretaceous rocks of j the Great Valley sequence in the Stanley Mountain area j northeast of Santa Maria, California. Folding and erosion of the upper thrust plate of Great Valley rocks has ex posed the Franciscan group as a fenster in the core of a broad northwest-southeast trending antiform, herein referred to as the Stanley Mountain antiform. Distribution of rock types in the Franciscan group is chaotic; lens-shaped tectonic inclusions of metagray- wacke, metaconglomerate, greenstone, metachert, amphibolite, and glaucophane schist are bounded by shear surfaces and commonly separated from one another by irregular layers of crushed argillite. Tectonically emplaced belts of serpentinite trend generally northwestward through Franciscan terrane. Shearing of metagraywacke evolves through slight cataclasis and rotation of detrital grains to pronounced cataclasis and incipient segregation of quartzofeldspathic and micaceous minerals. The meta- morphic mineral assemblage quartz-albite-chlorite-white mica-pumpellyite-lawsonite-stilpnomelane-sphene t aragonite indicates that Franciscan metagraywackes recrystallized under conditions of blueschist facies metamorphism. lii The Great Valley sequence includes three formations separated from one another by unconformities: (1) the Knoxville Formation of Late Jurassic age, which consists mainly of greenstone, chert, shale, and graywacke with subordinate conglomerate and limestone; (2) the Jollo Formation of Early Cretaceous age, made up of siltstone, shale, and thick conglomerate lenses; and (3) the Carrie Creek Formation of Late Cretaceous age, characterized by massive coarse-grained sandstone interbedded with thin layers of siltstone and conglomerate, Mineralogic and structural differences between Knoxville Formation and Franciscan group are distinct. Knoxville rocks recrystallized in the prehnite-pumpellyite facies under conditions of diagenesis or low grade regional metamorphism. Knoxville strata are exposed in a north-dipping homocline gently warped into open subsidiary folds. Thrusting of Great Valley sequence over Franciscan group caused (1) tectonic thinning of the Knoxville and Jollo formations; (2) pronounced brecciation of Great Valley rocks; (3) incorporation of quartz-mica schist into a discontinuous "friction carpet" of sheared Franciscan rocks beneath the thrust; and (4) emplacement of serpentinite (subsequently altered to silica-carbonate rock) as thrust slices. The thrust surface transects Knox iv ville strata at a low angle and intersects unconformities within the Great Valley sequence, "bringing Jollo Forma tion and Carrie Creek Formation into contact with the Franciscan group. Relative westward thrusting of the Great Valley sequence over Franciscan group took place in post-Cretaceous pre-Oligocene time. The depositional environment of the Franciscan group is interpreted as an oceanic trench which received sediment during Late Jurassic and Cretaceous time, while Great Valley sediments accumulated in a more stable environment on the continental flank of the trench. Down- warping of Franciscan trench fill was accompanied by penetrative shearing and blueschist metamorphism. The shearing, perhaps in response to underthrusting of the continent by the Pacific Ocean basin, produced a chaotic intermixing of Franciscan rock types. After high-angle faulting and folding had re-oriented shear surfaces and contributed to the chaotic nature of the Franciscan terrane, the Great Valley sequence was thrust westward over the Franciscan group in early Tertiary time. Oligo- cene and Miocene rocks were deposited upon the Great Valley allochthon and were deformed during the Pliocene. The contact between Franciscan group and Sierran- type basement, or between Franciscan group and Great Valley rocks deposited upon Sierran-type basement, repre sents an important boundary between rocks of oceanic and continental affinity. Geologists have recently interpreted this boundary as a thrust fault in areas north and east of the Nacimiento block. The present study extends thrust relationships between Franciscan group and Great Valley sequence to the Nacimiento block, and provides additional evidence that westward thrusting of continental rocks over oceanic rocks was a tectonic event of major significance during early Tertiary time. CONTENTS PAGE INTRODUCTION ........................................ 1 Statement of problem and general conclusions ................................. 1 Location ........................ 3 Methods of study . . .......................... 5 Acknowledgments .................... ..... 3 REGIONAL GEOLOGIC SETTING ............... .... 7 ROCK UNITS ................. 11 General statement .................... .... 11 Jurassic and Cretaceous systems ............. 15 Franciscan group ............................. 15 Metagraywacke ..... .................. 17 Metaconglomerate ........................... 40 Argillite................. 44 Greenstone .... 46 Bedded metachert ...... .. ..... 51 Metamorphism of "in situ" Franciscan r o c k s ................................... 57 Amphibolite........................ 68 Glaucophane schist........... 73 Serpentinized ultramafic rocks associated with the Franciscan g r o u p ............... 77 General statement ........................ 77 vii PAGE Serpentinite belts ............. 78 Isolated serpentinite lenses ............. 84 Pyroxenite ................. 86 Knoxville Formation ........................ 88 General statement .................... .. 88 Graywacke................................. 91 Conglomerate........... 101 Shale and nodular limestone............. 104 Greenstone . . . .. .. .................. 106 Bedded chert ............................... 114 Diagenesis of the Knoxville Formation . . 117 Jollo Formation............................. 121 Carrie Creek Formation ...................... 126 Tertiary and Quaternary systems ............. 129 Silica-oarbonate roek ............. ..... 134 STRUCTURE.......................................... 139 General statement . . . ...................... 139 Stanley Mountain antiform . . . ............. 140 Structure of the Franciscan group ......... 140 Tectonic inclusions ...................... 141 Foliation................................. 148 Penetrative shear .... 150 Mesoscopic folds .......................... 153 Faults...................................... 154 viii PAGE Structure of the Great Valley sequence * . 157 Unconformities ............................. 158 F o l d s ..................................... 158 Faults............. 159 Franciscan and Knoxville tectonic styles: a comparison............. 161 Thrust contact between Franciscan group and Great Valley sequence . ............. 162 General statement . • . .................. 162 Thickness changes of Knoxville and Jollo formations . e 165 Brecciation of Great Valley sequence .». . 168 Isolated occurrences of Franciscan quartz-mica schist ...................... 175 Presence of altered and unaltered ultra- mafic bodies along the Franciscan- Great Valley contact ......... ..... 176 Summary of thrust fault relationships • . 181 Geometry of thrust fault ........... 181 Time of thrust faulting.................... 184 Huasna syncline and East Huasna fault .... 185 GEOLOGIC HISTORY ................................... 187 Age of Franciscan g r o u p ...................... 187 Environments of deposition of Franciscan group and Great Valley sequence .................. 188 ix PAGE Pre-Oligocene deformation and metamorphism of Franciscan group . . . . . . . . . . 195 Pre-Oligocene deformation and diagenesis of the Great Valley sequence . ... 208 Thrust faulting ............................. 210 Late Tertiary and Quaternary history . . . » 211 THE FRANCISCAN-SIERRAN BOUNDARY ................ 212 Contact between Franciscan group and Great Valley sequence . * . .................. 213 Contact between Franciscan group and Sierran-type basement .................... 217 Offset of Franciscan-Sierran boundary by San Andreas f a u l t .................. 220 CONCLUSIONS...................................... 221 REFERENCES CITED ............................... 225 PLATE I . . . . k . J ... ... 235 PLATE IX ♦. . . . ... * I I 036 LIST OF FIGURES FIGURE PAGE 1. Location of Stanley Mountain area . . . 4 2. Distribution of Franciscan group in southern Coast Ranges ................ 8 3* Age relationships between Franciscan group and Great Valley sequence . . . 14 4. Tetnary diagram showing clastic ratios in Franciscan metagraywacke and Knox ville graywacke ................... 30 5. Estimated pressure-temperature fields for blueschist facies and prehnite- pumpellyite facies . . . . . . . . . . 60 6. Estimated position of Stanley Mountain metagraywacke within blueschist zones and mineral stability ranges established by earlier workers .... 63 7. Average compositions of Franciscan metagraywacke and Knoxville graywacke 98 8. Slab-like tectonic inclusions of Franciscan metagraywacke ............. 146 9. Fabric diagrams of Franciscan foliation and bedded metachert .................. 151 10. Fabric diagrams of Franciscan group and Knoxville Formation............. .. • 152 11. Mesoscopic fold in Franciscan group . . 156 12. Thrust contact between Knoxville Formation and Franciscan group .... 166 13. Franciscan group as a continental rise deposit .......................... 192 14. Franciscan group as an oceanic trench deposit.............................. 193 FIGURE xi PAGE 15. Thrust contacts between Franciscan group and Great Valley sequence, and between Franciscan group and Sierran-type basement in the Coast Ranges......... 214 16. Vertical section across western . . . California, showing relative westward thrusting of Great Valley sequence over Franciscan group ................ 219 LIST OP TABLES TABLE PAGE 1. Compositions of 28 Franciscan meta graywacke s ............. 27 2. Modes of Franciscan metabasalt and metadiabase............................. 49 3. Modes of Franciscan amphlbolite and glaucophane schist ...................... 69 4. Compositions of 9 Knoxville graywackes „ . 96 5. Modes of Knoxville greenstone ........... 109 6. Compositions of 9 normal Carrie Creek sandstones, and 9 brecciated sand stones collected at the Franciscan contact................................. 130 xiii LIST OF PLATES PLATE PAGE 1* Geologic map of the Stanley Mountain area............. In Pocket 2. Geologic structure sections across the Stanley Mountain area ......... In Pocket s * .. 3. Stratigraphic column, Stanley Mountain area ................. 13 4. Chaotic tectonic style of Franciscan g r o u p ............................... 16 5. Massive Franciscan metagraywacke . . . 19 6. Photomicrograph of Franciscan metagraywacke...................... 21 7. Photomicrograph of incipient shear surfaces in Franciscan metagray wacke ............................... 23 8. Photomicrograph of Franciscan meta graywacke bearing imprint of pronounced shear .................... 24 9. Photomicrograph of strongly foliated Franciscan metagraywacke ........... 25 10. Photomicrograph of albite surrounded by white mica and chlorite in Franciscan metagraywacke ...... 31 11. Photomicrograph of radiating sheaves and needles of pumpellyite in Franciscan metagraywacke ........... 34 12. Photomicrograph of lawsonite growing in detrltal albite in Franciscan metagraywacke ...................... 36 13. Photomicrograph showing prisms of lawsonite associated with vein quartz in Franciscan metagraywacke . 37 xiv PLATE PAGE 14. Photomicrograph of granular sphene in highly sheared Franciscan meta graywacke ........................... 39 15- Photomicrograph of fine-grained granular calcite replacing euhedral aragonite in Franciscan meta graywacke ........................... 41 16. Angular clasts in Franciscan meta conglomerate ........................ 43 17. Tectonic inclusion of metagraywacke enclosed in sheared argillite . . . 45 18. Sheared contact between Franciscan greenstone and metagraywacke .... 47 19. Photomicrograph of porphyritic meta- basalt from Franciscan group .... 50 20. Photomicrograph of albite surrounded by pumpellyite and chlorite in Franciscan metadiabase.......... . 52 21. Photomicrograph of lawsonite in Franciscan metadiabase ............. 53 22. Franciscan bedded metachert ......... 54 23. Contact between Franciscan meta graywacke and metachert-argillite sequence............................. 56 24. Photomicrograph of deformed silica- filled radiolarian tests in Franciscan metachert............. . , 58 25. Photomicrograph of Franciscan amphibolite, showing hornblende partially altered to glaucophane . . 72 26. Lens of quartz-white mica-glaucophane schist enclosed in sheared Franciscan metagraywacke . 74 27. Photomicrograph of glaucophane schist, showing prismatic and rhombic sections of glaucophane in quartz . 76 XV ! PLATE PAGE i 28. Steeply dipping lens of serpentinite . 79 j i i 29. Photomicrograph of bastite, showing i pseudomorph of antigorite after j pyroxene surrounded by veinlets of chrysotile...................... ®1 30. Photomicrograph of sheared serpentinite ........... 82 31. Lens of serpentinite oriented parallel to foliated metagraywacke......... 32. Base of Knoxville graywacke unit exposed in Alamo Creek canyon . . . 92 33. Photomicrograph of Knoxville gray wacke, showing Intact sedimentary framework.......................... 94 34. Rounded clasts in Knoxville intra- formational conglomerate ...... 103 35. Large limestone nodules In Knoxville shale unit........... . . ........... 105 36. Resistant andesite flow in Knoxville Formation............... 108 37. Photomicrograph of porphyritic Knoxville andesite . . . . . . . . . 110 38. Knoxville pillow l a v a ................ 113 39. Knoxville bedded chert Interlayered with paper-thin shale ............. 115 4-0. Sandstone and conglomerate In Jollo Formation...........» ............. 123 41. Interbedded siltstone and shale in Jollo Formation ..... ......... 125 42. Evenly bedded sandstone and siltstone in Carrie Creek Formation ......... 128 43. Silica-carbonate rock, showing porous texture .................. 136 xv i PLATE PAGE 44. Photomicrograph of silica-carbonate rock................................. 137 45. Monolith of Franciscan metagraywacke . 143 46. Slab-like tectonic inclusions of Franciscan metagraywacke . ....... 145 47. Boudinage and pinch-and-swell structure in Franciscan metagraywacke 147 48. Sole markings on surface of Franciscan metagraywacke ...................... 149 49. Folded Franciscan metagraywacke and argillite .................. 153 50. Gentle fold in steeply dipping Knox ville chert near contact with underlying greenstone .. . . . . . 160 51. Thrust contact between Knoxville shale and Franciscan metagraywacke .... 165 52. Brecciated Knoxville graywacke about 6 feet above Franciscan thrust contact............................. 169 53. Brecciated Carrie Creek sandstone about 10 feet above Franciscan thrust contact ...................... 171 54. Interbedded fine-grained sandstone' and micaceous siltstone in Carrie Creek Formation...................... 173 55. Sheared and contorted Carrie Creek siltstone about 20 feet above Franciscan thrust contact ......... 174 INTRODUCTION Statement of Problem and General Conclusions Contact relationships between eugeosynclinal rocks of the Franciscan group of Late Jurassic-Cretaceous age and miogeosynclinal rocks of the Great Valley sequence of comparable age are the subject of much current debate among geologists familiar with the Coast Ranges of Cali fornia. Contrasting ideas about the nature of this con tact are: (1) that Franciscan rocks grade upward into rocks of the Great Valley sequence (Taliaferro, 1943a, p. 218), (2) that the lower part of the Great Valley sequence rests unconformably upon the Franciscan (Anderson, 1945, p. 930), and (3) that the Great Valley sequence has been thrust westward over autochthonous Franciscan rocks (Bailey, Irwin, and Jones, 1964, p. 163). An ideal locality for studying the contact between the Franciscan group and Great Valley sequence is the southwest portion of the Branch Mountain quadrangle and an adjacent part of the Nipomo quadrangle in the southern Coast Ranges near Santa Maria. Here, urifossiliferous Franciscan rocks are in contact with formations of the Great Valley sequence containing fossils of Late Jurassic and Cretaceous age. Of special significance is the fact that Taliaferro (1943a, p. 197) cited this area as a prime example of gradational contact between Franciscan rocks and the Knoxville Formation of Late Jurassic age, which forms the lowest part of the Great Valley sequence. Based upon field and petrographlc work, two principal conclusions of the present study are: (1) that the Franciscan group and Knoxville Formation in the Branch Mountain-Nlpomo quadrangles are separate and distinct map- pable units which do not grade into one another (Plate 1); and that (2) the Franciscan group is in thrust contact with the overlying Knoxville Formation and Cretaceous rocks of the Great Valley sequence (Plate 2). In support of the first conclusion, comparison of Franciscan to Knox ville lithologies will emphasize differences in initial composition, metamorphism, and deformation. The existence of a thrust fault between the two rock units will be demonstrated In descriptions of outcrop patterns of Great Valley rocks, shearing at contacts, and distribution and geometry of serpentinite bodies and silica-carbonate rock. In subsequent discussions the informal term Franciscan "group" will be used to designate a chaotic assemblage of metagraywacke, metaconglomerate, argillite, greenstone, metachert, amphibolite, glaucophane schist, and serpentinite. As suggested by Ernst (1965» p. 880), the formal terms Franciscan Group and Franciscan Formation (American Commission on Stratigraphic Nomenclature, 1961) should be used only when mappable units within the Fran ciscan assemblage have been clearly distinguished. 3 Location The present study concerns a wedge-shaped area of 40 square miles enclosing a northwest-southeast trending belt of Franciscan rocks within the southern Coast Ranges about 15 miles northeast of Santa Maria, California (Fig. 1). This area, the southwest quarter of the U.S.GoS. Branch Mountain 15' quadrangle and a small part of the Nipomo 15' quadrangle to the west, is readily accessible from California Route 166 and several Improved dirt roads maintained by the United States Forest Service. "Stanley Mountain area" is an appropriate name for the study area consistent with references to the region by Taliaferro (1943a, p. 199) and Bailey and others (1964, p. 122). In the Stanley Mountain area, the topography is characterized by smooth rolling hills rising 1000 to 1500 feet above broad V-shaped canyons; maximum relief is 2000 feet. Rocks are best exposed along the steep sides of the Cuyama River gorge, which is cut across the structural grain of the area and in bottoms of major tributary canyons. On the grassy highlands and brush-mantled ridges, outcrops a'fre sparse, except for areas underlain by serpentinite„ + / y - 3 5 ° 0 0 ' Santa ° Maria 20 o Santa Barbara Figure 1. Location of Stanley Mountain area, southern Coast Ranges, California. 5 Methods of Study A total of 109 days were spent during summer and fall of 1966, in field mapping of the Stanley Mountain area on aerial photographs at a scale of approximately 1:25,000. At the end of each field day outcrop patterns, attitudes, and sample locations were transferred from photos to a topographic base map (assembled from 1952 editions of the Branch Mountain and Nipomo quadrangles enlarged to a scale of approximately 1:26,500). To aid in determining identity and distribution of minerals in rocks of the Stanley Mountain area, 94- thin sections were examined under the petrographic microscope. Modal analyses based upon 500 point counts of each section were in some cases checked against whole-rock X-ray dif fraction patterns. Acknowledgments A Predoctoral Traineeship from the National Aero nautics and Space Administration made the project possible by supporting the author's graduate study at the University of Southern California. A grant from the Penrose Pund of the Geological Society of America provided funds for field and laboratory work. A teaching appointment at the University of California, Los Angeles, furnished financial assistance during the latter stages of research and writ ing. The author gratefully acknowledges the help and advice of faculty and students in the Department of Geological Sciences, University of Southern California,, G. A. Davis, R. H. Merriam, G. Reaves, and V. A. Taylor reviewed and improved the manuscript. The author has benefitted from critical discussions with M. C. Blake, Jr., United States Geological Survey, Menlo Park, California; N. A. Bogdanov, Geological Institute, Academy of Sciences of the USSR; G. A. Davis, University of Southern California; and W. G. Ernst and Co A. Hall, Jr., University of California, Los Angeles. Members of the United States Forest Service, in particular John Graf at the Pine Canyon Guard Station in Los Padres National Forest, rendered assistance by granting access to government lands. Among private landowners whose cheerful cooperation and local knowledge greatly aided field work were E. Biaggini, R. Garcin, P. Glines, Ea Rice, J. Williams, R. Woods, and especially R. Hutchinson and the Oakley family. Mrs. Peter Kurtz drafted figures and plates and made helpful suggestions for improving format. Lowell Weymouth developed and processed photo micrographs. The writer thanks these individuals and institutions. 7 REGIONAL GEOLOGIC SETTING The Stanley Mountain area lies within the southern foothills of the Santa Lucia Range, one of a series of long northwest-trending ridges comprising the southern Coast Ranges where topographic patterns are at low angles to linear structural elements. A striking feature of the southern Coast Ranges is the presence of two distinct basement complexes, unrelated in origin, yet juxtaposed for hundreds of miles along the San Andreas and Nacimiento faults (Page, 1966, p. 255). One complex, consisting of eugeosynclinal Franciscan rocks of Jurassic-Cretaceous age, is distributed throughout two broad belts separated from each other by the other basement complex, a northwest- trending corridor of Cretaceous granitic intrusives and older metamorphic rocks called the Salinian block (Fig. 2). The Franciscan belt located opposite the San Andreas fault on the northeast side of the Salinian block forms the core of the Diablo antiform. The belt located opposite the Nacimiento fault on the southwest side of the Salinian block is called the Nacimiento block and includes Franciscan rocks of the Stanley Mountain area. Nowhere do granitic rocks intrude the Franciscan group. Many geologists attribute present distribution of basement complexes in the southern Coast Ranges to large strike- slip displacements on the San Andreas fault (Bailey and 8 v T A B L E N ^ MOUNTAIN Paso Robles' - 3 5 ° 30' -Q M ORRO o o •STANLEY MOUNTAIN 7K AREA o o Santa Maria A rea of Franciscan outcrop Fouit SANTA MARIA B A S IN 15 Miles A fter Geologic Map of California San Luis Obispo Sheet I2 1 °0 0 ' Figure 2. Distribution of Franciscan group in a portion of the southern Coast Ranges. Faults in Salinian block are not shown (after Jennings, 1958* 1989). others, 1964, p. 159). Sense of displacement and amount of movement on the Nacimiento fault is unclear. A thick section of miogeosynclinal sedimentary rocks of Upper Jurassic and Cretaceous age is well-exposed on the flank of the Diablo antiform along the western margin of the Great Valley, from which the term "Great Valley sequence" is derived. Rocks of similar age and lithology cover large areas of the Nacimiento block, but only Upper Cretaceous units of the Great Valley sequence rest on the Salinian block. The contact between Great Valley rocks and Franciscan basement is generally a fault (Bailey and others, 1964, p. 159)» whereas the contact between Great Valley rocks and crystalline basement of the Salinian block is clearly depositional. Several geologists have suggested that thrust faults carried rocks of the Great Valley sequence over Franciscan basement during latest Cretaceous or early Tertiary time (Irwin, 1964, p. 9; Bailey and others, 1964, p. 163-165; Dickinson, 1966a, p. 465). Marine sedimentary formations of Cenozoic age in the Coast Ranges represent almost every epoch and stage from early Paleocene to Pleistocene (Page, 1966, p. 263). Numerous unconformities, folds, thrust faults, steep reverse faults, and strike-slip faults record persistent crustal unrest, particularly in late Pliocene and Pleisto cene time. In the Ripomo quadrangle west of the Stanley 10 Mountain area, Hall and Corbato (1967, p. 562) reported an 18.000 foot-thick section of Cenozoic strata overlying 13.000 feet of Cretaceous strata belonging to the Great Valley sequence* Rocks peripheral to the Stanley Mountain area of the Nacimiento block are thoroughly imprinted with structural patterns characteristic of the southern Coast Ranges. Faults and fold axes trend northwesto To the north and east of the Stanley Mountain area the complex Nacimiento fault separates Franciscan rocks from the crystalline basement of the Salinian block (Fig. 2). Over a large area the fault is nearly straight, but in the Nipomo quadrangle it displays a sinuous surface trace and dips 45-60 degrees east (Hall and Corbato'’ , 1967, p. 577). To the south, Franciscan basement dips southward 15-20 degrees beneath folded and faulted Tertiary deposits of the Santa Maria basin (Canfield, 1939, p. 70). To the west, the Huasna syncline in Tertiary rocks is bounded by two high-angle faults, the East and West Huasna, which are subparallel to the Nacimiento fault and San Andreas fault (Hall and Corbato, 1967, p. 574). The Stanley Mountain area has been a training ground for students of geology under the supervision of N. L. Taliaferro (University of California, Berkeley), T. Clements (University of Southern California), and 0. A. Hall and 0. E. Corbato (University of California, Los Angeles). As an outgrowth of these mapping programs, several papers, both published and unpublished, have dealt with parts of the area but never the whole, Oakeshott (1929) undertook a thorough petrographic study of Francis can and Knoxville rooks cropping out in a small area 3ust south of Stanley Mountain. Taliaferro (1943a, p. 199) published a map of that part of the Stanley Mountain area located within the Nipomo quadrangle, According to Taliaferro (1943a, p. 197) rocks of the Franciscan group grade upward through a thick mass of greenstone into shales and sandstones of the Knoxville Formation of Upper Jurassic age. Easton and Imlay (1955» P* 2337) described locations of Upper Jurassic fossils discovered in the Stanley Mountain area of the Branch Mountain quadrangle. The authors assigned fossil-bearing rocks to the Franciscan group. Crandall (1961) examined stratigraphy of Upper Cretaceous rocks along the southwest border of the Stanley Mountain area. Recently, Hall and Corbato (1967) published a comprehensive report on stratigraphy and structure of Mesozoic and Cenozoic rocks in the Nipomo quadrangle. ROCK UNITS General Statement Within the Stanley Mountain area, sedimentary and volcanic rocks of Late Jurassic to Recent age exhibit a wide spectrum of composition and texture (Plate 3). The Franciscan group is structurally the lowermost unit, but not necessarily the oldest, because elsewhere in the Coast Ranges grossly similar rocks have yielded fossils of Late Cretaceous age (Fig. 3). Dominant rocks in the Franciscan group of the Stanley Mountain area are metagraywackes deformed by shear at all scales of observation. Above these rocks and in thrust contact with them are three broadly folded formations of the G-reat Valley sequence separated from each other by unconformities; (1) the Knox ville Formation of Late Jurassic age consisting mainly of greenstone, chert, and shale with subordinate graywacke, conglomerate, and limestone; (2) the Jollo Formation of Early Cretaceous age made up of siltstone, shale, and thick conglomerate lenses; and (3) the Carrie Creek Formation of Late Cretaceous age characterized by massive coarse-grained sandstone interbedded with thin layers of siltstone. The Knoxville Formation, as defined here, includes greenstone and fossil-bearing shale earlier assigned to the Franciscan group (Taliaferro, 1943a, p. 197; Easton and Imlay, 1955» p. 2337). The names Jollo Formation and Carrie Creek Formation have been proposed for Lower and Upper Cretaceous rocks respectively by Hall and Corbato (1967* P* 564-565). Oligocene and Miocene rocks characterized by numerous facies changes rest AUTOCHTHONOUS CENOZOIC SEDIMENTARY ROCK and ALLOCHTHONOUS MESOZOIC ROCKS o 1_ Ll) £ a> to >N GO to a> Q) CO a> o> o (A Formation Member Column E CO to 3 a> .§.£ 5g Description o 3 o Santa Margarita Fm. Unconsolidated sand, silt, cl ::Tfrisrri': 200' White,coarse-gr. ss. and < o *o IM o c a> O V_ a +- i — a) c. a> o o a > a. a. Monterey Formation 2500' Siliceous siltstone: well-bei brown, gray or white, com fractured, locally porous, w light orange-brown. Sandsl bedded, light brown,fine- tc grained. Porcelaneous she bedded, light arov. Chert bedded, brown laminated. a > T3 T3 Point Sal Formation rTmps-T 1000 ' Siltstone: thin-bedded, ligt dull white, soft, locally diatoi tuffaceous. Sandstone:eve white, fine- to medium-grair careous. Thin-bedded buff Rincon Fm. o o* Vaqueros Formation Sespe Fm. I —Tmr-z. 200' Claystone: brown,white, so "Ib-mvd" 600' Sandstone: massive, light b coarse-grained. Locally h i fine-grained. Brown siltstc • °Tos— - 300* Red and green coarse-gr. a> o. CL Carrie Creek Formation 3000' t n 3 O a) Sandstone: massive or thicl yellow-brown or yellow-gray,i pale orange brown, brick red yellow i coarse-grained, mo sorted, arkosic, abundant b ic calcareous cement. Some n fine-grained sandstone. Sill flaggy,dull gray or black, micaceous, locally siliceous bedded with brittle gray s i Cobble conglomerate: welT clasts in matrix of coarse-i sandstone with abundant bi TARY ROCKS AUTOCHTHONOUS MESOZOIC ROCKS ROCKS ■ FRANCISCAN GROUP D escription Tectonic Class System Column Description ilidated sand, silt, clasts. * S l oarse-gr. ss. and calc. silt. ; siltstone: well-bedded, liaht jray or white, conchoidally id, locally porous, weathering nqe-brown. Sandstone: well- light brown, fine- to medium- Porcelaneous shale: thin- liahtarav. Chert: thin- , brown laminated. > C o o zr ^ zr o ~T - • !: thin-bedded, light brown or 3, soft, locally diatomaceous or us. Sandstone:evenlv-bedded. ie- to medium-grained cal- Thin-bedded buff shale. O c in ie: brown,white, soft. ne: massive, light brown, soft, jrained. Locally hard and ined. Brown siltstone. green coarse-gr. ss., eg. J ie: massive or thick-bedded, own or yellow-gray,weathering ige brown, brick red or bright oarse -grained, moderate ly rkosic, abundant biotite, local us cement. Some medium gray ned sandstone. Siltstone: lull gray or black, brown, js, locally siliceous. Inter- with brittle arav shale, onglomerate: well rounded matrix of coarse-grained e with abundant biotite. thickness unknown IB® No stratigraphic order implied Tectonic inclusions of metagraywacke, metaconglomerate, argillite, greenstone, bedded metachert, amphibolite,glauco- phane schist. Belts of serpentinite. Pyroxenite. Metagray wacke: lenticular, rarely tabular bodies a few feet thick to several Mesozoic Sespe Fm. o> o. CL ZD Carrie Creek Formation tn 3 o CD O O CD O a> £ o Joiio Formation c_> V) <n O i_ 3 *3 CD CL Q. ID Knoxville Formation graywacke unit chert unit greenstone unit shale unit • . o > o • -Tos- & > k u c c - T" r~r • Jkgw A A A A A A 500' r ° FAULTS 300' 3000' 3000' 1200 ' 4 0 0 ’ 1500 Red and green coarse-gr. Sandstone: massive or thic yellow-brown or yellow-gray, pole orange brown, brick re( yellow; coarse-grained, m e sorted, arkosic, abundant bi calcareous cement. Some r fine-grained sandstone. Si] flaggy, dull gray or black, micaceous, locally siliceou; bedded with brittle gray § Cobble conglomerate: wpll clasts in matrix of coarse- sandstone with abundant b Siltstone and shale: evenly (beds 1-4"), olive, gray, or concretions of dark gray crystalline limestone wee creamy white. Conglomera (average 3 0 'thick) massiv thick bedded, overall dark black due to abundant gre and volcanic clasts; pebbl rounded, sometimes imbr Sandstone: 1-2' beds, lamim cross-bedding and graded yellow-brown, coarse-gr. p i Graywacke: massive, resis gray weathering same, co fractured, medium- to coarsi moderately sorted,arkosic, calcareous; interbedded wi upper part; tuf faceous in \ o \ Chert: even-bedded (ave.3“ ), I black or green-black,argill.,Rc Greenstone: andesitic and b flows, pillow lava in upper part, tuff. Dark green or gr amygdaloidal andesite and weathering red-brown. Flc thick. Pillows average 8"; c separated in green tufface White tuff beds weather br Mostly dark green shale wi limestone. Yellow-brown c j j and conglomerate. Buchia t STRATIGRAPH 1 C COLUMN, STANLE Geology by J. A. Bt P L A T E 3 in coarse-gr. ss.,cg. assive or thick-bedded, Dr yellow-gray,weathering rown, brick red or bright s-grained, moderately :, abundant biotite, local iment. Some medium gray andstone. Siltstone: ray or black, brown, cally siliceous. Inter- brittle gray shale. jmerate: wpll rounded ix of coarse-grained ti abundant biotite. shale: evenly-bedded ive, gray, or black ; > f dark gray finely tiestone weathering : . Conglomerate: lenses Ihick) massive and , overall dark greenish- Dbundant green chert dlasts; pebbles well letimes imbricate. 2' beds, laminated, local j and graded bedding, , coarse-gr. poorly sorted nassive, resistant, dark ing same, conchoidally dium- to coarse-grained, >rted,arkosic, locally terbedded with shale in Ffaceous in lower part. dded (ave.3“ ), lam., brown, -black,argill.,Radiolarians. indesitic and basaltic lava in upper and lower rk green or gray-green andesite and basalt jd-brown. Flows 3“IOO‘ . average 8"; completely green tuffaceous matrix. Is weather brick red. reen shale with nodular llow-brown graywacke irate. Buchia piochii. J o o o o £= cn V) Z3 o CD O O a> O o ■a ci o o C / J tn o thickness unknown / thickness unknown No stratigraphic order implied Tectonic inclusions of metagray wacke, metaconglomerate,argillite, greenstone, bedded metachert, amphibolite,glauco- phane schist. Belts of serpentinite. Pyroxenite. Metagray wacke: lenticular, rarely tabular bodies a few feet thick to several hundred feet thick in matrix of sheared argillite; exhibits boudinage, pinch- and-swell structure; dark greenish gray weathering pale olive or pale yellow-brown; highly resistant,medium- to coarse-grained,poorly sorted ; cataclastic textures common; locally a semischist. Metaconglomerate : crude ly layered, dark green gray;angular clasts average I" . Argillite: crumpled and sheared; discontinuous layers average 4". Greenstone: highly sheared dark green lenses weathering reddish brown. Relict basaltic and diabasic textures. A few indistinct pillows. Bedded metachert: lenticular bodies less than 15' thick; bed 1-3", intercalated with paper-thin argillite; pinch-and-swell structure common; generally pale yellow,orange, red; Radiolarians. Amphibolite: layered (2-10 mm) or unlayered, fine-to coarse-grained, nematobiastic or granoblastic. Glaucophane schist: layered lenses of medium-grained quartz-m ica- glaucophane schist; also blue-gray, fine-grained to aphanitic. Serpentinite: lenses up to I mile long; massive, resistant blocks embedded in aslickensided,soft, scaley matrix. Pyroxenite: partially serpentinized, light green, coarse - grained,clinopyroxenite envelops inclusions of olivine clinopyroxenite. No stratigraphic order implied fANLEY MOUNTAIN AREA, CALIFORNIA >y J. A. Brown, Jr. /968 AGE RELATIONSHIP BETWEEN FRANCISCN GROUP AND GREAT VALLEY SEQUENCE IN CALIFORNIA Franciscan group Great Valley sequence in *- CZ O O<0 C O A S T R A N G E S S A C R A M E N T O V A L L E Y Upper Cretaceous strata Q. C l 9 0 Horsetown Forma tion Franciscan group (undifferentiated) Paskenta Formation 135 J r . i n Q) u) CL rj CL i_ Knoxville Formation Figure 3. Diagram showing age relationships between Franciscan group and Great Valley sequence. Serrated line Indicates temporal equivalence (modified after Irwin, 1937, p. 2292). 15 unconformally upon the Franciscan group and Great Valley sequence. Recent landslide deposits are common, especial ly within the Franciscan terrane. Jurassic and Cretaceous Systems Franciscan group Sheared metamorphic rocks in the Stanley Mountain area have "been assigned to the Franciscan group. Since the rocks are exposed in a fenster and lack fossils as well as stratigraphic continuity, the metamorphic terrane cannot be directly correlated with the type "Franciscan i. Formation" on the San Francisco peninsula. Nevertheless, the lithology, textures, and mineral assemblages of the sheared metamorphic rocks are similar to properties which the writer has observed in rocks mapped as "Franciscan" by Coast Range geologists. Distribution of rock types in the Franciscan group is chaotic. Lens-shaped bodies of metagraywacke, meta conglomerate, metachert, greenstone, glaucophane schist, amphibolite, and serpentinite, varying in length from a few inches to several hundred feet, are separated from one another by thin, irregular layers of pervasively sheared argillite, averaging 4 inches in thickness, and are frequently bounded by slickensided surfaces (Plate 4). 16 T O Plate 4. Chaotic tectonic style of Franciscan group, showing lenticular competent rocks (tectonic inclusions) embedded in matrix of sheared argillite. Pencil point rests on greenstone. Lens slightly to left of center is metagraywacke. Light gray lenses at bottom and top are metacherts. Outcrop is located in bed of Aliso Greek 1000 feet from Cuyama River. Movements along these boundary planes have juxtaposed markedly different lithologies and, In most cases, completely obliterated bedding. As a result, the total thickness of the Franciscan group cannot be determined. Outcrops usually exhibit several joint sets as well as complex networks of quartz and/or carbonate veins. The blocky or hummocky character of the Franciscan terrane reflects the disorderly distribution of rock types, the locally inconsistent structural trends, and landsliding. Jumbled piles of resistant blocks involved in recent land- sliding occasionally augment the overall chaotic appearance of the Franciscan group. Good exposures of in_ situ. terrane in stream cuts, however, clearly show that chaotic structural style is a primary feature of these Franciscan rocks. The age of the Franciscan group in the Stanley Mountain area and in other parts of the Uacimiento block is unknown. To the northeast, Franciscan rocks along the east side of the Diablo antiform have yielded megafossils of Late Jurassic or Early Cretaceous age. Franciscan limestones in the San Francisco area contain Foraminifera of Late Cretaceous age (Bailey and others, 1964, p. 118, 122) . Metagraywacke. By far the most abundant rock type 18 in the Franciscan group is metagraywacke, accounting for more than 95 per cent of outcrop area. Where metagray wackes are massive, the quality of exposure depends more upon the extent of deformation than upon rock composition. Massive varieties cut by closely spaced joints and faults tend to crop out as irregular knobs or squat columns giv ing the hillsides a boulder-strewn appearance, whereas the more cohesive rocks form steep cliffs (Plate 5). Meta- graywackes interlayered with argillite are best seen in road cuts or stream beds. Lenticularity of metagraywacke units in the Stanley Mountain area is the product of intense deformation rather than the result of sedimentary processes. Surfaces of contact between metagraywacke and argillite are generally irregular, slickensided, or brecelated. Maximum thick ness of individual metagraywacke units as measured between successive argillite layers varies from an inch to several tens of feet. In some localities, faults have sliced lenses into blocks or slabs. The only primary bedding feature observed in Franciscan metagraywackes of the Stanley Mountain area is a single set of flute casts exposed in a roadcut on the east side of the Cuyama gorge. Freshly exposed metagraywackes are generally dark greenish gray, greenish gray, olive gray, or grayish olive green; weathered rocks are pale olive or pale yellowish brown. Color distribution tends to be uniform in hand Plate 5« Massive Franciscan meta graywacke. Outcrop is at east end of quarry on north hank of Aliso Greek 800 feet from Cuyama River. Location of sample D - 45a (see Plate 6 and Plate 13). 20 specimen. Both freshly broken surfaces and weathered surfaces tend to be smooth. Where sedimentary textures have been preserved, the clasts in metagraywackes are medium-grained (i?-^ irM), poorly sorted, and angular or subangular (Plate 6). Grain size may vary from 2 mm through silt-size in a single sample* As a rule the grains are randomly oriented, ex cept in a few samples where black argillite chips up to 1 cm in length define a faint lamination. A conspicuous textural feature of metagraywacke in the Stanley Mountain area is the reconstitution of clastic textures by shear under conditions of low-grade regional metamorphism. Modifications range from slight cataclasis of grain boundaries to complete recrystallization. Blake, Irwin, and Coleman (1967s p. 3) have classified metagray wackes of the northern Coast Ranges into “textural zones" modified from Turner's (1938, p. 163) breakdown of the Chlorite Zone in greenschists of southern New Zealand. Although textural zones cannot be mapped in the metagray wackes of the Stanley Mountain area, the concept is use ful for classifying individual samples, most of which fall into the category of textural zone 1 and textural zone 2 (Blake and others, 1967 > p. 3). As seen in thin section (Plate 6), the clastic texture of metagraywacke in textural zone 1 has been modified by slight cataclasis of grain boundaries due to I----------------------------------- 1 0 . 5 nun Plate 6. Photomicrograph of Franciscan meta graywacke (textural zone 1, according to Blake and others, 1967, p. 4"H Uniform white grains are sericitized albite; large dark gray, patchy grain at left center is a volcanic rock fragment„ Kicols crossed. Sample D - 45a from east end of quarry on north hank of Aliso Greek 800 feet from Cuyama River (see Plate 5)« 22 incipient shear. The amount of interstitial silty and argillaceous material has increased at the expense of abraded and recrystallized grain margins, but not enough to significantly reduce the average grain size of the sample. The term "semischist" is applicable to metagray wacke of textural zone 2, which bears the imprint of a secondary planar fabric observed in two stages of develop ment. Initially, abrasion and reorientation of grains along closely-spaced shear planes produces a subparallel alignment easily confused with primary lamination (Plate 7). Close inspection reveals that grain boundaries are shattered and that interstitial material is flattened in a plane parallel to the aligned grains. In the second stage of development, interstitial material has almost com pletely recrystallized to white mica. Schistosity and reduction of grain size are pronounced (Plate 8). Well-foliated metagraywackes of textural zone 3 occur in widely scattered outcrops. Complete re- crystallization of clastic grains has resulted in segrega tion of discontinuous folded layers of white mica, chlorite, and lawsonite, which alternate with layers of quartz 1-2 cm in thickness (Plate 9). \ To determine compositionNxf_the metagraywacke, 500 point counts were made on each of 28 thin sections. Whole rock samples as well as products of heavy mineral 23 -I 0 . 5 nun Plate 7. Photomicrograph of Franciscan meta graywacke. Elongate quartz grains are bounded by incipient shear surfaces diag nostic of initial stage of textural zone 2 (according to Blake and others, 1967, p. 4). Sample D - 2% collected from roadcut on east side of Calif. Rte. 166 about 3 miles northeast of intersection with Tepusquet Road. 24 1 0. 5 nun Plate 8. Photomicrograph of Franciscan meta graywacke, which bears imprint of pronounced shear diagnostic of advanced stage in textural zone 2. Light gray fuzzy material between grains is white mica. Black stringers mark shear surfaces stained by ferric iron. Nicols crossed. Sample D - 9 2 collected from east bank of Miranda Pine Creek 1200 feet southeast of Cuyama River. i 0 .5 mm Plate 9- Photomicrograph of Franciscan meta graywacke showing pronounced foliation representative of textural zone 3. Primary sedimentary textures have been completely destroyed by shearing and recrystallization. White mineral is quartz; dark material con sists of a fine-grained mixture of law- sonite, jadeitic pyroxene, and chlorite stained by ferric iron. Nicols parallel. Sample D - 89a collected from ledge on east side of Calif. Rte. 166 about 300 feet south of Aliso Creek. 26 separations were X-rayed in order to provide a rough check on identifications in 10 of the metagraywackes. Plagioclase compositions were determined with the aid of a Zeiss 4-axis stage following procedures outlined by Slemmons (1962). Metagraywacke consists of relict detrital grains and minerals formed by the reaction of these grains under conditions of regional metamorphism. Principal detrital constituents of Franciscan metagraywacke in the Stanley Mountain area are quartz, plagioclase, rock fragments, and interstitial fine-grained material, called matrix. Among the important metamorphic minerals are chlorite, white mica, pumpellyite, lawsonite, stilpnomelane, and sphene. Common vein minerals are aragonite, calcite, and quartz. Table 1 summarizes the composition of the meta graywacke . The tendency of some minerals to break down more readily than others in response to changes in temperature and pressure makes difficult the assessment of Initial detrital proportions in metagraywacke. For example, dur ing metamorphism such minerals as white mica, pumpellyite, and lawsonite may form at the expense of feldspar, while quartz remains relatively stable. The matrix is apt to encroach upon abraded grain boundaries and incorporate disintegration products of unstable rock fragments. Nevertheless, original sedimentary composition can be Component t P CO CM 1 P 0 to 1 p to 1 P to to i P JO 0 v 1 p A 10 1 p D-48 D-57 & 0 to 1 p H C- 1 P Francis a p c- i i p p Albite 37% 23 22 15 14 13 11 28 17 20 18 15 17 » Lithic frag, argillite tr 1 2 tr 1 1 2 2 4 0 3 1 0 n ■p chert 3 1 6 3 1 4 4 2 1 tr 5 4 0 43 qtz.-muse. tr 0 1 0 0 1 tr 0 0 1 tr 1 tr • P schist volcanic 1 X 0 1 0 1 1 0 2 0 1 tr 0 Quartz 43 20 24 19 39 26 26 41 27 23 30 44 26 Calcite 0 0 0 0 0 0 O' 1 0 0 0 0 0 Chlorate 0 16 12 38 15 16 25 12 14 20 18 14 10 Epidote 0 tr 1 3 0 4 1 tr 1 1 1 1 1 Iron stain 2 tr tr 5 3 2 2 1 1 2 2 2 3 Jadeitic px. 0 0 0 0 0 0 0 0 0 0 0 0 0 H c o Lawsonite 0 0 0 8 5 tr 1 0 tr tr 1 1 tr ■p ■H U 43 White mica 12 19 7 6 18 13 5 8 ■ 12 19 12 8 35 ® XJ c o Pumpellyite 0 15 24 1 tr 9 17 1 15 . 5 2 1 0 Sphene-leucoxene 1 2 tr tr '2 6 4 3 . 4 5 5. 6 5 Stilpnomelane 0 1 0 tr :2 4 tr 0 1 2 tr 2 0 Unknown 1 1 1 1 0 0 1 1 1 2 2 tr 3 Total t o t o t o T O 115$ T O T O T O T O T O T O T O T O Table 1. Compositions of 28 Fr Percentages based on 500 poln Samples D-45a and D-333 conta Tr 8 trace amounts. Franciscan Metagraywacke * j a * v* «o m c* <£ o- c- CO 00 00 P ft ft ft Q G D-91 D-92 a (O © i ft o 0 H 1 ft D-114 & 0 CM H 1 ft & w H CM 1 ft & 10 CM CM 1 ft D-266 0 10 1 f t to (0 to 1 ft 15 17 22 22 31 1 25 19 35 21 20 24 15 21 33 30 15 1 0 2 0 1 0 0 tr tr 1 tr 1 0 tr 0 1 2 4 0 3 3 5 0 1 1 2 2 3 2 1 4 1 5 3 1 tr 0 1 0 0 0 0 1 1 0 0 0 tr tr 1 tr tr 0 1 2 1 0 0 1 1 tr 1 tr 0 1 3 2 tr 44 26 27 34 20 37 30 39 35 21 38 28 27 35 30 39 28 0 0 0 0 0 0 0 0 tr 0 tr 0 0 0 0 6 0 14 10 8 25 22 18 19 13 14 20 1 3 14 14 24 8 11 1 1 0 tr 2 tr 1 1 0 1 tr 0 tr 2 tr tr 1 2 3 2 3 4 7 3 4 1 2 2 2 2 1 tr 1 6 0 0 0 0 0 - 13 0 0 0 0 0 0 0 0 0 0 0 1 tr 0 tr 0 7 tr 0 tr 1 0 1 1 tr 0 0 0 8 35 26 5 7 13 14 20 3 13 33 30 6 9 1 5 9 1 0 4 1 1 0 0 tr 6 9 0 tr 29 8 3 tr 18 6 5 3 3 5 3 6 tr 1 7 1 8 2 4 5 1 4 2 0 1 1 0 1 tr 1 0 1 0 0 2 0 0 0 0 tr 3 1 0 1 0 1 1 1 0 1 1 1 1 0 1 3 ebb TBB IBB ibb IBB TUU IBB IBB IBB IBB IBB IBB IBB IBB IBB IBB IBB of 28 Franciscan metagraywackes. 500 point counts per thin section. 333 contain aragonite in veins. Table 1 28 approximated from close examination of metagraywackes, particularly those of textural zone 1. Detrital quartz content of metagraywacke averages 31 per cent with a range of 19-44 per cent, and feldspar averages 21 per cent with a range of 1-37 per cent (Table 1). Assuming that the quartz remained stable and that most of the pumpellyite (6 per cent), lawsonite (1 per cent), and at least some of the white mica (13 per cent) were derived from the breakdown of detrital feldspar, the initial quartz-feldspar ratio probably approached a value of 1,0. The quartz, generally clear, shows undulating strain extinction. Plagioclase grains are anhedral and invariably scarred by recrystallization products, which exhibit all stages of development from minute isolated pods to dense Interlocking growths of such minerals as white mica, epidote, and pumpellyite. In twinned grains composition planes are often distorted. The average plagioclase compositions of 3 metagraywackes examined with a universal stage are An7> An5i and An6. Following procedures outlined by Slemmons (1962), 10 plagioclase grains were measured in each sample. The greatest range of composition within a sample is An2 to Anil. The al- bite composition of the feldspar is probably due to the breakdown of the anorthite molecule during regional metamorphism. Rock fragments range in abundance from 1 to 9 per 29 cent and average 4 per cent (Table 1), more than half of which is clear metachert. In order of decreasing abundance, other rock fragments ai^e volcanic rock, quartz- mica schist, and quartzite. On a ternary diagram showing proportions of stable grains, rock fragments, and feldspar in 28 metagraywackes (Pig. 4), the cluster of data points indicates an initial composition of arkosic wacke (Williams, Turner, and Gilbert, 1954, p. 292). Is used here, the term "matrix'* signifies a tough, dull green or dark brown interstitial paste consisting of comminuted clasts ( (O.Olmm) and microcrystalline aggre gates of chlorite, white mica, and pumpellyite, which collectively make up more than 30 per cent of the meta- graywacke (Plate 10). Calcite and recrystallized quartz are virtually absent from the matrix. The distinction between clasts and matrix is often arbitrary, since any grain too small to identify is relegated to the matrix. Moreover, the tendency of grain boundaries to blend Into the surrounding paste results in fuzzy ghost-like outlines which make differentiation of clast from matrix difficult. Chemical and/or mechanical disintegration of unstable clasts during burial (Cummins, 1962, p. 66) followed by formation of new minerals during regional metamorphism have obscured both relative abundance and composition of the original interstitial material. Minerals of the chlorite group, derived principally 30 Quartz, chert,quartzite 'Q UA RTZ \ W A C K E , FE LD- SPATHIC WACKE ARKOSIC WACKE LITHIC WACKE Feldspar Unstable fine-grained lithic fragments Plots of 9 Knoxville graywackes Approximate area encompassing Knoxville graywackes * Plots of 2 8 Franciscan m etagray wackes □ Approximate area encompassing Franciscan metagraywackes Figure A. Ternary diagram showing clastic ratios in 28 Franciscan metagraywackes and 9 Knoxville graywackes from the Stanley Mountain area. Plot at, apex repre sents a .-jadeite-bearing metagraywacke „ 31 ■ i 0.2 mm Plate 10, Photomicrograph of Franciscan meta- graywacke showing corroded albite grains irregularly e.mbayed in a matrix of white mica (stringy material, lower right) and chlorite (dark mottled area, upper left and right). Nicols parallel. Sample D - 85 collected from hillside 5500 feet due north of confluence of Ouyama River and Aliso Creek. from recrystallized matrix, range in abundance from 1-38 per cent with an average of 15 per cent. Chlorite most commonly appears as either minute ( <0.01mm) cloudy patches colored pale green or as nearly colorless stringers interleaved with white mica (Plate 10). Pleo- chroism is weak. In some metagraywackes the patches have coalesced into optically discontinuous mats, while in others the chlorite is thoroughly disseminated. A few relatively coarse (0.05-0.1 mm) discreet grains of chlorite showing distinct cleavage are usually present in each thin section. A conspicuous property of the chlorite is anomalous blue interference color, so pronounced that some grains appear isotropic. Most of the mineral is optically negative. According to Albee (1962, p. 863), chlorite exhibits abnormal blue or violet interference colors when the index of refraction is slightly higher than P = 1.631, at which the optic sign changes from positive ( (3 < 1,631) to negative ( (I ) 1.631)* In lieu of more esoteric terminology, Albee (1962, p. 868) suggests that the term "Pe-Mg chlorite" be applied to this variety, which is relatively rich in ferrous iron. White mica is ubiquitous in metagraywackes of the Stanley Mountain area. It not only accounts for about half the matrix, but also replaces feldspar on a limited scale. Ranging from 5-35 pan cent, the average white mica content of metagraywacke is 13 per cent. In rocks of textural zone 1, white mica assumes the form of tiny ( K 0.01 mm) anhedral platelets dispersed among the other matrix constituents. White mica content increases with advanced textural reconstitution of the metagraywacke, such that in textural zone 2, crude networks of micaceous layers 0.05-0.10 mm in thickness completely encircle detrital grains (Plate 8). Cleavage planes in the inter stitial white mica are invariably distorted. White mica replacing feldspar occurs in isolated patches or single flakes aligned parallel to cleavage surfaces of the host. Rarely are these flakes thick enough to show interference colors higher than first order yellow. Wo attempt was made to determine the composition of white micas through chemical analyses or X-ray techniques. Elsewhere in the Coast Ranges of California, rocks with mineral assemblages similar to those in metagraywacke of the Stanley Mountain area contain phengitic mica, representing solid solution between muscovite and celadonite (Ernst, 1963b, p. 1358). Pumpellyite most commonly appears as lemon green aggregates of radiating needles 0.005-0.10 mm in length, which grow inward from the margins to the centers of plagioclase grains (Plate 11). The average pumpellyite content of metagraywacke is 6 per cent, with a range of 0-29 per cent. When the mineral is abundant, it forms dense interstitial mats resembling chlorite, but close inspection reveals the presence of interlocking needles. t ------------1 0.1 mm Plate 11. Photomicrograph of radiating sheaves and needles of pumpellyite in Franciscan metagraywacke. Pumpellyite appears to be nucleated in matrix and en croaches on albite (?) grains at upper right. Nicols parallel. Sample D - 214b collected near nose of serpentinite mass about 8500 feet S60W of Shell Peak. The interstitial pumpellyite may be either a replacement of finely pulverized plagioclase or a reaction product of pre-existing argillaceous material. In addition to its characteristic acicular habit, another distinctive feature of the pumpellyite is pale yellow or anomalous brown interference color. Pleochroism from pale green to pale yellow green is barely perceptible, suggesting a low iron content of about 3 per cent weight Pe20^ (Coombs, 1933, p. 130). Although lawsonite occurs in more than half the metagraywackes studied in thin section, the average content is only 1 per cent with a maximum of 8 per cent. Most of the lawsonite is in the form of colorless stumpy to elongate prisms (average length 0.1mm, maximum 0.2mm) of high relief, replacing feldspar (Plate 12). Less easily detected are thin pseudohexagonal platelets, 0.01- 0.02 mm across, occasionally showing a very faint greenish tint. Orientation of lawsonite tends to be controlled by cleavage directions of the host plagioclase grain. Prisms and platelets are scattered throughout the matrix, but only in trace amounts. In rare cases lawsonite occurs with carbonate in veinlets (Plate 13) or as clustered prisms within fissile argillaceous rock fragments. Diagnostic optical properties of prismatic lawsonite are high positive relief, mottled texture, first order yellow or pale orange interference colors, and parallel extinc- 36 I ------------------ 1 0 .1 m m Plate 12. Photomicrograph of lawsonite (ir regular prismatic grains of high relief) rooted in a substrate of albite (speckled flat white mineral) in Franciscan meta graywacke. Nicols parallel. Sample D - 36 collected from ravine about 8000 feet N30E of confluence of Cuyama River and Aliso Creek. ^ 0,1 mm Plate 13. Photomicrograph showing prisms of lawsonite associated with vein quartz (flat white) in Franciscan metagraywacke. Fuzzy gray cluster of needles at lower left is pumpellyite. Large irregular grain at lower center is calcite or aragonite. Nicols parallel. Sample D - 45a collected from east end of quarry on north hank of Aliso Creek 800 feet from Ouyama River (see Plate 5). 38 tion. The majority of platelets are so thin that they show no interference color. Stilpnomelane, a metamorphic brittle mica somewhat resembling biotite, is invariably oriented parallel to incipient folia of chlorite and white mica in metagray wackes of textural zone 2. The average stilpnomelane content is 1 per cent, with a maximum of 4 per cent. More than half the metagraywackes contain flakes of this mineral, commonly interlaminated with chlorite. Grains vary in length from 0.1-0.5 mm. Identification of stilpno melane was based upon the frayed or braded appearance of flakes exhibiting pale yellow to dark red-brown pleo- chroism and high birefringence. Terminations of detrital biotite would tend to be abrupt or slightly rounded. In the extinction position, stilpnomelane lacks the mottled appearance characteristic of biotite (Hutton, 1938, p. 182). Granular sphene, often concentrated in aggregates along shear planes, is present in all of the Franciscan metagraywackes in amounts averaging 3 per cent and ranging up to 8 per cent. In thin section the mineral often assumes the form of cloudy brown beads varying in diameter from 0.05-0.20 mm (Plate 14). Many of these grains have been partially or wholly altered to leucoxene, a dull white or yellow-white opaque substance. Localization of sphene in zones of grain reorientation and cataclasis ^ 0*1 mm Plate 14. Photomicrograph of granular sphene in highly sheared Franciscan metagraywacke of textural zone 2. Scaley light gray patches surrounding sphene consist of chlorite. Nicols parallel. Sample D - 120b collected from ledge in bed of Cuyama River about 5200 feet S45E of Shell Peak. 40 indicates that most of the mineral is of metamorphic origin. Intricate networks of carbonate and quartz veins are prominent in the majority of metagraywacke outcrops. In some cases it is difficult to collect a hand specimen containing less than 50 per cent vein material. Calcite, the dominant carbonate, is normally medium-grained (1-5 mm) and euhedral, but is occasionally fine-grained and granular when replacing aragonite as shown in Plate 15. Here, veinlets of microcrystalline calcite replace a relict aragonite grain. The presence of calcite and aragonite was verified by X-ray in 2 metagraywackes. Vein quartz is generally arranged in mosaics of randomly oriented clear grains varying in size from 0.25-2.0 mm. When both carbonate and quartz are present in a vein, the quartz tends to line the walls of the vein. Metaconglomerate. A lens of Franciscan meta conglomerate 70 feet thick and at least 200 feet long is well-exposed in a roadcut on the steep east bank of the Cuyama River. The locality is unique because sole markings indicate that interlayered graywacke and argillite above and below the metaconglomerate are part of a bedded sequence. Elsewhere, intense deformation has obliterated bedding. The metaconglomerate consists of angular and & I ------------------------10.5 m m Plate 15* Photomicrograph of fine-grained, granular calcite (dark gray) replacing large, euhedral, twinned grain of ara gonite in vein which transects Franciscan metagraywacke. Nicols crossed. Sample D - 333 collected from north side of Ravine about 2000 feet S15W of Stanley Mountain. 42 subangular clasts ranging in size from %-6 inches and averaging 1 inch. Surrounding these is a greenish gray medium-grained matrix identical to the massive metagray wacke into which the metaconglomerate abruptly grades (Plate 16). Among the more abundant varieties of clasts in the metaconglomerate are chert (35 per cent of the clasts), metagraywacke (30 per cent), argillite (15 per cent), and mafic volcanics (10 per cent). Other lithologies repre sented are quartzite, limestone, and quartz-muscovite schist. About half the chert and most of the metagray wacke clasts are similar to rocks widely distributed throughout the Franciscan terrane. Bailey and others (1964, p. 41) concluded that some Franciscan conglomerates are of intraformational origin, since glaucophane schist clasts may be present. A different idea was advanced by Fyfe and Zardini (1967, p. 820), who contended that mineral assemblages of component clasts are the result of in situ metamorphism and selective metasomatism of clasts and matrix at the same time. Ernst (oral commun., 1967) believes that in situ metamorphism is the more important process, and that the role of metasomatism in the production of Franciscan metaconglomerate has not been conclusively demonstrated. In metaconglomerate of the Stanley Mountain area, similarity of matrix to metagray wacke enclosing the metaconglomerate indicates that Plate 16. Angular clasts in Franciscan meta- conglomerate. Roadcut on east side of Calif. Rte. 166 about 8000 feet S15E of Shell Peak. 44 matrix, clasts, and graywacke were metamorphosed at the same time. The presence of metagraywacke clasts, there fore, need not he considered evidence that the meta conglomerate is of intraformational origin. Argillite. Normally crumpled and sheared, dark gray or grayish tan argillite occurs in thin discontinuous layers separating lenses of metagraywacke (Plate 17). Argillite is tougher than shale, but lacks the consistently parallel cleavage surfaces characteristic of slate. Though widely distributed throughout the Franciscan terrane, argillite accounts for less than 5 per cent of the rocks of the Franciscan group. Where best exposed in stream beds, argillite layers vary in thickness from a fraction of an inch to 3 feet, but average only 4 inches. Discontinuous layers of argillite are frequently inter spersed with a chaotic Jumble of tectonic inclusions. The argillite fathers to a loose, gray, clayey soil often involved in landslides. Outcrops in which argillite is exposed form no consistent pattern in the Stanley Mountain area. In many instances argillite fills fractures which bound or penetrate graywacke blocks, indicating that dur ing deformation the argillite was highly mobile. Detached graywacke lenses often appear to "float1 1 in a matrix of Plate 17. Tectonic inclusion of metagraywacke (left of pencil) enclosed in sheared argil lite. Pencil tip rests upon irregular pod of greenstone. Outcrop on north bank of Aliso Creek about 3000 feet southeast of Cuyama River. 46 crushed argillite (Plate 17). Even where only mildly deformed, argillite breaks readily into chips less than an inch across. These have dull waxy or greasy surfaces on which individual grains are invisible to the naked eye. In a few outcrops the argillite is intercalated with thin (|-1 inch) lenses of drab brown siltstone. Greenstone. As used here, the term "Franciscan greenstone" refers to discontinuous lenses of highly deformed aphanitic metabasalt and fine-grained meta diabase which vary in length from a few inches to 200 feet (Plate 18). Fresh exposures of these rocks are dark green or grayish green and weathered outcrops are dark brownish green or reddish brown. Since the greenstone is highly resistant, it often occurs as prominent knobs dotting the hillsides or as ramparts forming waterfalls in the larger streams* Franciscan greenstone lacks a consistent pattern of distribution in the Stanley Mountain area. A few lenses of greenstone show vague pillow-like structures, but in most outcrops primary structures, if any, have been destroyed by shear. Veins of carbonate and quartz form intricate networks transecting the rock. In thin section relict igneous textures are some times quite distinct, but they are more often partially obscured or entirely effaced by physical reconstitution Plate 18. Sheared contact between Franciscan greenstone (upper half) and metagraywacke (lower half). Ledge in Aliso Creek about 3000 feet southeast of Cuyama River. and growth, of new minerals during metamorphism. Modes of three metabasalts and two metadiabases are listed in Table 2. The metabasalt in which texture is best preserved {sample D-18). appears to have been a vesicular augite basalt porphyry. Euhedral phenocrysts of plagioclase ranging in size from 0.5-3*0 mm are surrounded by an intergranular groundmass of corroded feldspar laths {average length: 0.05mm), augite, and pumpellyite (Plate 19)* Scattered throughout are amygdules of carbonate and chlorite. Relict textures of the remaining two meta basalts studied under the microscope are clouded by the growth of secondary quartz, chlorite, and pumpellyite. Both samples contain altered plagioclase phenocrysts surrounded by a groundmass of interlocking corroded feld spars, but one (sample 0-316) lacks interstitial augite, suggesting that the augite, or perhaps interstitial mafic glass, has been completely replaced by chlorite and pumpellyite (Bailey and others, 1964, p. 50). A greenstone displaying relict diabasic texture (sample D-177a ill Table 2) constituted a lens only 12 inches long oriented parallel to foliation of enclosing metagraywacke and argillite. As seen in thin section, the metadiabase consists of heavily altered plagioclase grains of albitic composition ranging in length from 0.5 mm to 1.5 mm. These are partly-surrounded by a mesostasis of chlorite and pleochroic yellow to blue-green pumpellyite 4-9 Mineral Franciscan Metabasalt Franciscan Metadiabase D-1 8 D-89b D-316 D-I20a D-l77a Albite 33* 19 30 9 23 Augite 15 8 0 25 0 Calcite 3* 0* 3 0 > Chlorite l*f 18 13 25 10 Epldote tr 0 0 0 tr Glauc ophane 0 0 0 tr 0 Lawsonite 0 0 0 8 0 White mica 0 0 0 11 1 Pumpellyite 30 28 28 12 38 Pyrlte 0 tr 0 1 0 Quartz 5 12 26 2 8 Sphene-leuc oxen© tr 15 tr 6 16 Unknown tr tr tr 1 tr Total 100 Too 100 100 100 Table 2. Modes of Franciscan netabasalt and metadiabase. Samples D-18 and D-89b contain aragonite In veins. Calcite and quartz are secondary. Percentages based upon 500 point counts per thin section. I -------------------1 0.5 nun Plate 19. Photomicrograph of porphyrltic metabasalt from Franciscan group, showing corroded albite phenocryst in inter- granular groundmass of albite, augite, and pumpellyite. Nlcols crossed. Sample D - 18 collected from knob about 3800 feet S50W of Stanley Mountain. 51 needles (Plate 20). The original texture of the meta diabase could have been either subophitic or intersertal, depending upon whether the mesostasis represents altera tion of pyroxene or mafic glass. The other metadiabase (sample D-120a) examined under the microscope exhibits relict ophitic texture. Hanging in size from 0.5-5 mm, fractured augite grains, partially replaced by chlorite and pumpellyite, surround plagioclase grains. The latter are in some instances almost completely altered to a mixture of white mica, pumpellyite, and lawsonite (Plate 21). Bedded metachert. Varicolored rhythmically bedded metacherts are exposed in blocky outcrops distributed randomly throughout the Pranciscan terrane of the Stanley Mountain area. Color commonly grades through shades of pale yellow-orange and red. Less common are green and gray metacherts. Anastomosing veinlets of carbonate and quartz frequently lace these rocks. Alternating with paper-thin argillite, beds of metachert range in thickness from 1 to 3 inches (Plate 22). Among conspicuous bedding irregularities are pinch-and-swell structures, discon tinuous layers, and wrinkled lenses. One or more of these features can be observed in nearly every outcrop. Bedding surfaces are quite rough. Within a sequence of bedded ■j 0 . 5 m m Plate 20. Photomicrograph of albite (large white grains) surrounded by a mesh of inter grown pumpellyite and chlorite (dark material) in Franciscan metadiabase. Fine pumpellyite needles are also enclosed in albite. Nicols parallel. Sample P - 177a from ledge in creek about 6300 feet N 35W of intersection of Calif. Rte. 166 and Tepus- quet Road. ^ 0,1 mm Plate 21* Photomicrograph of Franciscan meta- diahase showing suhparallel lawsonite prisms (high relief) formed from breakdown of feld spar, which is the large rectangular gray patch surrounding prisms at center of print. Sample D - 120a from east bank of Ouyama River about 5200 feet S45E of Shell Peak. Plate 22. Franciscan bedded meta chert, showing discontinuous nature of bedding and pinch-and- swell structure. Argillite is interlayed with metachert. Low cliff on northeast bank of Aliso Creek about 3200 feet southeast of Cuyama River. 55 metachert some layers may be tightly folded, but the deformation may not involve layers a few feet above or below. Bailey and others (1964, p. 63) suggested that such localized folds develop during submarine slumping prior to final hardening of the chert. Franciscan metacherts and associated argillites form discreet lenticular bodies normally less than 15 feet thick. Scattered outcrops of chert are difficult to correlate laterally across intervals of more than 100 or 200 feet. The stratigraphic relationship of metachert- argillite sequences to surrounding lithology is unclear in the Stanley Mountain area due to the highly deformed state of the rocks. The association of chert with volcanics, characteristic of other Franciscan terranes in California (Bailey and others, 1964, p. 35) is not common. The contacts between lenses of bedded metachert and the enclos ing metagraywacke normally parallel the metachert beds, but in some instances these contacts truncate beds at a high angle (Plate 23). Thin sections of Franciscan chert show round or elliptical outlines of silica-filled $.adiolarlan tests surrounded by a colorless matrix of microcrystalline chalcedony and quartz speckled with tiny grains of white mica, sphene, pumpellyite, chlorite, and hematite. X-ray analysis confirmed the presence of aragonite veins in one sample. Silica within the Eadiolaria, accounting for as Plate 23. Contact between metagraywacke (upper half) and bedded metachert- argilllte sequence. Contact truncates metachert at left and parallels chert at right, ledge in Aliso Creek about 2500 feet southeast of Cuyama River. 57 much as 20 per cent of the metachert, is coarser grained than silica in the matrix. White mica may constitute more than 15 per cent of the metachert, though individual platelets of mica are only a few microns thick. In some samples, mica parallels planes defined hy flattened Radiolaria, implying that the tests were deformed during metamorphism of the chert (Plate 24). Metamorphism of "in situ” Franciscan rocks. Fran ciscan rocks of the Stanley Mountain area may be sub divided into two broad categories: (1) in situ meta- morphic rocks, including metagraywacke, metaconglomerate, argillite, greenstone, and metachert; and (2) "exotic" metamorphic rocks of deep-seated origin, including amphibolite and coarse-grained glaucophane schist, which exhibit structures, textures, and mineralogy clearly out of place with respect to the surrounding in situ meta morphic rocks. The following discussion is concerned with in situ metamorphism; the origin of "exotic" metamorphic rocks will be considered in separate sections under the headings "amphibolite" and "glaucophane schist." Metagraywacke of the Stanley Mountain area is characterized by the metamorphic assemblage: quartz- albite-chlorite-white mica-pumpellyite-lawsonite- stilpnomelane-sphene £ aragonite. Among these minerals, 1 ------------------------------1 0 . 5 nrtn Plate 24. Photomicrograph of deformed silica- filled Radiolarian tests in Franciscan meta chert. Ricols parallel. Sample P - 90 collected from ledge in Aliso Creek about 2400 feet southeast of Cuyama River. 59 lawsonite and/or aragonite are considered diagnostic of the "blueschist facies,” which is indicative of meta- morphism at low temperatures and at relatively high pressures (Ernst, 1963a, p. 25-26; Bailey and others, 1964, p. 91). Figure 5 shows the estimated pressure- temperature field of the blueschist facies. Based on experimental evidence, Newton and Kennedy (1963, p. 2976) concluded that lawsonite may crystallize within the temperature range 200°-450° C at pressures of 3-8 kilobars. These observations were corroborated by Crawford and Fyfe (1965, p. 263) and Ghent (1965, p. 396- 397)« In a discussion of the kinetics of the aragonite- calcite transformation, Brown, Fyfe, and Turner (1962, p. 581) pointed out that aragonite, lawsonite, and jadeite + quartz would be stable at 200°-300° C under a pressure of 6-9 kilobars, based on phase equilibrium studies by several investigators. Assuming a geothermal gradient of 10° C/km, these conditions could exist at a depth of 20-30 km. Ernst (1965, p. 909) believed that blueschist metamorphism at Panoche Pass, California, involved temperatures of 200°-300° C at 7-8 kilobars (20-30 km). The paucity of ;]adeitic pyroxene in metagraywacke of the Stanley Mountain area would seem to indicate crystalliza tion near the lower pressure boundary of this range. As pointed out by a number of geologists (e.g. Ernst, 1963a, p. 4-5; Bailey and others, 1964, p. 106), 60 T e m p e ra tu re in degrees centigrade 0 100 200 300 400 300 Estimated P - T field for prehnite-pumpellyite facies (Knoxville Form ation) 2 4 6 8 Estimated P -T field for blueschist fa c ie s (Franciscan group) 10 12 Figure 5. Estimated pressure-temperature fields for blueschist facies, represented by Franciscan group, and prehnite-pumpellyite facies, represented by Knoxville Fm. Boundaries of prehnite-pumpellyite facies general- ized after Coombs and others (T959» P» 92), and Bailey and others (1964, p. 110); blueschist field after Cole man and Clark (1968, p. 54); geothermal gradient after Barth (196?, p. 332). rocks of the blueschist facies are chemically similar to those of the greenschist facies. Assuming that equilibrium was attained or closely approached, differing physical conditions must be responsible for the development of contrasting mineral assemblages of the two facies. High- density minerals such as lawsonite, jadeitic pyroxene, and aragonite indicate that the blueschist facies formed at higher pressures than the greenschist facies. Turner (1948, p. 99-100) suggested that pressure differences be tween the two facies are more crucial than temperature differences. Widespread occurrence— yet relatively low concen tration— of lawsonite in metagraywacke of the Stanley Mountain area and scarcity of jadeitic pyroxene suggest that pressure only slightly exceeded the stability range of the greenschist facies during metamorphism. The presence of abundant pumpellyite, not only in the Stanley Mountain area, but in other parts of the Nacimiento block as well (Blake, oral commun., 1967) supports this conclu sion. According to Ernst (1963a, p. 10), the stability field of pumpellyite "... could be reduced and possibly eliminated at higher pressures due to enlargement of the lawsonite PT stability field." The mineral assemblage of the metagraywacke in the Stanley Mountain.area, there fore, may represent physical conditions transitional between the chlorite zone of the greenschist facies and 62 the pumpellyite zone of the blueschist facies, as defined by Blake and others (1967, p. 3). Mineral assemblages of Franciscan greenstones in the Stanley Mountain area are representative of physical conditions intermediate between greenschist and blueschist facies metamorphism. Pumpellyite and chlorite are major constituents of each of the five greenstones examined in thin section (Table 2), averaging 27 per cent and 16 per cent respectively. Sphene is ubiquitous, but highly variable in abundance, and makes up an average of 7 per cent of the greenstones. Lawsonite prisms 0.1-0.2 mm in length account for 8 per cent of one metadiabase (Plate 21), in which thin (0.1 mm) stringers of glaucophane are also present. X-ray analysis of carbonate veins in two samples of metabasalt established the presence of aragonite. Epidote is present in only trace amounts in two of the samples. Of the foregoing minerals, lawsonite,- aragonite, and glaucophane are diagnostic of the blue schist facies. Metagraywackes of the blueschist facies have previously been subdivided into groups or subzones based upon mineralogy (Fig. 6). McKee (1962, p. 604-605), for example, recognized in the Pacheco Pass area southeast of San Francisco a progressive metamorphic sequence in rocks which he termed "subcrystalline metasedimentary rocks." The earliest reactions involved recrystallization of the Modified after Blake et a! (1967) Modified after Ernst (1965) Compiled from McKee (1962) Quartz Aibite Chlorite White mica Pumpellyite Lawsonite Calcite Aragonite BLUESCHIST FACIES Pumpellyite zone Lawsonite zone Summary of mineral parageneses of metagraywacke along eastern margin of northern Coast Ranges Estimated stability field of metagraywacke from Stanley Mountain area Aibite Jadeitic px. Glaucophane Lawsonite Quartz White mica Chlorite Stilpnomelane Sphene Calcite Aragonite Hematite GREEN SCHIST BLUESCHIST Paragenesis of minerals in m eta- graywackes of Panoche Pass, Calif. -------------------- major phases --------------------- minor constituents ---------------------accessory constituents co s c o o EC d UJ CO £ UJ UJ < t- c n >- EC O CD 3 CO UJ < I — CO >- EC « _ > DIAGNOSTIC MINERALS aibite or albite-lawsonite- Aibite Jadeite + quartz a > CO CO c CO (jadeite isograde) jadeite-lawsonite- glaucophane variable: mostly jadeite, lawsonite, glaucophane, garnet, epidote Subdivision of blueschist facies rocks, Pacheco Pass California Figure 6. Estimated position of Stanley Mountain metagraywacke within blueschist zones and mineral stability ranges established by earlier workers. CT\ ui matrix to microcrystalline muscovite, clay, quartz, aibite, chlorite, and pumpellyite. Lawsonite was the next mineral to form, growing in the matrix and within detrital aibite grains. Jadeite next crystallized entirely within aibite grains, followed by growth of subhedral glaucophane grains in the matrix. McKee (1962, p. 600) mapped a consistent pattern of "subcrystalline" metamorphic rocks suggestive of a regional variation in the PT conditions of meta morphism. Aibite or albite-lawsonite is the stable assemblage in the western half of the Pacheco Pass area, and jadeite-lawsonite-glaucophane is the stable assemblage in the eastern half. The boundary between the two as semblages was referred to as the "jadeite isograde." In terms of McKee's subdivision, metagraywackes of the Stanley Mountain area recrystallized on the aibite or albite-lawsonite side of the jadeite isograde (Pig. 6). Ernst (1965j p. 891, 893) divided Franciscan clastic metasedimentary rocks of the Panoche Pass area (south of Pacheco Pass) into two groups; aibite metagray wacke, and jadeitic pyroxene metagraywacke. Whereas clastic sodic plagioclase and lawsonite co-exist in aibite metagraywacke, the plagioclase is restricted to later veins in the thoroughly recrystallized lawsonite- bearing jadeitic pyroxene metagraywacke. According to Ernst (1965, p. 904), jadeitic pyroxene metagraywacke probably represents sediments buried more deeply than 6 5 aibite metagraywacke. In the Stanley Mountain area, wide spread incipient growth of lawsonite in detrital plagio clase and the virtual absence of jadeitic pyroxene (Table 1) are definitive features of aibite metagraywacke (Pig. 6). An important difference, however, is the fact that pumpellyite is abundant in metagraywacke of the Stanley Mountain area, but is absent from metagraywackes at Panoche Pass. Blake and others (1967* p. 3) distinguished two grades of blueschist metamorphism based on the pre dominance of pumpellyite or lawsonite in Franciscan meta graywackes exposed along the eastern margin of the Coast Ranges in northern California. The Influence of bulk rock composition on these zones apparently has not been evaluated. In the pumpellyite zone, felted aggregates of pumpellyite are scattered throughout a matrix of chlorite, muscovite, aibite, and minute hexagonal plates of lawsonite (Blake, 1965). Pumpellyite also replaces relict plagioclase grains. On a regional map, boundaries of the pumpellyite zone coincide roughly • Ith those of textural zone 1 (Blake and others, 1967f P- 2)> which in turn corresponds texturally, but not mineralogically, to chlorite subzone 1 defined by Turner (1938, p. 168). With increasing degree of recrystallization, the disappearance of pumpellyite and increase of lawsonite mark the transition from the pumpellyite zone to the lawsonite zone. 66 Based on the zonal classification by Blake and others (1967), most of the metagraywackes in the Stanley Mountain area fall within the pumpellyite zone of the blueschist facies (Fig. 6). A few of the more thoroughly reconstituted metagraywackes contain relatively high concentrations of lawsonite and little pumpellyite. These rocks represent metamorphic conditions transitional into the lawsonite zone. The Franciscan greenstones are texturally similar to rocks which Coleman and Lee (1963* P» 270) classified as type II metabasalts from the Cazadero area, California. Microscopic textures are volcanic, but most of the minerals } are metamorphic.' Mineralogically, Franciscan green- stones in the Stanley Mountain area represent a lower grade of blueschist metamorphism than the type II meta basalts, characterized by abundant glaucophane, some jadeitic pyroxene, and virtual absence of relict plagioclase. Ghent (1964, p. 102), who worked in the Black Butte area of northern California, described sheared lens-shaped boudins of metavolcanic rock enclosed in graywacke and argillite. Thin sections showed frac tured aibite grains, partially replaced by lawsonite, set in a network of chlorite, secondary quartz, sphene, iron ore, and aragonite. Both mode of occurrence and texture are similar to greenstone In the Stanley Mountain area. Also possibly analogous to this greenstone are mafic meta- volcanics of the Yolla Bolly area, northern California, which Blake (1965, p. 51) assigned to a textural subzone termed lawsonite-1. These metavolcanics lack foliation and contain augite and plagioclase incipiently replaced by pumpellyite, lawsonite, and scarce glaucophane. Metacherts in the Stanley Mountain area represent a lower grade of blueschist facies metamorphism than, for example, rocks classified as type III-B metachert by Coleman and Lee (1963, p. 277). Metacherts of the Stanley Mountain area appear to lack blue amphibole and garnet, often present in type III-B metachert. Whereas quartz grains in metachert of the Stanley Mountain area are nearly always smaller than 0.01mm, quartz grains in type III-B metachert range in size from 0.1mm to 0.4mm. According to James (1955> P- 1474), grain size in meta chert increases with increasing grade of metamorphism. Finally, quartz crystals in metachert of the Stanley Mountain area show no obvious preferred orientation. The foregoing discussion of metamorphism in the Stanley Mountain area has dealt with in situ rocks, which make up the bulk of the Franciscan terrane. Blocks of amphibolite and glaucophane schist associated spatially with in situ terrane are regarded as "exotic," i.e., as having formed under metamorphic conditions different from those recorded in the surrounding rocks. These two "exotic" rock types are described in the following 68 sections. Amphibolite. A northeast-southwest trending lenticular body of hornblende-rich epidote amphibolite, measuring 2500 feet in length and averaging 400 feet in width, crops out near the geographic center of the Franciscan terrane in the Stanley Mountain area. Sheared metagraywacke and greenstone border most of the amphibolite; a linear mass of serpentinite truncates the lens on the southwest. Wear the serpentinite contact, a highly resistant fine-grained amphibolite consists of alternating layers of aibite and layers of hornblende-epidote- chlorite-sphene. The remainder of the amphibolite is medium-grained, unlayered, and deeply weathered. A few boulder-like outcrops of the medium-grained variety of amphibolite occur along sheared margins of serpentinite masses elsewhere in the Stanley Mountain area. Modes of three amphibolites are listed in Table 3. The fine-grained amphibolite exposed near the southwestern end of the large lens exhibits a smooth weathered surface banded by dull white irregular aibite layers averaging 2 mm in thickness and dark green hornblende-rich layers averaging 5 mm in thickness. A thin section cut from a hornblende-rich layer (D-68a, Table 3) shows nematoblastic texture. Subparallel xeno- 69 Mineral Amphibolite Glaucophane Schist D-4-6 D-68a D- 1536 D-I65 D-175 D-177b Aibite 17* 1 2 0 0 0 Calcite 0 0 0 tr 0 0 Chlorite 7 k 6 0 3 0 Epidote 2 15 4 0 tr 0 Glaucophane 0 0 10 23 9 2k Hornblende ^5 43 k5 0 0 0 Iron stain 1 0 2 0 tr tr Lawsonite 2 0 0 0 tr k White mica 1 5 tr 15 23 22 Pumpellyite 19 0 12 0 0 0 Pyrite tr tr tr 0 0 1 Quartz 5 8 8 58 6^ k6 Sphene 1 22 9 2 1 2 St ilpnome1ane 0 0 0 1 0 0 Unknown 0 2 2 1 0 1 Total Too 100 100 100 100 100 Table 3* Modes of amphibolite and glaucophane schist from the Franciscan group. Percentages based upon 500 point counts per thin section. 70 "biastic hornblende grains 0.5-1*0 mm in length (43 per cent of sample) are intergrown with granular and idio- blastic sphene (22 per cent), xenoblastic epidote (15 per cent), white mica (5 per cent), chlorite (4 per cent), and interstitial sutured quartz (8 per cent). The horn blende is pleochroic as follows: X, pale yellow; Y, pale yellow; and Z, olive green. Small amounts of chlorite have replaced hornblende along cleavage planes. Though much of the sphene assumes the form of tiny beads (average diameter: 0.02 mm) enveloped in hornblende, a few excel lent rhombic sections measuring up to 0.5 mm in length are present. Epidote grains tend to be equidimensional with diameters of 0.5-1.0 mm. Platelets of white mica cluster in irregular patches and stringers. ■ A medium-grained, unlayered amphibolite (P-46, Table 3) weathering mottled green-gray was collected from a highly sheared, boulder-like outcrop on the margins of a serpentinite body near the mouth of Aliso Creek. The texture of the amphibolite is granoblastic. Component grains of xenoblastic hornblende (45 per cent of sample) and aibite (17 per cent) vary in size from 1 to 3 mm. Chlorite (7 per cent) replaces hornblende along grain boundaries and in irregular patches symmetrical to cleav age planes. Slender prisms and needles of pumpellyite (19 per cent) up to 0.3 mm in length thoroughly riddle the feldspar. Stubby prisms of lawsonite (2 per cent) less 71 than 0.1 mm in length, have replaced feldspar grains along cleavage planes. Other constituents are sutured q.uartz (5 per cent), epidote (2 per cent), white mica (1 per cent), and sphene (1 per cent). An amphibolite containing blue amphibole (D-153b, Table 3) crops out near serpentinite in the north-central part of the Franciscan terrane. The blue amphibole, which replaces hornblende, occurs as slender prisms in sinuated along hornblende cleavages and as irregular patches within the hornblende (Plate 25). The hornblende-rich epidote amphibolites of the Stanley Mountain area correspond to type IV metamorphic rocks of Coleman and Lee (1963» p* 266). Monoliths of type IV rocks have sheared contacts and exhibit structures, textures, and mineralogy clearly out of place with respect to surrounding rocks. Lee and others (1966, p. 150) sug gested that retrograde mineral assemblages found in these isolated blocks may have formed during upward tectonic transport along major fault zones. Essene, Fyfe, and Turner (1965> P» 699) pointed out that amphibolites of deep-seated origin can be converted to mineral assemblages typical of the blueschist facies, perhaps at higher and cooler levels of the crust. The blue amphibole-bearing amphibolite in the Stanley Mountain area appears to have recrystallized under conditions of blueschist facies meta morphism. In a discussion of retrograde metamorphism of 1 0.1 mm Plate 25. Photomicrograph of Franciscan amphibolite. Light gray wedge-shaped grain at left center is hornblende partially altered to glaucophane, which appears as dark ragged edges. Light gray mineral at right is also hornblende partially altered to glaucophane. White mineral is quartz. Nicols parallel. Sample I) - 153b collected from knob at edge of small serpentinite mass about 56OO feet S45W of Shell Peak. 73 pyroxene-hornblende rocks from the Healdsburg area, Borg (1956, p. 1576) described the replacement of hornblende by glaucophane. According to Ernst (1965* p. 890), crossite replaces hornblende in type IV epidote amphibolites at Panoche Pass, California. Glaucophane schist. Highly resistant knobs and lens-shaped boulders of glaucophane schist, ranging in length from 3 to 50 feet, are exposed in widely scattered localities within the Franciscan terrane of the Stanley Mountain area. Most of the glaucophane schist crops out near bodies of serpentinite or silica-carbonate rock. Slickensides furrow the peripheries of many of the lens shaped boulders, indicating that the glaucophane schists were tectonically emplaced into the more weakly meta morphosed surrounding rocks. Plate 26 shows a lens of banded quartz-rich glaucophane schist enveloped by boudins of sheared metagraywacke intercalated with crushed argillite. Another variety of glaucophane-bearing rock displays a fine-grained to aphanitic texture and weathers dark bluish gray. Modes of three samples of banded quartz-white mica- glaucophane schist are listed in Table 3. In general, 2-10 mm layers of Interlocking quartz (ave. grain diameter: 0.2mm) and sparse white mica alternate with 1-5 mm layers Plate 26. Lens of quartz-white mica- glaucophane schist enclosed in sheared Franciscan metagraywacke. Outcrop in creek about 6300 feet U35W of intersection of Calif. Rte. 166 and Tepusquet Road. 75 of idioblastic glaucophane prisms, ranging in length from 0.5-2.0 mm, enmeshed in abundant white mica (Plate 27)o Minor amounts of lawsonite and granular sphene are inter spersed throughout the glaucophane-rich layers, many of which show crenulations with amplitudes of 5-15 mm. Re crystallization of the rock apparently preceded folding, since no minerals transect the crenulations. Average contents of three principal minerals are: quartz, 56 per cent; white mica, 20 per cent; and glaucophane, 19 per cent. The abundance of quartz suggests that the banded glaucophane schist was derived from the metamorphism of argillaceous chert. Texturally, the schist resembles type III-B metachert described by Coleman and Lee (1963, p. 276-277) from the Oazadero area north of San Francisco. Banded glaucophane schist in the Stanley Mountain area does not exhibit gradational contacts with weakly meta morphosed cherts; metamorphic mineral assemblages in the schist are not in equilibrium with assemblages in sur rounding rocks. Field relationships of the schist, therefore, suggest tectonic transport of schist from zones of thorough recrystallization. From a structural point of view the banded glaucophane schists thus resemble type IV metamorphic rocks of Coleman and Lee (1963* P» 281). Highly slickensided, dark bluish gray, fine grained to aphanitic glaucophane-bearing rocks occur in I----------------------------------- 1 0 , 5 nun Plate 27. Photomicrograph of glaucophane schist, showing prismatic and rhombic sections of glaucophane in quartz and white mica. Uicols parallel. Sample D - 165 collected from top of small hill about 7000 feet S50W of Shell Peak. 77 I bold, smoothly weathered, boulder-like outcrops0 A thin section shows discontinuous feathery patches of fibrous | i glaucophane and chlorite surrounded by a nearly opaque dark ! brown mesostasis of chlorite, microcrystalline quartz, and 0,05 mm prisms of lawsonite.. Sprinkled throughout the mesostasis are rounded blebs of sutured quartz, which possibly represent relict amygdules. The high glaucophane content, estimated at 35 per cent, and massive fine grained texture suggest that these rocks were derived from metamorphism of mafic volcanics. Field relationships Indicate that the glaucophane-bearing rocks were tectoni- cally emplaced into the surrounding Franciscan terrane. Serpentinized ultramafic rocks associated with the Franciscan group General statement. Three modes of occurrence of serpentinized ultramafic rocks in the Stanley Mountain area are: (l) belts within the Franciscan terrane, (2) isolated lenses of serpentinite localized along the con tact between Franciscan rocks and Great Yalley sequence, and (3) a mass of partially serpentinized pyroxenite in apparent low-angle fault contact with overlying Knoxville greenstone. 78 Serpentinite belts. Belts of discontinuous slickensided serpentinite lenses trend generally north westward through Franciscan terrane. The largest of ap proximately 50 mappahle lenses is 5000 feet long with a maximum width of 1000 feet. Relationships between serpentinite bodies and topography indicate that most lenses dip vertically or steeply to the northeast. Narrow, boulder-strewn ridges of serpentinite scar the flanks and crests of rolling hills within the Franciscan terrane. From a distance these outcrops exhibit a dis tinctive light bluish gray color. Since weathered serpentinite supports only scanty vegetation, lenses are particularly conspicuous in brush-covered areas (Plate 28). Lenses consist of massive, resistant, bluish green and dark green blocks, spheroids, and ellipsoids of serpentinite scattered throughout a highly slickensided, soft, leafy or scaley matrix of serpentinite. Intricate networks of chrysotile veinlets commonly transect the massive serpentinite. The margins of the massive boulder like inclusions commonly exhibit polished and/or slicken sided surfaces. The surrounding matrix, accounting for more than 99 per cent of the serpentinite, weathers to shards with shiny curved slickensided surfaces. With few exceptions slickensided surfaces dip steeply, but strike varies to such an extent that conclusions about the orientation of poorly exposed serpentinite bodies cannot 79 Plate 28. Steeply dipping lens of serpentinite (middle dis tance) exposed in brush covered Franciscan terrain. View is toward southeast. Intersection of serpentinite mass and ravine is about 10,000 feet S70W of Shell Peak. 80 be based solely on these structural attitudes. The massive serpentinite may contain "bastite,** a term applied to pseudomorphs of antigorite after pyroxene. In the Stanley Mountain area, ultramafic rocks have been so thoroughly serpentinized that bastltes do not weather differentially on exposed surfaces. Instead, bastites appear as polygonal patches slightly lighter than the surrounding serpentine. A thin section (Plate 29) shows highly fractured anhedral bastites, laced with veinlets of chrysotile and bowlingite, enmeshed in fibro-lamellar antigorite sprinkled with magnetite. The pseudomorphs vary in diameter from 5 to 10 mm and make up 14 per cent of the rock. Shearing of matrix serpentinite has completely destroyed all textures of pre-existing ultramafic rock. Antigorite forms the bulk of the matrix; chrysotile is rare. Bowlingite, an iron-bearing alteration product of magnesium silicate, occasionally imparts a brown color to the rock. A thin section shows lenses of antigorite and bowlingite, 1-4 mm in length, bounded by closely spaced (0.5-2.0 mm) curved shear planes which appear braided in profile (Plate 50). Bailey and others (1964, p. 85-84) cite four criteria for determining the time of emplacement of serpentinized ultramafic rocks relative to development of structures In surrounding rocks. These criteria are: ^ 1 . 0 nun Plate 29. Photomicrograph of bastite. Large striated grain at center is pseudomorph of antigorite after pyroxene, surrounded by veinlets of chrysotile. Nicols crossed. Sample D - 31a collected from outcrop on east bank of Cuyama River about 3300 feet N45E of confluence of Cuyama River and Aliso Creek. f ------------1 1.0 mm Plate 30. Photomicrograph of sheared serpen tinite, showing closely spaced shear planes (black: stringers) enclosing antigorite and bowlingite. Nicols crossed. Sample D - 40a from west bank of Cuyama River about 1800 feet R20E of confluence of Cuyama River and Aliso Creek. (l) internal layering due to crystal settling, (2) symmetry of metamorphic aureole, if any, (3) shapes of serpentinite masses relative to structures in the enclos ing rocks, and (4) the coincidence of ultramafic masses with known faults„ Only the third criterion is applicable to serpentinite belts within the Franciscan terrane of the Stanley Mountain area, since internal layering is absent, serpentinite bodies lack metamorphic aureoles, and faults cannot be identified due to the absence of marker units and the overall sheared condition of the surrounding Franciscan rocks. As a rule, serpentinite lenses are oriented transverse to foliation in the enclosing meta- graywackes and associated rocks, which tend to be highly sheared and brecciated at serpentinite contacts. The traces of serpentinite belts are straight or slightly arcuate, whereas foliations in the host rocks lack con sistent structural grain. The foregoing relationships indicate that belts of serpentinite were tectonically emplaced into the Franciscan terrane subsequent to major deformation of the host rocks. In a discussion of the physical condition of serpentinite at time of emplacement, Bailey and others (1964, p. 87) concluded that serpentinites were intruded plastically as "cold intrusions," and cited as evidence the plastic behavior of this rock in landslides. Raleigh and Paterson (1965* P» 3982), who studied deformation of serpentinite in the laboratory, concluded that dehydra tion weakening of serpentine-hearing rocks accompanied by sliding on discreet fracture surfaces may occur at temperatures ranging from 300° C to 600° 0. These authors : excluded plastic deformation as a mechanism of serpentinite emplacement. Isolated serpentinite lenses. In a few widely scattered localities along the northeast margin of the Franciscan belt, lenses of slickensided serpentinite mark the contact between the Great Valley sequence and under lying Franciscan rocks. These lenses are concordant with foliation in the metagraywackes structurally beneath. The largest lens, viewed in profile at the contact between Knoxville Formation and Franciscan group, is at least 100 feet long and 20 feet thick, and dips 45° to the north east. Two lenticular serpentinite bodies, one structurally above the other, are exposed in profile in a roadcut on California Route 166. Both measure 15 feet in thickness, and are separated by 50 feet of sheared metagraywacke and gouge. The upper lens is at the contact between the Carrie Creek Formation and Franciscan group. The slicken sided surfaces within the lens and the foliation in the intervening metagraywacke dip 50° to the northeast (Plate Plate 31. Lens of serpentinite (light rock at left) oriented parallel to foliated meta graywacke (hammer) 50 feet below contact be tween Franciscan group and Carrie Creek Formation. Roadcut on east side of Calif. Rte. 166 about 8000 feet S75E of Shell Peak. 86 Pyroxenite. A pyroxenite body of unknown geometry is in apparent fault contact with Knoxville greenstone. Partially serpentinized, light green, coarse-grained clinopyroxenite envelops rare watermelon-sized inclusions of fresh, dark green, coarse-grained olivine clino pyroxenite. The pyroxenite crops out in the bottom and in. the lower walls of a narrow ravine; the greenstone is exposed in the upper parts of both walls. Thick brush and soil obscure the contact, but a small mass of slicken- sided serpentinite on one side of the ravine near the boundary between pyroxenite and greenstone suggests the presence of a low-angle fault. The minimum thickness of the pyroxenite body, measured from the bottom of the ravine, is 40 feet. The length of exposure along the ravine is about 200 feet. Grain sizes in the clinopyroxenite range from a few millimeters to more than an inch. Slightly curved crystal faces and cleavage surfaces indicate that the rock has been deformed. A thin section shows allotriomorphic- granular texture with fractured diopside (ave. diameter: 3 mm) constituting approximately 98 per cent of the rock. Exsolution lamellae are common, particularly where cleav age surfaces are distorted. Peathery apophyses of serpentine extend along cleavage surfaces from altered grain margins. Anhedral olivine, averaging 0„04mm in diameter, accounts for less than 1 per cent of the sample. 87 Blebs of goethlte are alteration products of diopside. Spheroidal and ellipsoidal inclusions of olivine clinopyroxenite range from 1 to 2 feet in diameter., The inclusions exhibit allotriomorphic-granular texture; maximum grain size is generally less than 5 mm. Estimated mineral percentages are: diopside, 90 per cent; enstatite, 5 per cent; and olivine, 5 per cent. The pyroxenes are generally unaltered, and are less deformed than the pyroxenes in the rock which encloses the inclusions. Anhedral olivine grains, 0.5 mm in diameter, are partially replaced by iddingslte. The inclusions of olivine clinopyroxenite may be relict layers disrupted during gravitational differentia tion of a mafic magma, or may be fragments of an earlier ultramafic rock intruded by clinopyroxenite. The absence of mesoscopic shear planes within the surrounding clino pyroxenite rules out tectonic emplacement of inclusions. Irvine (1965» p. 227) reported that blocks of olivine clinopyroxenite ranging in size from a few inches to several feet are included in a matrix of layered peri- dotite within the Duke Island ultramafic complex of southern Alaska. He considered the blocks to represent an older intrusion of ultramafic rock incorporated into a younger intrusion of layered peridotite. 88 Knoxville Formation General statement. The Knoxville Formation, the lowermost unit of the Great Valley sequence, consists of mildly deformed volcanic and sedimentary rocks of Late Jurassic age with a maximum thickness of about 3600 feet in the Stanley Mountain area*. These rocks are exposed in a broad belt on the northeast limb of the Stanley Mountain antiform. In this area, the Knoxville Formation is uncon- formably overlain by the Jollo Formation of Early Cretaceous age. Although the Knoxville Formation is well-exposed in large areas of the Coast Ranges of California, the nature of the contact between Knoxville and Franciscan rocks has been disputed. Bailey and others (1964, p. 145- 146) noted that one problem arises from lack of agreement about the lithologles that are diagnostic of the Knoxville Formation. Clastic rocks in the Knoxville are miogeo- synclinal in nature, yet in some places are interbedded with chert and volcanic rocks typical of an eugeosynclinal assemblage. In the Stanley Mountain area, Taliaferro (1943a, p. 197) believed that the intercalation of fossiliferous shale and sandstone with chert and volcanic rock indicated gradational relations between the Knoxville Formation and Franciscan group. Anderson (1945, p. 942) concluded that Knoxville rocks rest unconformably upon the Franciscan. He pointed out that because fossils found in the Stanley Mountain area appear elsewhere only in the upper one fourth of the Knoxville Formation, these rocks represent a westward transgression across Franciscan base ment. Bailey and others (1964, p. 122), in reference to the Stanley Mountain area, remarked that it would be "gross oversimplification" to include Upper Jurassic rocks in the Franciscan group simply because of the presence of volcanlcs and chert. In discussing the distribution of Mesozoic rocks in the Coast Ranges, Brown (1964, p. 9) and Bailey and others (1964, p. 163) suggested that the con tact between Knoxville rocks and Franciscan group may be tectonic. Page (1966, p. 272) agreed with this hypothesis and concluded that in all probability the base of the Jurassic part of the Great Valley sequence has never been seen in the Coast Ranges. Field work undertaken during the present study of the Stanley Mountain area yielded strong evidence for a tectonic contact between the Knoxville Formation and Franciscan group. Little difficulty was encountered in following the contact across which differences in petro graphy and structural style can be readily mapped. The contact relationships are described in detail in the section on structure. The Knoxville Formation in the Stanley Mountain area has been subdivided into four stratigraphic units, each characterized by a’dominant lithology mappable over 90 a wide area (Plate 3). The lowermost part of the formation :consists of dull gray shale interbedded with subordinate amounts of nodular limestone, conglomerate, and fossill- ferous graywacke. Maximum observed thickness of this "shale unit" is approximately 500 feet; the base is in fault contact with the Franciscan group. The shale unit grades upward through a 5 to 10-foot interval into a unit consisting of interlayered pillow lavas, tuff beds, and flows of andesitic and basaltic composition. This "green stone unit," approximately 1500 feet thick, grades upward through a 1 to 3-foot interval into an even-bedded brown "chert unit" having a maximum thickness of 400 feet. The uppermost unit of the Knoxville Formation is the "gray wacke unit," consisting of dark gray, calcareous, massive graywacke interbedded with minor amounts of shale and flaggy tuffaceous sandstone. Although the graywacke unit reaches a maximum thickness of 1200 feet in the northern part of the Stanley Mountain area, it thins eastward and disappears beneath the angular unconformity marking the contact between the Knoxville Formation and Carrie Creek Formation. In order to evaluate differences and/or similarities between the Knoxville Formation and Franciscan group, Knoxville lithologies will be described, and then compared to corresponding Franciscan rock types. These comparisons furnish a basis for determining whether or not the contact between Knoxville Formation and Franciscan group can be considered gradational,, Graywacke. Two varieties of graywacke in the Knox ville Formation are: (1) a bedded fossiliferous graywacke, weathering yellow-brown, interlayered with shales, lime stones, and conglomerates of the Hshale unit,*' and (2) a massive dark gray type making up the bulk of the "gray wacke unit". From bedded graywacke cropping out about one mile southwest of Shell Peak, Easton and Imlay (1955, P° 2539) collected pelecypods and ammonites of Late Jurassic (Tithonian) age. The graywacke beds are commonly 2-4 feet thick, sometimes flaggy, and can be traced for several hundred feet where exposures permit. Frequently, the graywacke grades upward or downward into cherty con glomerate. Beds may be scarred or pocked due to dif ferential solution of calcite cement. On weathered surfaces, individual grains appear in sharp relief, giv ing the rock a sandpaper texture. Examination of freshly broken surfaces reveals that fractures tend to pass around grains rather than through them. The resistant massive variety of graywacke, weathering dark gray, crops out in cliffs and steep ravines along the northwest side of Alamo Creek canyon (Plate 32). At intervals of 10 to 20 feet, the graywacke is interbedded with 2 to 6 inch layers of hard gray siliceous siltstone. The massive graywacke also Plate 32. Base of Knoxville gray- ' wacke unit exposed on northwest side of Alamo Creek canyon. Thick resistant graywacke is interbedded with shale and tuffaceous sandstone. Open 6 inch notebook provides scale in lower center. exhibits a sandy surface texture. In outcrop, the chief difference between Knoxville graywackes and Franciscan metagraywacke is that the former generally show undisturbed sedimentary textures, while the latter bears the imprint of pervasive shear. The sandy, mottled, yellow-brown appearance of bedded graywacke exposed in the lower part of the Knoxville Formation contrasts strongly with the polished slicken- sided surfaces exhibited by the dull green-gray Franciscan metagraywacke. Microscopic examination of 7 bedded graywackes and 2 massive graywackes from the Knoxville Formation revealed similar textural features. Clasts are medium- to coarse grained, poorly sorted, and subangular to subrounded„ If present, detrital mica and platy shell fragments tend to be subparallel. Most laminations are produced by subtle gradations in clast size. Grain boundaries are slightly serrated (Plate 33), yet more distinct than the margins of Franciscan clasts. Whereas Knoxville grains have retained original detrital shapes, most Franciscan grains have been reoriented and/or crushed during movement along closely spaced shear planes. Principal detrital components of Knoxville gray wacke are quartz, plagioclase, rock and fossil fragments, and small amounts of biotite. Interstitial material has been completely recrystallized to calcite, chlorite, white 94 ■10.5 mm Plate 33. Photomicrograph of Knoxville gray wacke showing intact sedimentary framework characterized by sharp grain boundaries. Rounded mosaic grain at left center is quartz-muscovite schist. Light gray interstitial mineral of high relief is calcite. Kicols crossed. Sample D - 160 from top of small hill about 6400 feet S60W of Shell Peak. 95 mica, and sphene. Calcite cement is abundant in most of the samples, as shown in Table 4, which summarizes the content of 9 Knoxville graywackes. Quartz content ranges from 9-39 per cent and averages 19 per cent. Except for a few bubble-shaped inclusions a micron or two in diameter, quartz is general ly clear. Replacement of grain margins by calcite cement has smoothed the angular outline of some clasts. The quartz grains appear to be unstrained. Plagioclase makes up an average of 19 per cent of the rocks examined and ranges from 6 to 30 per cent. The grains are essentially undeformed, showing few examples of patchy extinction and distorted composition planes so characteristic of Eranciscan feldspars. In Knoxville gray wackes many of the plagioclase grains have a corroded appearance resulting from partial replacement by calcite. Universal stage measurement of 10 plagioclase grains in each of 2 samples of Knoxville graywacke from the shale unit yielded average compositions of An6 and An5. The sample with the higher range contained grains of Anl and Anl3. The highly sodic and uniform composition of the plagioclase suggests that the anorthite molecule broke down during diagenesis and/or low grade regional meta morphism o Rock fragments make up an average of 18 per cent of the graywacke and range from 6 to 37 per cent. In order Knoxville Graywacke Samples Component JO a rH 1 P D - 1 5 7 0 to rH 1 P « rH to rH 1 P D - 2 6 7 Cl t- <M l P o> Cl i p CD lO (0 1 p D - 3 7 7 Alblte 6% 2 4 16 1 7 2 5 16 2 4 30 1 6 Blotite 0 3 0 0 0 0 0 0 tr Fossil 1 0 2 1 tr 0 tr tr 0 Lithie. frag. rH chert 5 6 1 2 6 20 1 3 7 2 6 u ■p qtz.-muse. tr tr 2 2 2 tr 0 tr tr ® P schist shale 11 0 3 3 2 0 2 2 2 volcanic 1 tr 4 2 1 3 6 6 3 1 8 Muscovite 0 tr 0 0 0 0 0 0 0 Quarts 1 6 3 9 1 5 2 0 1 5 3 6 9 13 1 3 Calcite 4-5 tr 3 2 2 1 1 4 3 7 4 0 1 5 Chlorite 7 1 8 8 2 0 1 9 1 9 8 6 2 1 Epidote tr 1 1 tr 0 0 tr 0 1 rH 0 Iron stain 4 1 2 4 2 1 3 1 1 ■P u •p JSf c Muscovite 3 7 2 3 tr 1 2 1 2 w ■o c o SB Sphene- leucoxene 0 0 tr 0 1 2 1 2 4 Unknown 1 1 1 1 0 2 1 tr 1 Total 1 0 0 1 0 0 1 0 0 Too 100 1 0 0 1 0 0 Too 1 0 0 Table 4. Compositions of 9 Knoxville graywackes. Samples D-358 and D-377 are from the graywacke unit; the remaining samples were collected from the shale unit. Percentages based upon 500 point counts per thin section. 97 of decreasing abundance, these consist of chert, 9 per cent; volcanics, 6 per cent; shale, 3 per cent; and traces of quartz-muscovite schist. Fossil fragments are mainly broken pelecypod valves consisting of calcite. Internal shell structures are well preserved. One graywacke contained 3 per cent detrital biotite in the form of relatively unaltered grains with distinct broken ends. Plotted on a ternary diagram (Figure 4), points representing proportions of quartz, feldspar, and lithic fragments in 9 Knoxville graywackes fall mainly within the field of arkosic graywacke, as defined by Williams, Turner, and Gilbert (1954, p. 292). Quartz-feldspar ratios are highly variable, perhaps due in part to observed preferential replacement of the two components by calcite. Comparison of Knoxville graywackes with Franciscan metagraywackes (Fig. 7) indicates that Knox ville rocks have higher proportions of unstable lithic fragments relative to quartz and feldspar. This dif ference may reflect either dissimilarity in maturity of sediments or simply that Franciscan rocks have undergone more severe metamorphic reconstitution than Knoxville rocks. The writer favors the latter alternative because he has observed that in Franciscan metagraywackes the lithic fragments are often comminuted and easily mistaken for matrix. The fact that Knoxville graywackes have appreciably higher proportions of chert, a stable lithic FRANCISCAN METAGRAYWACKE KNOXVILLE GRAYWACKE ( Percent (X = trace) ) 5 10 15 20 25 30 c Percent (X = trace) 5 10 15 20 25 30 ' FELDSPAR o LITHIC FRAGMENTS mm ‘ l _ - < C D Cl (CHERT : m\ QUARTZ IH ffiffliH tH n n H H H n B ' CALCITE X CHLORITE m a i m m m s s m H M B H " o EPIDOTE X 1 _ a , ■o i a . o LAWSONITE PUMPELLYITE SPHENE-LEUCOXENE STILPNOMEL ANE WHITE MICA I . 1 1 .1 n 8 Figure 7. Average composition of 28 Franciscan metagraywackes and 9 Knoxville gray wackes from the Stanley Mountain area (see Tables 1 and 4). 99 fragment, indicates that source terranes from which Knox ville and Franciscan sediments were derived may have differed slightly. Assuming that some calcite and most chlorite, white mica, and sphene have been derived from recrystallization of interstitial argillaceous material, the average matrix content of Knoxville graywacke exceeds 20 per cent. Calcite has been included in the discussion of matrix, because indeterminate amounts of calcareous clay were undoubtedly mixed with sand fractions during deposition. Calcareous shales and limestones are Interbedded with Knoxville graywackes in many parts of the Stanley Mountain area. Calcite ranges in abundance from 1 to 45 per cent, averaging 22 per cent. The mineral varies in habit from tiny (0.01 mm) blebs replacing feldspar to larger interstitial mosaics made up of euhedral grains 1-2 mm in diameter. Much of the calcite appears to have precipitated from solutions circulating through pores in the graywacke. In Franciscan metagraywacke, interstitial calcite appeared in only 4 of the 28 samples examined in thin section; only one sample contained more than trace amounts. The chlorite content of Knoxville graywackes ranges from 6 to 21 per cent and averages 14 per cent. Most of the mineral is a product of recrystallization of interstitial argillaceous material. In some cases chlorite has replaced margins of mafic rock fragments, causing out 100 lines of these fragments to appear fuzzy in thin section. Smaller interstices tend to contain clear, lime colored, optically continuous chlorite, whereas larger interstices accommodate diffuse patches of cloudy brownish chlorite intergrown with sparse white mica and microcrystalline calcite. White mica is relatively scarce in Knoxville gray wacke, accounting for an average of 2 per cent of the rock with a maximum of 7 per cent. Most of the white mica is interstitial; feldspar grains contain only trace amounts of sericite. Comparison of average mineral content of Knoxville graywacke with average mineral content of Franciscan meta graywacke shows major differences, as illustrated in Figure 7. Mondetrital minerals restricted to Franciscan metagraywacke are: pumpellyite, 6 per cent; lawsonite, 1 per cent; and stilpnomelane, 1 per cent. Of these, lawsonite is diagnostic of blueschist facies metamorphism and is present in 17 of the 28 Franciscan metagraywackes studied in thin section. Whereas Franciscan metagraywacke; contains 13 per cent nondetrital white mica, Knoxville graywacke contains 2 per cent. This dissimilarity in white mica content indicates that Franciscan rocks represent a higher grade of metamorphism than Knoxville rocks. Textural differences between Franciscan and Knoxville rocks also indicate independent metamorphic histories; most 101 Franciscan metagraywackes have been reconstituted by pervasive shear, whereas sedimentary textures in Knoxville graywackes are Intact. Conglomerate. Two beds of conglomerate, both 20 feet thick, are intercalated with graywacke, shale, and nodular limestones in the "shale unit." The lower of the conglomerates is a chert pebble conglomerate, which can be traced for several miles. In places the chert conglomerate has been faulted out against the underlying Franciscan group. The upper conglomerate is a dis continuous intraformational cobble conglomerate separated from the chert conglomerate by 20 feet of shale and nodular limestone. The intraformational conglomerate occurs approximately 400 feet below the contact between shale unit and greenstone unit. Subrounded, well sorted, dark green-gray chert pebbles averaging 6 mm in diameter account for about two thirds of the clasts In the lower conglomerate. The chert contains undeformed radlolarian and foraminiferal tests. Other lithic components are quartzite, dark porphyritic volcanic rock, graywacke, and traces of granitic rock. These are embedded in a tough, quartz-rich, slightly calcareous matrix weathering brown. The intraformational conglomerate contains sub- 102 angular cobbles of laminated limestone weathering white, and subrounded cobbles of chert conglomerate, dark porphyritic volcanic rock, tuff, and graywacke (Plate 34). The limestone closely resembles the nodular limestones included in underlying shale; the cobbles of chert conglomerate are identical in composition to the lower conglomerate bed previously described. Clasts of the - intraformational conglomerate stand out in strong relief from a coarse-grained matrix which weathers yellow-brown. Knoxville conglomerate contrasts markedly in texture and composition with Franciscan metaconglomerate. Whereas clasts in Knoxville conglomerate tend to be sub- rounded, Franciscan clasts are subangular or angular. Knoxville conglomerate lacks the platy black argillite fragments which are so common in Franciscan meta conglomerate. Since the matrix of Knoxville conglomerate weathers more readily than its Franciscan counterpart, Knoxville clasts stand out prominently, whereas Franciscan clasts are flush with the matrix and difficult to see. Calcite is present in the matrix of Knoxville conglomerate, but absent from Franciscan metaconglomerate. Knoxville conglomerates lack pumpellyite, but Franciscan meta conglomerate contains this mineral, which probably contributes to the dark green color of the rock. 103 Plate 34. Rounded clasts in Knoxville intra formational conglomerate. Light gray clasts are limestone, and vesicular clasts (upper center) are greenstone. Black clasts are chert. Base of 20-foot cliff on north side of ravine about 7500 feet S65W of Shell Peak. 104 Shale and nodular limestone. Within the shale unit of the Knoxville Formation, fissile drab gray to dull black shales, interlayered with laminated limestone nodules, account for an aggregate thickness of approxi mately 400 feet. Shales of similar appearance in the lower part of the graywacke unit have an overall thickness of about 100 feet. Individual shale beds vary in thickness from 5 to 60 feet. The shale splits readily into soft, irregular plates ,2-3 mm in thickness and 5-20 mm in length. These form distinctive talus slopes normally free of vege tation. The highly contorted nature of the shale is in part due to folding against more competent graywacke and greenstone, and in part due to surface slump brought about by weathering. Limestone nodules are normally ellipsoidal with diameters averaging 12 inches; rare limestone nodules may be as much as 6 jfeet in length (Plate 33). Conchoidal fracture is common. The limestone tends to be finely crystalline (ave. grain size: less than 0.25 mm) and argillaceous. Brown calcite veins consisting of crystals 2-3 mm in length transect many of the nodules. Pelecypods were collected from nodular limestones in a narrow strip of the shale unit exposed southwest of Stanley Mountain. W. P. Popenoe (oral commun., 1968) identified these fossils as Buchla piochii of Late Jurassic age. Argillaceous rock in the Knoxville Formation is 105 Plate 35. Large limestone nodules in Knox ville shale unit. Shallow ravine in large field about 3100 feet S35W of Shell Peak. 106 distinctly different from argillaceous rock in the Franciscan group. Knoxville shale is thick bedded (5 to 60 feet), soft, and interlayered with nodular limestone. Franciscan argillite layers rarely exceed 1 foot in thick ness. Limestone is extremely rare in the Franciscan group of the Stanley Mountain area, A few clasts of carbonate are present in Franciscan metaconglomerate; the only other limestone encountered in Franciscan rocks during the present study was a single lens measuring 2 inches in thickness and 3 feet in length. Greenstone. Interlayered andesite and basalt flows, pillow lavas, and tuffs comprise the Knoxville greenstone unit, which crops out over an area of approximately 8 square miles. The gross structure of the unit is a homo- cline dipping moderately to the north. In a few areas, layers in the greenstone steepen to vertical or dip south ward, but dense brush and deeply weathered outcrops make delineation of small-scale folds extremely difficult. Fart of the greenstone unit, particularly that exposed in the Cuyama River gorge, lacks primary planar features. Elsewhere, thick sequences of interlayered pillow lavas and tuff beds furnish excellent structural attitudes clearly showing that the greenstone is conformable with underlying shale and overlying chert. In two localities, tuff near the base of the greenstone unit is interbedded 107 with shale. Knoxville greenstone is characterized by continuity over a wide area, tabular geometry, and clearly defined stratigraphic boundaries. In contrast, Franciscan green stone occurs as small sheared lenses difficult to cor relate from one outcrop to the next. Spilitic andesite and basalt flows, varying in thickness from a few feet to more than 100 feet, make up more than 70 per cent of the Knoxville greenstone unit (Plate 36). In some cases, differential weathering of composite flows delimits thicknesses, but more often dimensions of flows can be found only by estimating intervals between successive horizons of pillow lavas. The andesite and basalt is dark green or gray-green weathering brown or red-brown. The tops of flows tend to be vesicu lar and/or amygdaloidal. The mineral contents of three greenstones classified as andesites are listed in Table 5. Though these rocks appear to be mildly altered, relict igneous textures are well preserved. Sample D-19 (Table 5) consists of euhedral albite (An 4) phenocrysts, 0.1-1.0 mm in size, embayed in a pilotaxitic groundmass of 0.05 mm subhedral feldspar laths with interstitial chlorite and sphene (Plate 37)• Plagioclase accounts for 70 per cent of the rock, chlorite 17 per cent, and sphene 6 per cent. Amygdaloidal epidote, 1-5 mm in diameter, makes up 4 per cent of the sample, and Plate 36. Knoxville greenstone. Resistant layer at center is andesite flow inter- bedded with tuff. North bank of Pish Greek about 6900 feet N50W of Shell Peak. 109 Mineral Knoxville Greenstone Andesltlc D-19 D-121t Flows D-122 Pillow Lava D-145 Altered Vltrlc Tuff D-364 Albite 10% 50 19 12 8 Auglte 0 0 0 9 0 Calclte 0 0 0 2 0 Chlorite 17 12 6 1 7 Epidote b 13 bz 3 tr Iron stain 1 2 tr 1 1 Prehnite 0 0 0 0 b6 Pumpellyite 0 2 0 57 7 Pyrite 1 tr 0 0 0 Quartz 1 16 32 15 31 Sphene 6 b tr tr tr Unknown 0 1 1 0 0 Total 100 Too 100 100 100 Table 5. Modes of Knoxville greenstones, Including andesltlc flows, pillow lava, and altered vltrlc tuff. Calclte and quartz are amygdales. Percentages based upon 500 point counts per thin section. 110 j — ----------- 1 0,3 m Plate 37. Photomicrograph of Knoxville ande site, showing euhedral alhit-' phenocryst surrounded by a pilotaxitic groundmass of albite, chlorite, and sphene. Dark, oblong, mosaic patches at lower left and upper left center are amygdules of chlorite. Nicols crossed. Sample D - 19 collected from hill side about 2600 feet S50W of Stanley Mountain. Ill ; small amounts of pyrite and amygdaloidal quartz are dispersed throughout. Sample D-121b (Table 5) contains I badly corroded sereate plagioclase phenocrysts smaller than j i 0.25 mm enmeshed in a trachytic groundmass of fused feld- j spar laths and the following interstitial minerals: ! quartz, 16 per cent; epidote, 13 per cent; chlorite, 12 per cent; sphene, 4 per cent; and pumpellyite, 2 per cent. Total feldspar content is 50 per cent. Some of the quartz may be primary in origin. Sample D-122 (Table 5) consists of plagioclase phenocrysts smaller than O.25 mm partially replaced by epidote and quartz. The phenocrysts are ir regularly embayed in a highly silicifled intergranular [ groundmass of subhedral plagioclase laths, epidote beads (ave. diameter: 0.05 mm), and chlorite. Both phenocrysts and clear vein feldspars have an average composition of An 6. The most abundant mineral is amygdaloidal epidote (42 per cent) followed by quartz (32 per cent), plagio clase (19 per cent), and chlorite (6 per cent). Basalt constitutes a very small percentage of Knox ville greenstone in the Stanley Mountain area. One extremely dense sample contained 5 per cent olivine pheno crysts, 0e25-0.50 mm in diameter, surrounded by an inter granular groundmass transected by veins of epidote. Oakeshott (1929)» who examined a basalt from the Stanley Mountain area in thin section, reported that much of the feldspar was altered to chlorite, which formed about half the rock. Crudely layered pillow lavas account for approxi mately 25 per cent of the greenstone and are particularly abundant near its base and top. Tabular sequences of pillow lavas vary in thickness from 5 to 150 feet. Occasionally, a few highly-deformed isolated pillows occur scattered along the boundary between andesite flows. Individual pillows are blocky, spheroidal, ellipsoidal, or even disc-shaped, but rarely cuspate, so that they cannot be used to determine facing. A powdery green tuffaceous matrix surrounds and completely separates adjacent pillows (Plate 38). Long dimensions of individuals range from 4 to 18 inches and average 8 inches. Vesicles or amygdules of chalcedony and calclte cluster along the periphery of some pillows and frequently dot the interior, but are not present in all pillow lavas. Fresh exposures commonly exhibit a lighter shade of green than adjacent flows of andesite or basalt, A single sample of pillow lava studied in thin section (D-145, Table 5) contains a few phenocrysts of auglte and highly altered plagioclase grains, less than 0.3 mm in diameter, surrounded by an intergranular to intersertal groundmass consisting of corroded plagioclase laths 0.1-0.2 mm In length, pumpellyite, auglte, epidote, and chlorite. Yellow-green feathery bundles of nearly submicroscopic pumpellyite needles, comprising 57 per cent Plate 38. Knoxville pillow lavas. Height from bottom to top of photo is 8 feet. Northwest bank of steep ravine about 5900 feet S85W of Shell Peak. 114 of the rock, probably replace mafic glass. Neither the feldspar (12 per cent) nor augite (9 per cent) in this sample appear to serve as a substrate for growth of pumpellyite. Amygdules of fibrous brown chalcedony, granular quartz, and calcite reach a maximum diameter of 5 mm and make up 17 per cent of the pillow lava. A soft, porous, medium-grained white tuff, which weathers brick red, crops out near the base of the green stone unit southeast of Stanley Mountain. The tuff band, more than 50 feet thick, can be traced for nearly 0.5 mile along strike. Near the upper contact of the green stone unit, a dense, laminated, fine-grained vitric tuff 10 feet thick is interbedded with vesicular andesite flows. A thin section (D-364, Table 5) shows that the rock con sists of 31 per cent detrital quartz; 8 per cent detrital feldspar; 46 per cent prehnite, showing "bow-tie1 ' structure (Kerr, 1959 > p. 418); 7 per cent pumpellyite; and 8 per cent chlorite. Dispersed throughout are trace amounts of deep green palagonite. Bedded chert. Dark-colored, evenly bedded Knoxville cherts, conformably overlying the greenstone unit, are well exposed in Alamo Creek canyon in the northern part of the Stanley Mountain area (Plate 39)• Their dominant color is dark brown; other colors are deep green, dull reddish brown, and black. Thicknesses of individual chert beds, Plate 39» Knoxville bedded chert interlayered with paper-thin shale. Northwest bank of Alamo Creek about 3100 feet N55E of the confluence of Alamo and Jollo Creeks. 116 averaging 3 inches, tend to remain constant where traced across an outcrop. Laminations defined by faint color bands 2-10 mm in thickness also show excellent lateral continuity. Interbedded black shales are normally so thin that in many cases it would be difficult to pass a knife blade between the smooth surfaces of successive chert layers. Small-scale effects of deformation are seen only near the greenstone contact, where dips steepen and beds are gently folded. Thin sections of Knoxville chert show round radio- larian tests filled with clear chalcedony and chlorite embayed in a brownish matrix of chalcedony, chlorite, and iron oxide. Radiolaria range in abundance from 10 to 20 per cent. A few detrital grains of feldspar and quartz are present. Knoxville cherts contrast markedly with their Franciscan counterparts. Whereas Knoxville cherts are dark, evenly bedded, and mildly deformed, Franciscan meta cherts are light colored, lenticular, and sometimes tightly folded. Perhaps the most significant difference is evident in the regional patterns of chert distribution. The Knox ville chert-shale sequence Is a tabular stratigraphlc entity with a thickness of 400 feet and a lateral extent of three miles. It is clearly conformable with the under lying volcanics and overlying massive graywacke. On the other hand, Franciscan metachert-arglllite sequences occur 117 as small lenticular bodies having no consistent strati- graphic or structural relationship with the surrounding rocks. Under the microscope, the siliceous matrix of Knox ville chert is brown, whereas the Franciscan metachert matrix is colorless. As pointed out by Bailey and others (1964, p. 64), chemical differences between Knoxville and Franciscan cherts in the Stanley Mountain area do not support Taliaferro's (1943a, p. 197-200) conclusion that the Knoxville grades into the Franciscan. Chemical analy sis of one Knoxville chert sample from the Stanley Mountain area (Bailey and others, 1964, p. 63) showed that the rock contained appreciably less silica, and more aluminum, iron, and magnesium than the average Franciscan metachert from other parts of California. Perhaps the color difference reflects presence of more argillaceous impurities in Knox ville chert than commonly found in Franciscan metachert. Radiolarian tests in two thin sections of Knoxville chert are undeformed. In one of two thin sections of Franciscan metachert, Eadiolarian tests are flattened parallel to planes of preferred mica orientation. Dlagenesls of the Knoxville Formation. The as semblage quartz-albite-white mica (muscovite)-chlorite t calcite indicates that Knoxville graywackes have re- 118 crystallized In either the zeolite facies, greenschist facies, or in a facies transitional "between the two. Ac cording to Pyfe and others (1958* p. 217) complete albiti- zation of plagioclase takes place in the highest stage of zeolite facies alteration. The average plagioclase composition in Knoxville graywackes is An6. Hence, the lower grade limit of the graywackes is near the boundary between zeolite facies and greenschist facies. The ab sence of metamorphic biotite dictates the upper grade limit, which is the boundary between the quartz-albite- muscovite-chlorite subfacies and quartz-albite-epidote- biotite subfacies of the greenschist facies (Fyfe and others, 1958, p. 219). The mineral assemblage in Knoxville graywackes may be considered either “metamorphic" or "diagenetic," depending on criteria used to distinguish processes of metamorphism from diagenesls. Turner (in, Pyfe and others, 1958, p. 215) suggested that .the term "metamorphic" is applicable to rocks in which albitization of plagioclase is extensive. Crook (in Packham and Crook, i960, p. 404) believed that mineral reactions are diagenetic until the stage is reached where original rock fabric is extensively altered. Since Knoxville graywackes are texturally un modified, constituent minerals would be classed as "diagenetic" according to Crook's definition. Also in context of this definition, minerals in highly sheared 119 Franciscan rocks would be called "metamorphic." Hence, in the Stanley Mountain area, textural distinctions are im plied in use of the terms Knoxville "graywacke" and Franciscan "metagraywacke." The soda-rich plagioclase in Knoxville andesite may have originated from either albitization during or soon after solidification, or from reactions which took place during diagenesis or low grade regional metamorphism (Williams and others, 1954-, p. 59). Oakeshott (1929) described an extrusive spilitic andesite in greenstones presently mapped as Knoxville in the Stanley Mountain area. He reported that alteration of andesine to water-clear albite began in fractures and spread into feldspar grains as irregular patches. Packham and Crook (i960, p. 405) suggested that spilites may be the result of diagenetic modification rather than products of magmas of unusual composition. The presence of pumpellyite in Knoxville greenstones indicates that the rocks have recrystallized in response to diagenesis or low grade regional meta- morphism. The albite may be a product of these reactions. In Knoxville greenstone the mineral assemblage albite-pumpellyite-chlorite-sphene-epidote t prehnite t quartz is diagnostic of the prehnite-pumpellyite zone, which Coombs (in Coombs, Ellis, Fyfe, and Taylor, 1959, p. 68) considered to be transitional between the zeolite facies and greenschist facies. To accommodate rocks of the ■ 120 | prehnite-pumpellyite zone found in the New Zealand geo- syncline, Ooombs (I960, p. 340-34-1) described and defined I a prehnite-pumpellyite metagraywacke facies# Among rocks i assigned to this facies were pumpellyite-bearing spilitic lavas. Packham and Orook (i960, p. 405-406), who recog nized a prehnite-pumpellyite facies in sedimentary and volcanic rocks of New South Wales, Australia, suggested that pumpellyite-and albite-bearing andesite may be a diagenetically altered andesite. The widespread occurrence of epidote in Knoxville greenstones of the Stanley Mountain area indicates that these rocks may represent a higher grade subfacies within the prehnite-pumpellyite metagraywacke facies defined by Coombs (I960, p. 342). The sequence pumpellyite, preh- nite, epidote, is one of decreasing hydration in response to increasing pressure and temperature (Coombs and others, 1959> p* 60). Packham and Crook (i960, p. 403) remarked that increase in epidote content signals a transition from the prehnite-pumpellyite facies to a higher grade of meta morphism equivalent to the greenschist facies. Estimated temperatures and pressures of recrystal lization in the zeolite facies are 200°-300° C and 2-3 kllobars; estimated temperatures and pressures under which greenschist minerals form are 300°-500° C and 3-8 kilobars (Turner and Verhoogen, i960, p. 532, 534). Temperature- pressure fields for a transitional facies (prehnite- 121 pumpellyite facies) have not been defined experimentally. Extrapolation of temperature-pressure conditions from zeo lite and greenschist experimental data indicates that pressure requirements for growth of minerals in Knoxville rocks would be considerably less than the 6-9 kilobars necessary for the formation of blueschist facies minerals in the Franciscan group. Figure 5 is a temperature- pressure diagram comparing probable stability fields for Knoxville and Franciscan minerals. To prove that the contact between Knoxville Forma tion and Franciscan group is gradational, one must demonstrate that pressure gradients changed abruptly through a stratigraphic interval of a few tens of feet. Since such an abrupt change seems unlikely, the contact must be either unconformable or tectonic. Jollo Formation The Jollo Formation of Early Cretaceous age con sists of siltstone, shale, and conglomerate lenses and un- conformably overlies the Knoxville Formation in the north east limb of the Stanley Mountain antiform. Here, the Jollo Formation is approximately 3000 feet thick (Hall and Corbato, 1967* p. 564). On the southwest limb of the anti form, patches of the Jollo Formation in fault contact with the Franciscan group vary in thickness from a few feet to about 200 feet. 122 Angular discordance between the Jollo Formation and underlying Knoxville graywacke unit ranges from 10 to 25 degrees. The upper contact is probably also unconformable. Though there is a sharp lithologic break between siltstones of the Jollo Formation and coarse-grained sandstones of the overlying Carrie Creek Formation, the two formations appear to be structurally concordant In the few places where the contact is exposed. Nevertheless, regional relationships suggest an unconformity, since the Carrie Creek Formation overlaps the Jollo Formation toward the east. Taliaferro (1943a, p. 199) first used the name "Jollo conglomerate" to designate conglomerate and sand stone of Early Cretaceous age exposed in the Stanley Mountain area. Hall and Corbato (1967, p. 564) employed the name Jollo Formation as a local stratigraphic name for Lower Cretaceous rocks in the Jollo Creek area. The most conspicuous rock types in the Jollo Forma tion are resistant, dark gray, pebble conglomerates with interbedded light brown coarse-grained sandstone (Plate 40). In the northern part of the Stanley Mountain area, three distinct conglomerate-sandstone lenses averaging 30 feet in thickness crop out near the base of the formation. These form sharp contacts with intervening siltstone beds, 50 to 60 feet thick, and persist laterally for more than 3 miles before lensing out. Along the Franciscan contact to the south, where exposures are poor, Isolated blocks of Plate 40. Sandstone and conglomerate In Jollo Formation. Note graded bedding ir, sandstone. Southwest side of ravine about 4800 feet S15E of Stanley Mountain. 124 conglomerate are frequently the only indication that the Jollo Formation is present„ Pebbles are well rounded and oriented randomly where the conglomerate is massive, but are imbricated where the conglomerate is interbedded with sandstone* Estimated pebble contents are as follows: dark green to black chert, 50 per cent; dark aphanitic or porphyritic volcanic rock, 30 per cent; and dark brown medium-grained sandstone, 10 per cent. Other clasts found in the conglomerate are shale, limestone, quartz-muscovite schist, and rare granitic rock. The chert and volcanic clasts are similar to rocks in the Knoxville Formation. Clastic components of the conglomerates do not appear to have been derived from the Franciscan group. Interbedded with conglomerates of the Jollo Forma tion are layers of yellow-brown, laminated, coarse grained poorly sorted sandstone averaging one foot in thickness. The sandstone commonly exhibits cross-bedding and graded bedding. Detrital quartz ranges in abundance from 30 to 50 per cent, with lesser amounts of feldspar (15-25 per cent), lithic fragments (5-10 per cent),and heavy minerals (5-10 per cent). A matrix of chlorite and muscovite makes up 10-15 per cent of the sandstone, and calclte may be present as a cementing agent. Evenly bedded (2-6 inches) olive gray or black calcareous siltstone and shale form the bulk of the Jollo Formation (Plate 41). Concretions of dark gray finely Plate 41. Interbedded siltstone (resistant layers) and shale in Jollo Formation. Southwest side of ravine about 5000 feet S15E of Stanley Mountain. 126 crystalline limestone weathering creamy white occur through out the siltstone-shale sequence. These vary in diameter from 6 to 24 inches, and are often transected hy coarsely crystalline brown calcite veins. Carrie Creek Formation i The Carrie Creek Formation of Late Cretaceous age rests unconformably upon the Jollo and Knoxville forma tions, and is in fault contact with the Franciscan group. It consists of massive, yellow-brown or yellow-gray, coarse-grained arkosic sandstone interbedded with thin layers of dark gray, fine-grained sandstone, brown silt- stone, and conglomerate. Hall and Corbato (1967, p. 565) proposed the name Carrie Creek Formation and designated the type area as the Carrie Creek area in the eastern part of the Nipomo quadrangle. They suggested that rocks of the Carrie Creek Formation might be equivalent in age and lithology to the Atascadero Formation of Fairbanks (1904, P. 3). In the Stanley Mountain area rocks correlative with parts of the Carrie Creek Formation reach a thickness of more than 3000 feet; the top of the formation is not ex posed. These rocks were initially mapped as "Cretaceous sandstone" by Oakeshott (1929) and later mapped as "Cretaceous undifferentiated" by Taliaferro (1943a, p. 199). On the southwest limb of the Stanley Mountain antiform, Crandall (1961) collected Late Cretaceous fossils from rocks which Hall and Corbato (1967, p. 566) considered correlative with the Carrie Creek Formation. Field work undertaken during the present study revealed no significant lithologic break between Upper Cretaceous rocks dated by Crandall (1961) and rocks designated Lower Cretaceous on the San Luis Obispo Sheet (Jennings, 1958)<> The rocks formerly designated Lower Cretaceous, therefore, are re assigned to the Carrie Creek Formation of Late Cretaceous age. The lithology most characteristic of the Carrie Creek Formation in the Stanley Mountain area is massive yellow-brown or yellow-gray, coarse-grained, calcareous, arkosic sandstone. This rock weathers pale orange-brown, bright reddish brown, and sometimes bright yellow. At stratigraphic intervals of 4 to 6 feet the sandstone is commonly intercalated with siltstone beds typically less than 6 inches thick (Plate 42). Sandstone clasts are moderately sorted to well sorted, angular, and oriented randomly in a matrix of microcrystalline muscovite and chlorite. Interstitial calcite grains average 0.25-0.50 mm in diameter and range up to 1.5 mm. Under medium magnifi cation grain margins are distinct and slightly serrated due to encroachment of matrix minerals. The average composi tion of 9 unbrecciated sandstones examined In thin section is: quartz, 33 per cent; feldspar, 29 per cent; calcite, Plate 42. Thick, evenly hedded, re sistant sandstone interbedded with siltstone in Carrie Creek Forma tion, exposed in northwest bank of Jollo Creek. 129 12 per cent; chlorite, 8 per cent; lithic fragments, 6 per cent; muscovite (recrystallized in matrix), 4 per cent; and detrital mica (principally biotite), 4 per cent (Table 6). Minor detrital accessories are epidote and sphene. Light brown flaggy siltstone and medium gray, flaggy, laminated, fine-grained sandstone units varying in thickness from 4 to 50 feet are exposed only in road cuts and along stream bottoms. These units occur in the Carrie Creek Formation near the Franciscan contact, but are ab sent in the north where the Carrie Creek Formation un- conformably overlies the Jollo and Knoxville formations. Siltstones and shales could not be traced laterally due to the poor exposure. Resistant cobble conglomerate, a minor constituent of the Carrie Creek Formation, is particularly well- exposed in roadcuts on California Route 166. Well rounded cobbles are principally dark amygdular volcanic rocks (20- 50 per cent), granitic rocks (10-50 per cent), chert (5-60 per cent), graywacke (5-10 per cent), and small amounts of quartzite. A light gray very coarse-grained sandstone containing up to 10 per cent purplish biotite constitutes the conglomerate matrix. Tertiary and Quaternary Systems Sedimentary rocks of Oligocene and Miocene age are Component D-103 D-115b Normal Carrie c- o o> 03 to to i —i i —i i —i i i t q » n Creek i —i r - i 1 « Sandstone CD CM lO to rH 02 1 1 ( = » Q D-378 Blotlte 1# 2 3 4 1 1 4 3 3 Epldote 2 0 1 1 0 0 1 2 3 Feldspar 36 35 22 37 28 19 26 33 22 Lithlc frag, chert 3 5 4 2 6 1 1 2 3 « —i CD granitic tr tr tr 1 O O 0 tr 1 43 tH F - , qtz.-muse. 0 3 2 2 1 O 0 2 . 1 4> © o schist shale tr 1 1 2 tr O 0 1 1 volcanic 1 1 1 tr 1 tr 0 0 2 Muscovite tr 1 1 1 O tr 3 t r tr Quartz 28 34 27 32 38 33 •57 34 32 Silt 3 tr 0 tr t r 0 1 0 0 Calcite 0 4 27 0 10 29 11 4 22 Chlorite 13 7 6 9 6 12 5 11 4 ' M M —' ' M M — 1 Iron stain 5 3 2 2 2 3 5 tr 1 * * as 43 •H Muscovite 5 4 3 3 5 1 3 7 3 u 43 © •d Sphene 1 0 tr 3 0 1 2 1 1 c o t s Unknown 2 tr O 1 2 0 1 0 1 Total TO T TOT TOT TOT TOT TOT TOT 1 0 0 100 Table 6. Compositions of 9 normal and 9 brecclated sandstones coll< contact. Note the high chlorite pies, percentages based on 500 ] Brecciated Carrie Creek Sandstone c t i «j X> lO to Sandstone lO m 0 3 CO LO lO 0 3 CO C- to I Q O 03 V* 0 3 lO lO 03 Ol tO Q Q tr tr tr tr tr tr 36 26 36 20 33 22 27 37 37 31 27 31 tr tr tr tr tr tr tr tr 32 45 32 34 •37 29 39 33 26 42 29 40 11 22 18 11 20 16 14 15 20 20 tr tr tr TOT TOT lOO loo TOT TOT TOT TOT TOT TOT TOT TOT ons of 9 normal Carrie Creek sandstones, sandstones collected above the Franciscan e high, chlorite content of brecciated sam- s based on 500 point counts per thin section. 131 | well exposed in the Huasna syncline, which is separated ; i from the Stanley Mountain antiform by the northwest trend- / i ing East Huasna fault (Hall and Corbato, 1967, p. 576). In I the Huasna syncline the contact between Tertiary and older j rocks is not exposed; on the northeast side of the fault, conglomerates of the Sespe Formation of Oligocene age rest unconformably upon the Carrie Creek Formation, and small patches of the Santa Margarita Formation of upper Miocene age rest unconformably upon the Knoxville and Carrie Creek Formations. The oldest Tertiary unit in the Stanley Mountain areh is the Sespe Formation of Oligocene age, which consists of red-brown, coarse-grained, poorly sorted sandstone inter bedded with conglomerate containing well rounded granitic clasts and angular clasts of Carrie Creek sandstone. The Sespe, which overlies the Carrie Creek Formation with an angular discordance of 5-15 degrees, is confined to a small rectangular patch bounded on the southwest by the East Huasna fault and on the northeast by a parallel high angle fault. Maximum thickness of the Sespe is about 300 feet. The Vaqueros Formation of Oligocene and Miocene age consists of massive, conchoidally weathered, soft, light brown, coarse-grained sandstone and massive, well-indurated,; light brown, fine-grained sandstone. The coarse-grained sandstone crops out in prominent bluffs on the north bank of the Cuyama River; the fine-grained variety persists in a 132 narrow belt along the East Huasna fault to the northwest and southeast of Alamo Creek. The formation is at least 600 feet thick. The contact between the Vaqueros Pormation and underlying Sespe Pormation was not exposed in the Stanley Mountain area. Hall and Corbato (1967, p. 567) reported that in the Nipomo quadrangle, the contact is conformable in some localities and unconformable in others. Two hundred feet of poorly consolidated, con- choldally fractured, yellowish brown claystone and soft white tuff make up the Rincon Pormation of early Miocene age. The Rincon Pormation conformably overlies the Vaqueros Pormation on a narrow ridge on the north bank of the Cuyama River; a few hundred feet to the north the Rincon pinches out between the Vaqueros and overlying Point Sal formations. The Point Sal Pormation of middle Miocene age comprises 1000 feet of light brown and dull white, evenly bedded, quartz-rich, fine-grained sandstone, white flaggy siltstone, and buff colored shale. Well sorted and sub rounded sandstone grains are sometimes embedded in a diatomaceous or slightly calcareous matrix. Sandstone beds range in thickness from 1 to 3 feet, whereas siltstone and shale beds are commonly less than 6 Inches thick. The lower contact of the Monterey Pormation of middle and late Miocene age was mapped at the first occur rence of porcelaneous shale and/or chert above the sand stones, siltstones, and shales of the Point Sal Formation* According to Hall and Corbato (1967» p. 562), the Monterey Formation attains a maximum thickness of 5500 feet in the Nipomo quadrangle; only the lower part of the formation extends into the Stanley Mountain area, where thickness was estimated at 2500 feet. The formation consists of flaggy (1-4- inch beds), light brown, conchoidally fractured, porous siltstone, porcelaneous shale, brown chert, and well bedded white claystone. The siltstone weathers to a dis tinctive light orange-brown color. Flat-lying erosional remnants of brilliant white, friable sandstone of the Santa Margarita Formation cap a ridge of Knoxville greenstone northwest of Shell Peak. The sandstone, less than 20 feet thick, covers only a few acres. The presence of giant oysters, Orassostrea titan (Conrad), establishes a late Miocene age for the Santa Margarita Formation in the Stanley Mountain area (C. A. Hall, oral commun., 1968). To the west, in the Huasna syncline, the Santa Margarita formation of Mio-Pliocene age conformably overlies the Monterey Formation (Hall and Corbato, 1967, p. 570). In the southeastern part of the Stanley Mountain area, 200 feet of gently dipping, thick bedded (2-5 feet), cavernous, white, coarse-grained cal careous sandstone and soft brown claystone assigned to the Santa Margarita Formation rest unconformably upon the Carrie Creek Formation of Late Cretaceous age. 134 Quaternary deposits include stream terrace deposits along Alamo Creek and the Cuyama River, landslides within the Franciscan terrane, and alluvial deposits in the present stream courses. Many landslides appear to he initiated in shear zones peripheral to serpentinite belts. Silica-carbonate Rock Silica-carbonate rock, a product of hydrothermal alteration of serpentinite (Bailey and others, 1964, p. 87), forms discontinuous patches of resistant, quartz- veined, cavernous brown blocks. These are localized along a high angle fault parallel to the East Huasna fault, and along the contact between Cretaceous rocks and Franciscan group on the southwest limb of the Stanley Mountain anti form. Individual patches of silica-carbonate rock usually cover less than an acre. Outcrop patterns indicate that some bodies of silica-carbonate rock are lenticular. The largest lens, exposed in the south bank of Aliso Creek at the contact between Carrie Creek Formation and Franciscan group is about 50 feet thick and 1000 feet long. Silica-carbonate rock consists of quartz, chalce dony, opal, magnesite, calcite, and dolomite, with traces of relict serpentine. Gradational relationships between nearly fresh serpentinite and silica-carbonate rock are recorded in rocks of the Stanley Mountain area. In 135 exposures showing initial stages of alteration, veins of milky quartz and carbonate cut slabs of foliated, dense, aphanitic dark green-gray serpentinite. Irregular clots and stringers of granular black chromite (ave. grain size: 0.05 mm) contribute to the dark color of the rock. A thin section shows feathery veins of cryptocrystalline carbonate interwoven around 2-5 mm lentils of relict anti- gorite and bowlingite. X-ray analysis indicates that magnesite is the dominant carbonate, although calcite and dolomite are also present. The most common variety of silica-carbonate rock appears as gnarled, brittle, orange-brown blocks on which intricate networks of iron-stained quartz veins stand out in bold relief (Plate 43). The silica-carbonate rock inherits the sheared appearance of serpentinite from which it was derived. Pods of relict serpentinite ranging in thickness from a few inches to 3 feet are scattered throughout the rock. Though chalcedony is the common variety of silica, well-formed quartz crystals up to 10 mm in length line cavities in some silica-carbonate bodies. Silica-carbonate rock representing complete altera tion of serpentinite consists of sieve-like chalcedony patches partitioned by dendritic carbonate veins, in which grains range up to 0.25 mm in diameter (Plate 44). Another common texture is a dense, dull white mosaic of chalcedony and carbonate grains averaging less than 10 microns in Plate 43. Silica-carbonate rock. Porous texture is due to solution of carbonate. White vein material is quartz. Outcrop on northwest side of canyon about 7600 feet N42W of intersection of Calif. Rte. 166 and Tepusquet Road, ft • ' ^ 0.5 mm Plate 44. Photomicrograph of silica-carbonate rock. Veinlets consist of magnesite and calcite; intervening speckled material is chalcedony. Nlcols crossed. Sample D - 169 collected from ledge on north bank of Cuyama River about 3000 feet R5W of intersection of Calif. Rte. 166 and Tepusquet Road. diameter. The association of silica-carbonate with valuable deposits of cinnabar has fostered much interest in the origin of silica-carbonate. Bailey and Everhart (1964, p. 63) considered that alteration of serpentinite to silica- carbonate requires simple dehydration and carbonatization without appreciable change in volume. They pointed out that hydrothermal solutions responsible for conversion of serpentinite to silica-carbonate are not genetically related to emplacement of serpentinite. According to Bailey and Everhart (1964, p. 60-61), carbonate replaces antigorite and chrysotile in initial stages of alteration. With continued alteration, minute quartz grains replace serpentinite; later, quartz recrystallizes into larger crystals. Magnetite disappears during early stages of alteration, but chromite remains unaltered. Since silica- carbonate rock is associated with cinnabar mineralization of Tertiary or younger age, many geologists working in the Coast Ranges have concluded that hydrothermal solutions involved in the genesis of silica-carbonate rock are related to Tertiary or Quaternary volcanism. East of San Jose, California, serpentinite in contact with rhyolite of probable Miocene-Pliocene age has been altered to silica- carbonate rock (Crittenden, 1951* P* 25). In the Mayacmas district north of San Francisco, silica-carbonate rock has been assigned a Pliocene or younger age (Bailey, 1946, p. 139 208; Tates and Hilpert, 1946, p. 241). STRUCTURE General Statement The Stanley Mountain area is part of the Nacimiento block, one of two broad belts in the southern Coast Ranges underlain by Franciscan basement. The Nacimiento fault forms the northeast boundary of the Nacimiento block and brings Franciscan basement into juxtaposition with Cretaceous granitic intrusives and older metamorphic rocks of the Salinian block (Fig. 2). The Stanley Mountain area lies between the Nacimiento fault and the Huasna syncline, which together with other high angle faults and fold axes in adjacent areas trend to the northwest. Outcrop patterns and bedding attitudes of Mesozoic rocks in the Stanley Mountain area delineate a broad northwest-trending, doubly plunging antiform, herein called the "Stanley Mountain antiform." The northwest trending East Huasna fault, a high angle fault along which right lateral movement may have occurred (Hall and Corbato, 1967, P. 576), transects the southwest limb of the anti form and is oblique to the inferred trace of the axial plane (Plate l). Southwest of the fault, Oligocene and Miocene rocks of the Huasna syncline dip southwestward at moderate angles. 140 Stanley Mountain Antiform The form of the Stanley Mountain antiform is best shown by bedding attitudes of the Great Valley sequence and by the trace of the contact between Great Valley rocks and the Franciscan group. The trace of the axial plane was inferred to be symmetrical to this contact. On the basis of field and petrographlc evidence, the contact between Franciscan rocks and structurally higher rocks of the Great Valley sequence is interpreted by the writer as a thrust fault. Hence, the term "antiform" is used instead of "anticline" to avoid the implication that Franciscan rocks are necessarily older than Great Valley rocks. The following discussion of the Stanley Mountain antiform will concern six main topics: (1) structure of the Franciscan group; (2) structure of the Great Valley sequence; (3) comparison of Franciscan and Knoxville tectonic styles; (4) evidence for a thrust contact between Franciscan group and Great Valley sequence; (5) geometry of the thrust fault; and (6) time of thrusting. Structure of the Franciscan group The tectonic style of the Franciscan group is chaotic; the rocks appear to have been deformed by multiple tectonic events. Penetrative shearing under conditions of blueschist metamorphism produced a chaotic mixture of 141 tectonic inclusions of diverse eugeosynclinal rock types (Plate 4). Sheared contacts between tectonic inclusions constitute a crude foliation, but one having unsystematic orientation (Plate 1). Some variability in foliation may be due to the lenticular shape of tectonic inclusions, but most of the variability is probably the result of re orientation of shear surfaces during subsequent high-angle faulting and folding. Axes of rare mesoscopic folds plunge in all directions. Orientations of slickensided surfaces and joints are variable. The only mappable structural grain is provided by northwest-trending belts of serpentinite within the Pranciscan core of the Stanley Mountain antiform. Tectonic inclusions. Metagraywacke, metaconglomer ate, greenstone, metachert, amphibolite, and glaucophane schist occur as discontinuous bodies which are generally lenticular in shape, but which vary considerably in detail. Hast (1956, p. 401) suggested that the term "tectonic inclusion" be applied to any isolated body of rock formed by tectonic disruption of a layer which was originally more or less extensive. Within the framework of East’s definition, the bulk of the rock masses in the Pranciscan group in the .study area can be considered tectonic inclusions. These include slabs of metagraywacke, lenses 142 I exhibiting pinch-and-swell structure, and boudins. Tectonic inclusions range in size from lenticular frag ments a few inches in length (Plate 17) to monoliths with diameters of several hundred feet in plan view (Plate j 45). Highly irregular layers of sheared and crushed argillite, ranging in thickness from a fraction of an inch to 3 feet, normally separate tectonic inclusions. Dickinson (1966a, p. 462), who mapped highly deformed Pranciscan rocks in the Table Mountain area (Pig. 2), applied the term "tectonic breccia" to diverse lenticular blocks and reserved the term "cataclasite" for interven ing crushed argillite. Por purely descriptive purposes these terms adequately express the tectonic style of the Pranciscan group in the Stanley Mountain area. A draw back to Dickinson's terminology is a genetic implication: that faulting was the dominant process in deformation of the Pranciscan group. Although numerous slickensided surfaces attest to the frequency of faulting in the Stanley Mountain area, other processes such as penetrative shear ing and folding could also have been instrumental in deforming Pranciscan rocks. In profile, tabular tectonic inclusions of meta graywacke are usually less than 2 feet thick; length is generally less than 10 times the thickness. The slabs terminate abruptly against small faults, and the ends may be either squared or wedge-shaped depending upon the angle 143 Plate 45* Pranciscan metagray wacke exposed as large monolith on east side of Ouyama River canyon. Calif. Rte. 166 in foreground. 144 of intersection between slabs and faults. Plate 46 shows slabs of metagraywacke interlayered with sheared argillite. Figure 8 is an inked drawing enlarged from Plate 46 to highlight the deformational style. Lens-shaped tectonic inclusions of metagraywacke vary in length from a few inches to at least 100 feet. Some tectonic inclusions exhibit pinch-and-swell structure (Plate 47). Joints at right angles to the long axes of many tectonic inclusions are commonly coated with quartz and carbonate. The vein material is sometimes deformed, indicating that the metagraywacke continued to flow after fracture. The writer believes that the joints and veins resulted from extension of originally continuous layers of metagraywacke in a matrix of incompetent argillite during penetrative shearing of the rocks. Lens-shaped tectonic inclusions, which are the products of this style of deformation in the Stanley Mountain area, are considered to be boudins. The consensus of modern opinion regards the origin of boudinage as due to the extension of competent layers within an incompetent medium (Rast, 1956, p. 405). Ramberg (1955» p. 513) believed that tension joints in boudinage function as sinks for diffusable matter. Attenuation at the extremity of boudins is fre quently pronounced in the Stanley Mountain area (Plate 47). Plate 46. Slah-like tectonic inclusions of Franciscan meta graywacke (see Figure 8). Note extension joints transverse to long axes of tectonic inclusions. Outcrop in Aliso Creek 600 feet west of outlier of Santa Margarita Formation exposed in southeast corner of Stanley Mountain area. Grayw acke Argillite Form line Fault Figure 8. Drawing of slab-like tectonic inclusions of Franciscan metagraywacke (see Plate 46). Note exten sion joints transverse to long axes of tectonic in clusions. Plate 47. Boudinage and pinch-and-swell structure in Pranciscan metagraywacke* Transverse veins may be extensional features. Outcrop on southwest bank of Aliso Creek, about 2100 feet southeast of Cuyama River. Foliation. Planar and curved surfaces separating rocks of different composition in the Franciscan group are defined as "foliationThe relationship between folia tion and original bedding is conjectural. Conclusive evidence that foliation parallels bedding could be seen in only one outcrop, a roadcut on California Route 166, where slickensided shear surfaces are parallel to a dis continuous horizon of sole markings (Plate 48) and to crude layering in metaconglomerate. Elsewhere, rare occurrences of argillite chips in metagraywacke represent a turbidite texture (R. A. Bogdanov, oral commun., 1968), but these fragments show no consistent relationship to adjacent shear surfaces. Foliation enclosing lenticular bodies of bedded metachert may either parallel or transect individual chert beds, as illustrated in Plate 23. Foliation is highly variable in aspect. As illustrated in Plate 18, foliation at contacts between monoliths of graywacke and greenstone is irregular. Planar foliation is rarely seen (Plate 46). By far the most common type of foliation is the undulating contact between sheared argillite and boudins, or between argil lite and lenses exhibiting pinch-and-swell structure (Plate 47). These undulating surfaces may be smooth, scarred by slickensides, or smeared with argillite. Where shearing has been particularly intense, the outer 1-5 mm of a tectonic inclusion is a rind of cataclasite Plate 48. Sole markings in Pranciscan metagraywacke. Pencil points to surface of sole markings (grooves) parallel to shear surfaces in dark argillite at right. Markings indicate that bedding is overturned. Road- cut on east side of Calif. Rte. 166 about 8000 feet S15E of Shell Peak. 150 consisting of comminuted inclusion material commingled with argillite. Strike and dip of foliation is highly variable in outcrops showing abundant lens-shaped tectonic inclusions; representative attitudes are difficult to measure. On the geologic map of the Stanley Mountain area, strike and dip of Pranciscan foliation and bedded meta chert do not define a consistent structural grain. Fold axes cannot be inferred from the pattern of these symbols. One hundred measurements of foliation in Franciscan rocks were plotted and contoured on a TT-diagram (Fig. 9a); orientations of 50 metachert beds were plotted and con toured on a separate TT-diagram (Fig. 9b). In structural analysis, foliation is distinguished from metachert beds, because, in some outcrops, foliation truncates these beds. Both TT-diagrams exhibit maxima which are in part co incident and which provide indirect evidence that on a regional scale foliation is generally parallel to bedding. A TT-diagram combining data from foliation and metachert beds exhibits a very poorly defined "b" axis for the Stanley Mountain terrane (Fig. 10a). Penetrative shear. Shear in metagraywacke- argillite sequences Is normally penetrative on the scale of thin sections. Spaced less than 1 mm apart, shear surfaces In tectonic inclusions of metagraywacke are 151 w- N Figure 9. Fabric diagrams of Franciscan foliation and bedded metachert. Figure 9a. TT-diagram for 100 Fran ciscan foliations (lower hemisphere, equal-area net). Contours: 5-3-1 per cent per 1 per cent area. Meso scopic fold axes shown as small "x,s., f Figure 9b. TT-diagram for 50 bedded metacherts. Contours: 7-5-3-1 per cent per 1 per cent area. Mesoscopic fold axes shown as small circles. A N 152 w N B w Figure 10. Fabric diagrams of Franciscan group and Knox ville Formation. Figure 10A. TT-dlagram for 100 Fran ciscan foliations combined with 59 metachert beds (lower hemisphere, equal-area net). Contours: 5-3-1 per cent per 1 per cent area. _b: regional fold axis Inferred from girdle maximum. Mesoscopic fold axes in foliation shown as small f ’x's." Mesoscopic fold axes in bedded metachert shown as small circles. Figure 10B. TT-diagram for 100 Knoxville beds. Contours: 9-7-5-3-1 per cent per 1 per cent area. Point maximum represents homoclinal structure. Mesoscopic fold axes shown as small Mx's." parallel or subparallel to the foliation. Imprint of shear upon metagraywacke evolves through slight cataclasis and rotation of grains (Plate 6) to pronounced cataclasis and incipient segregation of quartzofeldspathic and micaceous minerals (Plate 9). Blake and others (1967, p. 3) have zoned progressive reconstitution of Franciscan metagraywackes into textural zones 1, 2, and 3* Rocks of textural zone 1 show no evidence of cataclasis; rocks of textural zone 2 exhibit distinct cataclastic texture; rocks of textural zone 3 are intensely crumpled fine grained schists. The majority of metagraywackes in the Stanley Mountain area fall into textural zone 2. Though textural zones have been mapped in the northern Coast Ranges (Blake and others, 1967? p. 2), areal changes in reconstitution of metagraywacke in the Stanley Mountain area appear to be unsystematic. Mesoscopic folds. Folds with directly measurable axes were encountered in only 14 outcrops, of which 5 were lenticular bodies of Interbedded metachert and argillite. Sequences of interlayered metagraywacke and argillite exhibited the remainder of the folds. In metacherts, broken chevron folds with amplitudes of less than 2 feet are restricted to discrete horizons and die out rapidly above and below. Antiforms and synforms in metagraywacke- argillite sequences range in amplitude from a few inches to 6 feet. The folds occur in limited structural hori zons; folds do not diminish in amplitude in a particular direction, hut instead terminate abruptly against shear surfaces which constitute regional foliation. As illu strated in Plate 49, these folds tend to be tight with curved axial surfaces. Figure 11, an inked drawing en larged from Plate 49, elucidates the style of one of these folds. Attenuated, lens-shaped tectonic inclusions of metagraywacke delineate the limbs and hinge area. Axes of mesoscopic folds in Franciscan rocks plunge in many different directions and show no tendency to parallel a regional fold axis, b, inferred from a TT-diagram of foliation and metachert beds (Fig. 10a). Faults. In the majority of outcrops, faults transect and disrupt foliation. It is impossible to categorize the faults by magnitude or sense of displace ment, since tectonic inclusions cannot be used as marker horizons. Zones of brecciation vary in width from a few inches to several feet. Fault surfaces are nearly always curved. A single outcrop may exhibit as many as 4 fault surfaces striking in different directions. Joints, normally planar, are so numerous that some outcrops of metagraywacke appear finely faceted. Quartz and carbonate are the usual Joint fillings. In some instances, thin layers of argillite injected along Joints can be mistaken 155 Plate 49. Folded Franciscan metagraywacke and argillite (see Figure 11). Ledge in stream ted about 5600 feet N15W of junc tion of Calif. Rte. 166 and Tepusquet Road. Graywacke Argillite ~c Form line Fault Figure 11. Drawing of mesoscopic fold In Franciscan metagraywacke and argillite (see Plate 49). h ui o\ for foliation. Northwest trending belts of discontinuous serpentinite lenses delineate the only structural grain apparent on a geologic map of Franciscan terrane in the Stanley Mountain area. As discussed previously, the internal structure of the serpentinite and contact relationships with surrounding rocks indicate that the serpentinite has been tectonically emplaced. The steeply dipping serpentinite belts, therefore, probably mark the trace of high angle fault zones. The temporal relation ship between the belts and development of smaller faults visible in single outcrops is unclear. Structure of the Great Valley sequence On the limbs of the Stanley Mountain antiform, the structure of the Great Valley sequence is essentially homo- clinal (Plate 2). Dips generally vary between 20 and 60 degrees, but steepen near some high angle faults. Forma- tional contacts within the Great Valley sequence are un- conformable. Removal of beds at these Mesozoic erosion surfaces has caused pronounced thinning of the Great Valley sequence from northeast to southwest across the crest of the antiform. On the northeast limb of the anti form, open subsidiary folds with wavelengths of more than a mile parallel the inferred axial trace of the antiform. 158 Mesoscopic folds are limited to the Knoxville Formation, in which chert and shale are deformed locally at contacts with greenstone. Unconformities. Great Valley rocks exposed on the northeast limb of the Stanley Mountain antiform attain a maximum thickness of 9600 feet, distributed as follows: Knoxville Formation of Late Jurassic age, 3600 feet; Jollo Formation of Early Cretaceous age, 3000 feet; and Carrie Creek Formation of Late Cretaceous age, 3000 feet. On the southwest limb, however, the Knoxville Formation and Jollo Formation have an aggregate thickness of only 300 feet. Much of the thinning has been accomplished by truncation of strata at unconformities, as map patterns clearly indicate. The base of the Carrie Creek Formation trends generally west to east across the northern part of the Stanley Mountain area and progressively overlaps the Jollo Formation, the graywacke unit of the Knoxville Formation, and the upper part of the greenstone unit. Folds. The undulating outcrop pattern of Knoxville chert reflects open subsidiary folds on the northeast limb of the Stanley Mountain antiform. Local dip reversals in the eastern part of the greenstone unit also suggest the presence of subsidiary folds, which have been generalized 159 in cross section (Plate 2). These folds probably trend west northwest, since deflections of the chert unit are very similar in style to deflections of Oarrie Creek units r'T associated with west northwest trending folds north of the Stanley Mountain area (Hall and Corbato^, 1967). Mesoscopic folds in Knoxville chert and shale are localized along contacts with relatively undeformed green stone. These folds are an isolated phenomenon; most of the cherts and shales along the contacts are undeformed. Axes trend generally east-west and are horizontal or sub horizontal. As shown in Plate 50, folds in chert are gentle asymmetric flexures with amplitudes of a few feet. Shales tend to be tightly folded, the intensity of deformation diminishing away from the greenstone. Local stresses originated when competent greenstone offered resistance to translation of relatively incompetent chert and shale during regional flexural slip folding (Hills, 1963, P. 284). Faults. A north-south trending high-angle fault offsets Knoxville rocks in the northern part of the Stanley Mountain area between Fish Creek and Corral Creek, but does not displace strata of the overlying Carrie Creek Forma tion (Plate 1). The geometry of the Knoxville chert unit provides a clue to the sense of movement; the chert is Plate 50. Gentle fold in steeply- dipping Knoxville chert near contact with underlying green stone. Southwest bank of Pish Creek about 3300 feet S30E of confluence of Pish and Alamo creeks. only a few feet thick on the west side of the fault, but is more than 100 feet thick on the east. This disparity Indicates a large pre-Late Cretaceous vertical component of movement. Franciscan and Knoxville tectonic styles: a comparison In the Franciscan group, contacts between dis continuous rock bodies are shear surfaces which may or may not parallel original bedding. Several sets of high- angle faults, characterized by crush zones and slicken- sided surfaces, displace tectonic inclusions in nearly every outcrop. The structure of the Knoxville Formation Is rela tively simple. Distinctive stratigraphic units, which can be traced for miles are exposed in a north-dipping homo- cline gently warped into open subsidiary folds. Sedi mentary textures are well preserved. In Figure 10, a TT-diagram of Franciscan foliation is compared to a TT-diagram of Knoxville bedding. A girdle maximum of foliation suggests a northwest plunging regional axis, b. Mesoscopic folds are unsystematic in trend and plunge. The absence of a girdle maximum for Knoxville bedding is a reflection of homoclinal structure. Mesoscopic fold axes plunge generally westward at low angles. 162 The contrasting tectonic styles of Franciscan group and Knoxville Formation, and the contrasting mineralogy, imply as has petrographic evidence that the contact between the two units is either unconformable or tectonic. The possibility that the contact is grada tional, as suggested by Taliaferro (1943a, p. 218), ap pears remote. Evidence supporting the existence of a tectonic contact will be discussed in the following section. Thrust contact between Franciscan group and Great Valley sequence General statement. In the Stanley Mountain area the contact between the Franciscan group and overlying volcanic and sedimentary rocks of the Great Valley sequence is interpreted by the writer as a thrust fault. Formations in thrust contact with the Franciscan group are the Knoxville Formation (Late Jurassic), the Jollo Forma tion (Early Cretaceous), and the Carrie Creek Formation (Late Cretaceous). Folding and erosion of the alloch- thonous Great Valley rocks has exposed the autochthonous Franciscan group as a fenster in the core of the broad northwest-southeast trending Stanley Mountain antiform. A tectonic contact between Franciscan group and Great Valley sequence is indicated by: (1) thickness changes of the 163 Knoxville shale unit and Jollo Formation along the con tact; (2) pronounced brecciation of Great Valley rocks, particularly sandstones and siltstones of the Carrie Creek Formation; (3) isolated occurrences of Franciscan quartz-mica schist along the contact; and (4) discon tinuous lenticular bodies of silica-carbonate rock localized along the contact. Thickness changes of Knoxville and Jollo formations. The shale unit of the Knoxville Formation, in places more than 500 feet thick, is not everywhere present between the overlying Knoxville greenstone and underlying Franciscan group. As clearly shown on the geologic map, the pattern of thickening and thinning is unsystematic. Well layered tuffs and flows in the overlying greenstone unit are conformable with shales; distinctive beds of chert conglomerate and intraformational conglomerate within the shale unit can be correlated across intervals where green stone rests directly on Franciscan metagraywacke. Hence, neither intrusion of greenstone nor buttressing of Knox ville sediments against Franciscan basement could have caused the pronounced thickening and thinning of the shale unit. The alternative explanation is that the contact between shale unit and Franciscan group is a fault locally oblique to bedding (Plate 2). In places, the fault passes upward through the shale unit into the greenstone. This 164 fault is exposed in a small outcrop about 3000 feet south east of Stanley Mountain (Plate 51). Figure 12 is an inked drawing enlarged from Plate 51. A gouge zone 6 feet in thickness separates tightly folded Knoxville shale from massive Franciscan metagraywacke. Embedded in the gouge is a lens of slickensided metagraywacke dipping 45 degrees northward beneath the Knoxville. Scattered patches of Jollo Formation consisting of siltstone with lenses of conglomerate crop out between the Franciscan group and Carrie Creek Formation along the southwest limb of the Stanley Mountain antiform. Contacts between the patches of Jollo Formation and overlying Carrie Creek Formation are poorly exposed, and structural data are scarce. Three possible explanations for erratic distribution of the Jollo Formation are: (1) that Jollo sediments were deposited in isolated pockets on an erosional surface of high relief; (2) that Jollo sediments were locally removed by erosion prior to deposition of the Carrie Creek Formation; and (3) that a low angle fault truncated the contact between Jollo and Carrie Creek formations, bringing both units into contact with the Franciscan group. Changes in thickness of the Jollo Formation along the contact with Franciscan rocks are abrupt (Plate 2, sections B-B1 and 0—0*). For example, more than 200 feet of siltstone and conglomerate crop out on one side of a Plate 51* Thrust contact between Knoxville shale (dark rock, left center) and Francis can metagraywacke (light rock, lower right). Sheared lens of metagraywacke in fault zone at center. Embankment is 4 feet high (see Figure 12). Roadcut in jeep trail about 4000 feet S65E of Stanley Mountain. C OV E RE D COVERED Knoxville shole s Knoxville graywacke Franciscan metagraywacke Gouge ■ ■ n Form line Trace of shear surface Figure 12. Drawing of thrust contact between Kr.cxville Formation and Franciscan group (see Plate 51). 166 167 canyon, but do not appear on the opposite side, where Carrie Creek sandstones rest directly upon Franciscan rocks. If an erosional surface of high relief existed during initial Jollo sedimentation, this surface should be expressed at the unconformable contact between Jollo Formation and Knoxville Formation exposed in the north western part of the Stanley Mountain area. This contact is fairly well exposed, and the trace of the contact shows no marked irregularities. Pronounced thickening and thinning of the lower part of the Jollo Formation, there fore, does not appeaij? to be a primary sedimentary feature. An alternative explanation for erratic distribution of Jollo rocks along the Franciscan contact is that erosion locally removed the Jollo Formation prior to deposition of the Carrie Creek Formation. Marked angular discordance between the two Cretaceous formations would best account for variations in thickness of Jollo sediments. In the northwest part of the Stanley Mountain area, however, the Jollo and Carrie Creek formations are structurally con cordant; a regional overlap suggests that the contact is a subtle angular unconformity. A contact of this nature would not cause pronounced thickening and thinning of a formation over distances of a few hundred feet. A low angle fault contact between Great Valley rocks and Franciscan group best explains the patchy distribution of the Jollo Formation above that contact. 168 In two localities, Jollo beds can be observed dipping into the contact. Brecclatlon of Great Valley sequence. The tectonic style of Great Valley rocks at the Franciscan contact tends to differ from the tectonic style of rocks structurally higher In the Great Valley sequence. Closely spaced (1-2 inches) slickensided surfaces commonly inter lace sandstones cropping out along the contact. Esti mated thicknesses of brecciated zones range from 20 to 100 feet. Within these zones, intensity of deformation de creases away from the contact. Brecciated Great Valley rocks are restricted to the Franciscan contact, but not all parts of the contact are associated with brecciated Great Valley rocks. The overall appearance of Knoxville graywacke Interbedded In the shale unit changes as the Franciscan contact is approached. Laminations and pebbly layers are conspicuous sedimentary structures in graywacke far removed from the contact. Within 30 feet of the contact, however, fractures clearly offset the laminations and transect individual pebbles. At the contact, where shear surfaces are spaced a few inches apart, sheared pebbles are the only traces of the original sedimentary fabric. Plate 52 shows a Knoxville fault breccia partially cemented by calcite. Plate 52. Brecciated Knoxville graywacke about 6 feet above Franciscan thrust con tact. Outcrop is located about 4000 feet S35W of Stanley Mountain. 170 Sandstones of the Carrie Creek Formation tend to be evenly bedded (1-4 feet) or massive, as illustrated in Plate 42. At the Franciscan contact, these sandstones are brecciated (Plate 53) in a style identical to the deforma tion of Knoxville graywacke described above. Jumbled masses of shattered and slickenslded Carrie Creek sand stone form small talus piles at the base of some outcrops. Carrie Creek sandstones vary in color with increas ing brecclation from yellow-brown to grayish yellow or even gray. To test whether sandstones have undergone chemical as well as physical reconstitution, a group of 9 normal samples and a group of 9 brecciated samples were examined in thin section. The brecciated samples were cut by shear planes generally a few millimeters apart and transected by irregular networks of quartz- and calcite veins. Compositions of the 18 sandstones are listed in Table 6. The 9 normal samples contain an average of 8 per cent chlorite, whereas the 9 brecciated samples contain an average of 14 per cent. Of the 9 brecciated sand stones, 6 samples have higher chlorite contents than the maximum amount (13 per cent) observed in any one unsheared sample. Chlorite percentages in the two groups of samples were evaluated by the Mann-Whitney U test, a nonparametric statistical test (Siegel, 1956, p. 119) • The data support the hypothesis that brecciated sandstones will contain more chlorite than the normal sandstones. Stated dif- Plate 53. Brecciated Carrie Creek sandstone about 10 feet above Franciscan thrust contact. Out crop is located on west bank of Aliso Creek about 900 feet north west of outlier of Santa Margarita Formation near southeast corner of Stanley Mountain area. Compare with Plate 42. 172 ferently, the probability that brecciated and normal samples will have similar distribution of chlorite is equal to, or less than, one per cent. The color transi tion from yellow-brown to gray with increasing brecciation appears to be a function of increasing chlorite content. In the Carrie Creek Formation of Late Cretaceous age, siltstones far removed from the Franciscan contact are evenly bedded. Plate 54 shows a sequence of hard, siliceous, fine-grained sandstone beds, 1-6 inches thick, intercalated with thin siltstone layers, which tend to part along micaceous laminae. In contrast, siltstones at the Franciscan contact are highly deformed. Plate 55 depicts tightly folded, lenticular siltstone, which breaks into disc-shaped, plate-sized fragments smoothed and I p ; polished by slickensides. Gradations from evenly bedded siltstone to contorted siltstone have not been observed, due to poor exposure. Siltstones of contrasting tectonic styles are identical in composition, and the contorted siltstone occurs only along the Franciscan contact, Cataclastic deformation of Jurassic-Cretaceous rocks of the Great Valley sequence indicates that the contact between Great Valley rocks and Franciscan Group is tectonic. Deformed sandstones and siltstones are limited to a zone 20-100 feet thick at the base of the thrust plate of Great Valley rocks. Local shear stresses, which caused brecciation, originated from friction along the thrust Plate 54. Carrie Creek Formation, showing interbedded fine-grained sandstone (light gray) and mica ceous siltstone (dark gray) several hundred feet above Fran ciscan contact. Ledge is in ravine about 7500 feet N60E of Shell Peak. Plate 55. Sheared and contorted Carrie Creek siltstone approximately 20 feet above Franciscan thrust contact. Compare with Plate 54. Outcrop is at bottom of deep ravine about 10,600 feet N40E of confluence of Cuyama River and Aliso Creek. 175 | j surface separating the Great Valley sequence from autoch thonous Franciscan rocks. I i I t Breccia zones marking the base of thrust plates are well documented in geologic literature although not all low angle thrust faults exhibit breccia zones. The thick ness of a breccia zone indicates neither the magnitude of displacement nor the thickness of the thrust plate. Isolated occurrences of Franciscan quartz-mlca schist. Broken slabs of banded quartz-mica schist con taining traces of glaucophane crop out in six locations along the contact between Great Valley sequence and Franciscan group. The schist does not occur elsewhere in the Stanley Mountain area. Four of the localities are on the southwest limb of the Stanley Mountain antiform. The quartz-mica schist parts readily along 1-5 mm bands of white mica interlayered with 10-20 mm bands of white quartz, and is easily distinguished from darker colored tectonic inclusions of glaucophane schist. Isolated occurrences of quartz-mica schist at the contact between Great Valley sequence and Franciscan group must be considered fortuitous if the contact is regarded as depositional. On the other hand, if the contact is regarded as tectonic, the distribution of quartz-mica schist can be directly related to processes by which the 176 schist may have been formed and emplaced. The schist exhibits textural and mineralogical similarity to parts of the South Fork Mountain Schist of northern California described by Blake and others (1967, p. 3, 4-). They considered the South Fork Mountain Schist to be a meta- morphic facies of the Franciscan, formed in response to tectonic overpressures developed beneath a thrust plate of Klamath terrane. The isolated blocks of quartz-mica schist in the Stanley Mountain area may be tectonic rem nants of a more extensive body of schists developed be neath the thrust plate of Great Valley rocks. Continued movement of the thrust plate brecciated the schist zone and incorporated the resultant fragments into a shear zone or “friction carpet" beneath the fault surface. Presence of altered and unaltered ultramafic bodies along the Franciscan-Great Valley contact. Ultramafic bodies have been reported along thrust surfaces in many parts of the world (Heard and Rubey, 1966, p. 752). In the Stanley Mountain area, bodies of serpentinite, silica-carbonate rock, and pyroxenite occur discontinuous- ly along the contact between Franciscan group and Great Valley sequence. Hydrothermal alteration of serpentinite forms silica-carbonate rock, which generally consists of quartz, chalcedony, magnesite, and calcite. Formation of silica- carbonate rock is related to neither the original process of serpentinization nor to the emplacement of serpentinite. A body of silica-carbonate rock simply Indicates the existence of serpentinite during some interval of the geologic past. Lenticular bodies of silica-carbonate rock occur discontinuously along the trace of the contact be tween Franciscan group and Cretaceous rocks of the Great Valley sequence; these occurrences are limited to the southwest limb of the Stanley Mountain antiform. Only a few of the lenses are large enough to appear on the map. Small patches of silica-carbonate rock also delineate the trace of a high angle fault which parallels the East Huasna fault and truncates parts of the contact. Ser pentinite in belts within the Franciscan terrane has not been altered to silica-carbonate rock. The belts trend beneath rocks of the Great Valley sequence without off setting them. The distribution of silica-carbonate rock along the trace of the contact between Franciscan group and Great Valley sequence could be explained by; (l) deposition of Cretaceous rocks on the Franciscan group, with the present trace of the eroded contact coinciding by chance with a serpentinite belt altered to silica-carbonate rock; or (2) emplacement of serpentinite (subsequently altered to silica-carbonate rock) as fault slices during thrusting of the Great Valley sequence over the Franciscan group. 178 If the contact between Franciscan group and Great Valley sequence is depositional, lenses of silica- carbonate rock along the trace of the contact ought to have the same orientation as lenses of unaltered ser- pentinite far removed from the contact. Relationships be tween silica-carbonate rock and topography indicate that the lenses dip southwestward, probably less than 4-5 degrees. The largest body of silica-carbonate rock, cropping out over an area of about 25.acres, may be seen in profile looking north from California Route 166. As illustrated in Plate 2, section C-C1, the silica-carbonate rock dips at a low angle beneath Cretaceous rocks of the Great Valley sequence. In the southeastern part of the Stanley Mountain area, a lens of silica-carbonate rock exhibiting a linear outcrop pattern is exposed in the southwest bank of Aliso Creek. The southwest dip of the lens may be seen in the sides of small gullies. Lenses of unaltered serpentinite enclosed in Franciscan metagray- wacke and associated rocks tend to dip vertically or steeply to the northeast. If the contact between Francis can group and Great Valley rocks is considered deposition al, the distribution of silica-carbonate rock must be explained by the following sequence of events: (1) deposition of Cretaceous rocks upon a Franciscan erosion surface exposing serpentinite belts, one of which dips southwestward; (2) hydrothermal alteration of only the 179 southwest dipping belt of serpentinite; and (3) erosion of the contact such that the present trace of the contact exactly coincides with the belt of hydrothermally altered serpentinite (silica-carbonate rock). The writer favors the more simple explanation that serpentinite was em- placed as fault slices during thrusting of the Great Valley sequence over the Franciscan group, A thrust origin for the serpentinite from which silica-carbonate was derived explains why lenses of silica-carbonate rock dip at relatively low angles. Emplacement of thrust slices post-dated emplacement of the serpentinite in belts within the Franciscan terrane, since these belts do not offset the Great Valley sequence. In the southeast corner of the Stanley Mountain area, a relatively large patch of silica-carbonate rock -sits on a hilltop encircled by Franciscan terrane. The silica-carbonate rock exhibits relict shear surfaces which are distinctly shallow-dipping, though strike is erratic. In contrast, slickensided surfaces in the fresh ser pentinite of interior belts dip at steep angles. The flat- lying attitude of the isolated hilltop patch of silica- carbonate rock suggests that the rock is an erosional rem nant of a thrust slice, which was emplaced as serpentinite and subsequently altered. Nearby, a small klippe of Carrie Creek sandstone indicates that the projected thrust surface is locally coincident with the present erosion surface of Franciscan rock. Small patches of silica-carbonate rock mark the trace of a high angle fault parallel to the East Huasna fault. Since the traces of the two faults intersect at a low angle north of the Stanley Mountain area, they are probably contemporaneous branch faults. The East Huasna fault truncates strata of late Miocene age. Silica- carbonate rock, though extremely brittle, is not brec- ciated along the high angle fault. Perhaps serpentinite was emplaced as high angle fault slices and subsequently altered. If so, alteration of serpentinite to silica- carbonate rock was a post-late Miocene event. Presumably, alteration of serpentinite along the contact between Franciscan group and Great Valley sequence occurred at the same time. Isolated, unaltered, thrust slices of serpentinite also occur along the contact between Carrie Creek Forma tion and Franciscan group on the northeast limb of the Stanley Mountain antiform. Two of these are well exposed in a roadcut on California Route 166. Slickensided sur faces in the serpentinite and foliation in the metagray- wacke dip 50 degrees toward the northeast (Plate 31). The pyroxenite cropping out in a small fenster between Shell Peak and the Cuyama River may also be a thrust slice. 181 Summary of thrust fault relationships. Diverse field evidence indicates that the contact between Francis can group and Great Valley sequence is tectonio. The fault has sliced out lower units of the Knoxville Forma tion and parts of the Jollo Formation, and caused pro nounced brecciation of overlying sandstones and sllt- stones. Isolated blocks of quartz-mica schist along the sole of the thrust may be relicts of a more extensive zone of recrystallization underlying the thrust plate. Thrust slices of serpentinite, altered to silica-carbonate rock, are exposed at the contact on the southwest limb of the Stanley Mountain antiform. Isolated lenses of fresh ser pentinite are thrust slices on the northeast limb. Geometry of thrust fault The contact between Franciscan group and Great Valley sequence is a broadly folded thrust surface which defines the shape of the Stanley Mountain antiform (Plate 2). The attitude of the thrust surface was directly measured in four localities on the northeast limb of the antiform. Foliation in Franciscan metagraywacke and ser pentinite lenses in the thrust zone dip 45 to 54 degrees northeastward, but the trace of the thrust contact on topography indicates that the fault locally dips at lower angles. On the southwest limb of the antiform, the trace 182 of the thrust contact forms pronounced "V's" which-denote dips consistently less than 25 degrees. The Great Valley sequence thins appreciably from northeast to southwest across the Stanley Mountain anti form. The Knoxville Formation of Late Jurassic age is 3600 feet thick on the northeast limb, but is less than 100 feet thick in two small patches on the southwest limb. On the northeast the Jollo Formation of Early Cretaceous age is 3000 feet thick, but to the southwest the formation occurs in small patches less than 200 feet thick. To interpret the angular relationships between the thrust surface and strata of the Great Valley sequence, it is necessary to determine whether the major cause of thinning was tectonic or due to removal of section by erosion be neath Mesozoic unconformities. The thrust fault truncates at least 500 feet of Knoxville Formation, since in places the shale unit is missing and greenstones rest directly on Franciscan meta- graywacke. The thickness of greenstone removed by fault ing is indeterminate due to the absence of consistent marker horizons. In the northwestern part of the Stanley Mountain area, the Carrie Creek Formation of Late Cretaceous age rests unconformably upon the Jollo Formation. To the southeast, near the Cuyama River, the Carrie Creek Forma tion rests unconformably upon greenstone. Farther to the 1 183 southeast, the Carrie Creek Formation rests unconformably upon a small patch of Knoxville shale unit in thrust con tact with the Franciscan* More than 6000 feet of Great Valley strata, therefore, were removed by erosion prior to the deposition of the Carrie Creek Formation. Erosion, rather than faulting, was the dominant process responsible for thinning of the Great Valley sequence in the Stanley Mountain area. Structure section A-A1 (Plate 2) best illustrates the relationship between thrust fault and stratigraphic units of the Great Valley sequence. Due to the uncertain nature of a contact between the Jollo Formation and a small patch of Knoxville shale unit on the southwest limb of the antiform, two interpretations of section A-A* are presented. Both interpretations show that units within the Great Valley sequence are overlapped by unconformities, and that the folded thrust surface truncates Knoxville strata at a low angle and intersects these unconformities, bringing all formations of the Great Valley sequence in tectonic contact with the Franciscan group. The only dif ference between the two interpretations is that one treats the Jollo-Knoxville contact on the southwest limb of the antiform as an unconformity, whereas the other treats the small body of Knoxville shale as a thrust slice. 184 Time of thrust faulting The time of thrust faulting in the Stanley Mountain area can he bracketed by determining the age of the young est formation of the Great Valley sequence and the age of the oldest unit deposited upon both the Great Valley sequence and Franciscan group. The Carrie Creek Formation of Late Cretaceous age is the youngest stratigraphic unit in thrust contact with the Franciscan group. Crandall (1961) collected foraminifera of Campanian age from rocks correlative with the Carrie Creek Formation. The oldest Tertiary unit resting unconformably upon the Carrie Creek Formation is the Sespe Formation. In Ventura County the type section of the Sespe Formation yields a mammal as semblage ranging in age from late Eocene to early Miocene (Savage, Downs, and Poe, 1954, p. 43). In the Santa Ynez Mountains of Santa Barbara County, the Sespe grades laterally into a marine formation containing fossils, of Oligocene age (Hall and Corbato, 1967, p. 567). The Sespe Formation in the Stanley Mountain area is correlative with the Lospe Formation in the Santa Maria basin, as described by Woodring and Bramlette (1950, p.. 13). The Lospe rests unconformably on the Franciscan group. Since the Sespe and its equivalent rest unconformably upon both the Great Valley sequence and Franciscan group, the Sespe Formation must be autochthonous. Hence, in the Stanley Mountain area 185 thrusting was a post-Cretaceous (Campanian) to pre- Oligocene event. Huasna Syncline and East Huasna Fault The northwest trending East Huasna fault forms the ■boundary between the Stanley Mountain antiform arid Huasna syncline, which is actually a pair of doubly plunging en echelon synclines with an associated anticline (Hall and Corbato/, 1967* p. 576). Only the northeast limb of the syncline, cut by the East Huasna fault, is exposed in the Stanley Mountain area. Sedimentary rocks of Oligocene and Miocene age generally strike parallel to the fault and dip 30 to 75 degrees. An open subsidiary anticline near the Cuyama River trends parallel to the fault. Tertiary rocks in the northeast limb of the Huasna syncline rest unconformably on Cretaceous rocks of the Great Valley sequence, but overlap Franciscan rocks to the southwest; the contact between Cretaceous and Franciscan group is buried beneath Tertiary sediments. The East Huasna fault is a prominent physiographic as well as structural boundary. Tertiary sediments on the southwest side support steep narrow ridges, whereas Mesozoic rocks to the northeast uphold smooth rolling hills. Alignment of ravines and saddles marks the fault trace, which is generally straight. According to Taliaferro (1943c, p. 443), the Bast Huasna fault Is a high angle reverse fault which developed in late Pliocene time. He believed that irregularities in Tertiary sediment thick ness near the basin margin determined the position of folds. Tightening of the folds initiated high angle faulting. The East Huasna fault has been mapped over a distance of 25 miles (Jennings, 1958). Its southeastern end is near the Cuyama River. In the Stanley Mountain area the Bast Huasna fault exhibits apparent vertical displacement of at least 1000 feet. Evidence for determining the absolute sense of movement is tenuous. Hall and Corbato/ (1967, p. 576) cite stratigraphic and structural relationships supporting strike-slip displacement. Facies are different on op posite sides of the fault, and traces of minor fold axes along the fault show right drag. A high angle fault, parallel to the East Huasna fault, cuts the southwest limb of the Stanley Mountain antiform and shows an ap parent vertical displacement of at least 500 feet. Slices of Franciscan metagraywacke and silica-carbonate rock crop out along the fault trace. Surface traces of the two faults intersect at a low angle in the Uipomo quadrangle; the faults are probably contemporaneous. 187 GEOLOGIC HISTORY Age of Franciscan Group The age of the Franciscan group in the Stanley Mountain area is unknown. Taliaferro (1943a, p. 197) and Easton and Imlay (1955» P* 2337) assigned a Late Jurassic (Tithonian) age to Franciscan rocks, hut the present study has shown that fossiliferous strata reported by earlier workers are part of the Knoxville Formation. Elsewhere in the Coast Ranges of California, Franciscan Rocks have yielded fossils ranging in age from Late Jurassic to Late Cretaceous (Bailey and others, 1964, p. 113-122). Foraminifera from limestones in the San Francisco area have afforded most of the diagnostic age determinations (Graham, 1962, p. 100). Megafossils are rare. From a locality in the northern Coast Ranges, Ghent (1963, p. 41) collected an Early Cretaceous pele- cypod, Buchia crassicollis, coated with a fine-grained matrix containing lawsonite. Buchia piochii, a pelecypod of Late Jurassic age, has been found in Franciscan rocks of the northern Coast Ranges. Bailey and others (1964, p. 141) speculated that the relatively high detrital K-feldspar content of Fran ciscan rocks of the Hacimiento block may indicate a mid- Cretaceous age for these rocks. They drew an analogy be tween the Franciscan group and rocks of the Great Valley 188; sequence, in which average detrital K-feldspar content of graywackes increases markedly with decreasing age of deposition. Though K-feldspar content may be a useful j indicator of age in the Great Valley sequence, Gluskotter (1964, p, 343) has questioned the applicability of this criterion to Franciscan graywackes. He emphasized that orthoclase can be a mobile secondary mineral, and observed that in some samples orthoclase replaces matrix and detrital albite. Staining procedures outlined by I Bailey and others (1964, p, 139) would not differentiate detrital from secondary K-feldspar. It may be premature, therefore, to assign a mid-Cretaceous age to Franciscan rocks of the Nacimiento block on the basis of K-feldspar content. Until diagnostic fossils are found, these rocks must be considered as Jurassic and/or Cretaceous in age. Environments of Deposition of Franciscan Group and Great Valley Sequence Prior to the discovery that the Franciscan group and Great Valley sequence are age equivalent, geologists believed that sites of deposition of Franciscan and Great Valley sediments were coincident; Great Valley rocks were simply deposited upon eroded Franciscan basement. Citing angularity of detrital grains and scarcity of fossils, Davis (1918, p. 29) proposed that the Franciscan group is 189 a continental-shallow marine deposit derived from granitic highlands. Reed (1933, P« 93) suggested that the Francis can was deposited in basins similar to those existing in early Tertiary time. Taliaferro (1943a, P* 213) believed that Franciscan rocks accumulated in a geosyncline occupying the present site of the Coast Ranges. Sediments were derived principally from a western sialic landmass and deposited upon the Sur series, a basement complex of metamorphosed Paleozoic rocks. The realization that the Franciscan group and Great Valley sequence are contemporaneous engendered the problem of relating the two depositional environments (Fig. 3). Geologists could no longer assume that Great Valley rocks were deposited upon eroded Franciscan base ment. Irwin (1957, p. 2295) postulated that Great Valley sediments were accumulating on a uniformly subsiding continental shelf or slope, while the Franciscan was being deposited along a tectonically active, volcanic belt to the west. Drake, Ewing, and Sutton (1959, p. 180) refer red to the Franciscan group as a deep water, eugeosyn- clinal assemblage laid down at the base of a continental slope. They noted that the association of spilites with radiolarian cherts and pillow lavas indicates a submarine environment. Dietz (1963, p. 329, 332) suggested that “Franciscan-like" geosynclinal sediments can be deposited as a continental rise at the base of the continental 190 slope. In support of turbidity current deposition of Franciscan sediments in a deep water environment, Bailey and others (1964, p. 36) cited angular clastic grains in abundant matrix; lack of large-scale cross-bedding and ripple marks; and absence of indigenous shelly fauna. Relict sedimentary textures in Franciscan metagray- wacke of the Stanley Mountain area, and the association of metagraywacke with greenstone and radiolarian-bearing metachert, point toward turbidity current deposition in a deep water environment. Detrital grains are angular, poorly sorted, and embedded in a matrix which can account for as much as 40 per cent of the metagraywacke. Locally abundant argillite chips appear to be remnants of con volute bedding (N. A. Bogdanov, oral commun., 1968). Sole markings have been observed in a single locality (Plate 48). The assemblage of blueschlst minerals in the Fran ciscan group suggests that a tectonic model for the environment of deposition allow the base of the sedimentary pile to reach a depth of at least 20 km. As pointed out by Bailey and others (1964, p. Ill), part of this depth may be Initial owing to deposition in an oceanic deep, part may represent thickness of sediments, and part may represent downbuckling. Two environments favoring ac cumulation of thick sedimentary sequences are: (1) a continental rise (Dietz, 1963, p. 329); and (2) an oceanic trench (Ernst, 1965» p. 908). These models are illustrated in Figures 13 and 14, respectively. Relating these models to the geologic history of the Coast Ranges, the Great Valley sequence would represent a continental shelf deposit east of the continental rise (Fig. 13)» or a deposit on the continental side of an oceanic trench (Fig. 14). Radiometric dates indicate that hurial metamorphism was a latest Jurassic-earliest Cretaceous event (Lee and others, 1964, p. 105). Evidence has also been cited for blueschist metamorphism in the northern Coast Ranges in response to what Blake and others (1967, p. 7) have interpreted as tectonic overpressures generated by thrust ing of the Great Valley sequence during Late Cretaceous time. The writer believes that Blake and others have presented a strong case for tectonic control of blueschist metamorphism in the northern Coast Ranges, but finds no evidence that blueschist metamorphism in the Stanley Mountain terrane was directly related to overthrusting of the Great Valley sequence; grade of metamorphism does not increase upward toward the base of the thrust plate. The writer favors a trench model for Franciscan deposition be cause active downbuckling of the trench would create a high pressure, low temperature environment for burial metamorphism of the blueschist type. Vesiculated greenstones, accounting for less than 10 per cent of all greenstones in the Franciscan, Continental Rise Continental Shelf Slope ^ r sea level GREAT VALLEY SEQUENCE F R A N C IS C / GROUP CONTINENTAL* CRUST OCEANIC CRUST SC A L E 5 Miles MANTLE Horizontal and vertical Figure 13. Idealized vertical section across Mesozoic continental margin, showing Franciscan group as a continental -rise deposit and Great Valley sequence as shelf deposit. 192 Oceanic Trench sea level GREAT VALLEY SEQUENCE ^ A FRANCISCAN /G RO UP A zeolite / f prehnite-pumpellyite ✓ greenschist /i ’ ,O V ' amphibolite * % blueschist MANTLE • • -eclog ite SCALE 10 Miles Horizontal and vertical Figure 1A, Idealized vertical section across Mesozoic continental margin, showing Franciscan group as an oceanic trench deposit (modi fled after Ernst, 1965, p. 908). 193 194 group, indicate that seamounts may have risen from parts of the trench in which sediments accumulated. According to Moore (1965* P» 43) the presence of vesicles indicates that lavas were extruded at depths of less than 2 miles. Bailey and others (1964, p. 123-124) considered the Great Valley sequence of Late Jurassic and Oretaceous age to he a miogeosynclinal assemblage deposited on the continental shelf and slope. Rocks of the Great Valley sequence have an aggregate thickness of more than 45,000 feet. In general, the Great Valley sequence contains more fine-grained rocks than the Franciscan, more conglomerate, and many more fossils. Sandstones of the Great Valley sequence are more uniformly and more evenly bedded than metagraywackes of the Franciscan. Based on the foregoing comparison, the writer agrees that Great Valley rocks represent a more stable environment of deposition than Franciscan rocks, but hesitates to apply the term "mio- geosyncllnal, , to Great Valley rocks in the Stanley Mountain area. Here, the Knoxville Formation contains appreciable thicknesses of submarine lava flows and r.adiolarlan cherts. Similar lithologies are found in the Knoxville elsewhere in the Coast Ranges (Taliaferro, 1943a, p. 198; Bailey and others, 1964, p. 125; McKee, 1966). Perhaps Knoxville rocks were deposited on the continental flank of an oceanic trench, where water depths fluctuated dur ing Late Jurassic time. The size (0.5-2 mm) and percent- 1951 ages (5-10 per cent) of vesicles in Knoxville greenstones indicates that lavas were extruded at depths of less than one mile (Moore, 1965» p. 4-3); the occurrence of in situ pelecypods in Knoxville graywacke also suggests depo- i sition in relatively shallow water. In contrast, Knox ville radiolarian cherts probably accumulated at depths of more than two miles, where calcium carbonate dissolves in sea water. During deposition of Cretaceous rocks of the Great Valley sequence in shallow water, volcanic activity was confined to the trench axis where Franciscan sedi ments were accumulating. Pre-Oligocene Deformation and Metamorphism of Franciscan Group The Franciscan group in the Stanley Mountain area and coastal parts of the Nacimiento block consists of a chaotic mixture of tectonic inclusions embedded in sheared argillite. Several hypotheses have been advanced to ex plain the erratic distribution of exotic blocks and the apparent structural confusion in many Franciscan terranes. Brown (1964, p. 11) interpreted chaotically deformed Franciscan rocks as a "friction carpet" beneath a thrust plate of Great Valley sequence. Along the Stony Creek fault zone in the northern Coast' Ranges, the friction carpet consists of masses of intensely altered volcanic rock, glaucophane schist, conglomerate, and graywacke. Dickinson (1966a, p. 470) remarked that Franciscan "tectonic breccia" near Table Mountain (Fig. 2) may pos sibly have been formed by repeated and ancient movements along .the adjacent San Andreas fault system. Hsu (1966, 1967) drew analogies between chaotically deformed parts of the Franciscan group and argille scagliose, a term which Apennine geologists apply to gravity slide masses thought to have been displaced several miles from their original depositional sites. Page (1966, p. 260) speculated that some large-scale submarine sliding may have occurred, but stressed that the chaotic appearance of Franciscan rocks resulted mainly from overprinting of several tectonic events following depositional disturbances. As applied to the Stanley Mountain area, the foregoing explanations for the chaotic appearance of the Franciscan group can be evaluated in light of field and petrographic studies. If the highly sheared, chaotic assemblage of Franciscan rocks in the Stanley Mountain area represents a "friction carpet" beneath a thrust plate of Great Valley sequence, the intensity of deformation should increase downward from the thrust contact. Brown (1964, p. 11) remarked that the "friction carpet" of Franciscan rocks -beneath the Stony Creek thrust fault in the northern Coast Ranges grades downward through 800 feet into relatively undeformed sedimentary rocks of the lower plate. Grada tional relationships of this type are not evident in the Stanley Mountain area. Tectonic inclusions of glaucophane schist occur more frequently along the contact than else where, and blocks of quartz-mica schist are found only at the contact, but the style of deformation of Franciscan rocks does not change systematically with respect to the thrust contact. One might argue that the entire Francis can group represents a complex zone of imbrication, the base of which is not exposed. For example, an imbricate zone several thousand feet thick lies below the Shuksan thrust in the northern Cascades and involves slices of crystalline basement, ultramafics, volcanics, graywackes, and other rocks, some derived from the upper plate (Misch, 1966, p. 123). The base of the Great Valley sequence in the Stanley Mountain area, though locally brecciated, is relatively intact with respect to the underlying sheared Franciscan rocks. Moreover, no slices of Great Valley rocks are mixed with tectonic inclusions. It seems un likely that thrusting of the little deformed Great Valley sequence could cause imbrication of Franciscan rocks through a thickness of at least 1500 feet (the maximum relief of Franciscan terrane). Dickinson (1966a, p. 469) mapped a thrust contact between Franciscan “tectonic breccia" and Panoche Group of Late Cretaceous age on Table Mountain in the Diablo Range and concluded that pas sage ". . .of mudstones in the lower Panoche Group can 1 9 8 hardly be called upon to brecciate underlying argillite, graywacke, and greenstone.1 1 Dickinson (1966b, p. 723) noted'similarities be tween the structural style of deformed rocks in the San Andreas fault zone and the style of Franciscan rocks in Castle Mountain Range, California. Both terranes are pervasively interlaced with closely spaced shear surfaces which separate thoroughly shattered diverse rock types. Dickinson suggested that ancient deformation in the old San Andreas fault zone caused shearing of Franciscan rocks. The writer has observed striking similarities between the structural style of Franciscan rocks in the Stanley Moun tain area and the Portal Schist in the San Andreas fault zone west of Palmdale, California. The Portal Schist, which Evans (1966, p. 16) differentiated from the Pelona Schist on petrologic criteria, is particularly well exposed on Ritter Ridge. Here, sheared mica schist encloses lenses and slabs of more coherent schist and quartz. Evans (1966, p. 100) constructed a TT-diagram of shear foliations related to movements in the San Andreas fault zone. A pronounced point maximum indicates that shear surfaces strike subparallel to the San Andreas fault and dip 65 degrees to north-northeast. In contrast, a TT-diagram of Franciscan foliation in the Stanley Mountain area (Fig. 9a) shows a diffuse girdle maximum which reflects the low- angle dip of many foliations. Though rocks in strike- 199 slip zones and Franciscan group may exhibit similar chaotic aspect in outcrop, structural analysis indicates a funda mental difference in the orientation of foliation. Based on the foregoing comparison, it seems doubtful that strike slip movements alone could have caused chaotic deformation of Franciscan rocks in the Stanley Mountain area. The following descriptions of gravity slide masses in widely separated parts of the world will furnish a basis for evaluating the role of gravity in deformation of Franciscan rocks in the Stanley Mountain area. Allochthonous terrane consisting of chaotic masses of ophiolites and radiolarian cherts embedded in crumpled variegated shales characterizes the foothills structure belt of southeast Turkey (de Righi and Cortesini, 1964, p. 1922). ' ’Free gliding” of an incompetent shaley series in a submarine environment occurred in Late Cretaceous time. Abundant slickensided surfaces in shales and ophiolites indicate that extensive internal differential movements affected the sliding masses. The Danau Formation of probable Mesozoic age in North Borneo is a chaotic assemblage of slickensided radiolarian cherts, serpentinite, peridotite, jadeitite, basalts, spilites, and diabases— all intimately mixed with crumpled flysh-like sandstone and shale. Extrusive rocks are characterized by abundant pumpellyite. Reinhard and Wenk (1951, p. 92) concluded that neither thrusting nor 200 folding could have produced the style of deformation exhibited by the Danau Formation. They suggested that ophiolites penetrated and crumpled unconsolidated sedi ments and/or that massive submarine landslides occurred. Gravity sliding along argillaceous horizons was closely related to basin geometry and cycles of sedi mentation recorded in the Kama Beds of Frecambrian age in South West Africa. According to Korn and Martin (1959, p. 1062), a deformation was strongest where the basin floor was most steeply inclined. Chaotic structural style is restricted to the central part of the basin where beds are torn, dragged, and rolled out. Fold axes are dif ferently oriented at different tectonic levels. Korn and Martin (1959, p. 1073) remarked that structural features resemble those in other folded belts; cleavage planes more or less parallel axial planes, pebbles are elongated either parallel to fold axes or at right angles to them, and thicker beds are sheared and covered with slickensides. The intensity of metamorphism which accompanied deforma tion was low, as shales were changed to slates or phyl- litic shales. Marchetti (1956, p. 211) mapped gravity slides ("olistostromes") in Tertiary rocks of central Sicily. Rootless blocks of sandstone, limestone, dolomite, quartz- ite, and basalt, which range in size from a few cubic inches to more than half a cubic mile, "float" in a matrix 201 of brown shales. Episodes of submarine sliding, flowing, and slumping coincided with well-defined tectonic peaks of Alpine orogeny in late Miocene and Pliocene time. The argille scagliose of the northern Apennine Mountains of Italy is a complex of contorted claystone and abundant exotic rocks varying from pebble size to mountainous masses and sheets many cubic kilometers in volume. Maxwell (1959> p. 2711) reported that Italian geologists have interpreted the argille scagliose as a gigantic submarine gravity slide which moved intermittently northeastward in late Oligocene and Miocene time. Among the exotic rocks are ophiolite (including serpentinite), limestone, sandstones, and granite. Page (1963, p. 666) described the orientation of allochthonous blocks as follows: Discrepancies in attitude between laterally neighboring rock masses are the rule rather than the exception. Blocks of limestone at approximately the same structural horizon show beds dipping in various directions without rhyme or reason, like so many chips of wood floating in a stream. According to Elter and others (1964, p. 3), metamorphism of the Tuscan autochthon exposed in the Apuane north of Pisa was partially due to movement of allochthonous Ligurian complexes. The metamorphic rocks consist of marbles, phyllites, and quartzites. The foregoing descriptions have dealt with chaotic terranes Interpreted by earlier workers as gravity slides. 202 All of these terranes contain abundant crumpled argil laceous sediments mixed with exotic blocks. The writer notes that argillite accounts for less than 5 per cent of Franciscan rocks in the Stanley Mountain area and that discontinuous argillite layers have an average thickness of only 4 inches. This low argillite content is one dif ference between Franciscan terrane and gravity slides of the argille scagliose type. Hsu {1967j P* 286) stated that similarities between Franciscan group and argille scagliose are pinch-and-swell structure and boudinage exhibited by blocks of various lithology and different size embedded in a sheared shaley matrix. He believed that subparallelism of shallow- dipping Franciscan shear planes and bedding planes indicates horizontal tectonic transport, and that local reorientation of shear planes to steep dips during Ceno- zoic deformation misled several authors to postulate vertical tectonic transport of Franciscan rocks. Accord ing to Hsu, Franciscan rocks were involved in two episodes of westward gravity transport. The first took place in Late Jurassic time when Franciscan rocks, cor relative with early Mesozoic rocks (Tuolumne Group and Mariposa Slate), slid westward from the Sierra Nevada. After deposition of Upper Jurassic and Lower Cretaceous formations of the Great Valley sequence upon an eroded Franciscan, renewed gravity sliding in Late Cretaceous 203 time incorporated some fossiliferous Great Valley blocks into a chaotic terrane and transported other Great Valley units as large slabs in "piggy-back1 ' fashion. . Hsu be lieved that high-pressure metamorphism accompanied both episodes of deformation. The writer agrees that the chaotic style of the Franciscan group in the Stanley Mountain area warrants consideration of gravity sliding as a possible mechanism of deformation, but feels that Hsu's working hypothesis for the tectonic history of western California is over simplified. For example, nowhere has a pre-Knoxville un conformity between Franciscan group and Great Valley sequence been conclusively demonstrated. Also, syn- kinematic blueschist metamorphism would require the base of the gravity slide to be depressed to depths of as much as 20 kilometers (Bailey and others, 1964, p. 110). At this depth, gravitational movement accompanied by mixing of superjacent fossiliferous strata is difficult to visualize. The writer's interpretation of the signifi cance of gravity sliding in Franciscan deformation is dis cussed in the following paragraphs. In the Franciscan group, penetrative shear surfaces parallel the margins of tectonic inclusions. Lawsonite is more abundant in highly sheared metagraywackes (samples D-34, D-36, and D-89a, Table 1) than in metagraywackes less thoroughly reconstituted (samples D-28, D-45a). These 204 relationships suggest that pronounced textural and mineral- ogic changes accompanied formation of tectonic inclusions. Chaotic terranes interpreted as gravity slides in other parts of the world have not undergone synkinematic high pressure metamorphism. The slide masses in South West Africa (Korn and Martin, 1959 > p. 1073) and in the Apuane of northern Italy (Biter and others, 1964, p. 3) yield evidence for synkinematic metamorphism— hut at low pressures and temperatures. The high pressures which existed during deformation of Franciscan rocks in the Stanley Mountain area seem difficult to reconcile with gravity transport. Exotic blocks of amphiholite and coarse-grained glaucophane schist in the Stanley Mountain area represent type IV metamorphic rocks (Coleman and Lee, 1963, p. 300), which are rocks of deep-seated origin containing relict mineral assemblages of higher grade than the assemblages in the enclosing metagraywackes. These rocks might be considered analogous to basement blocks of Hercynian granite and gneiss incorporated into the allochthonous scisti galestrini (Lower Cretaceous) of the northern Apennines (Maxwell, 1964, p. 6). The structure and mineralogy of exotic blocks in the Franciscan group, how ever, indicates that they moved upward along steeply dipping shear zones and underwent metamorphism during emplacement (Lee and others, 1966, p. 150). Upward move ment of blocks is not compatible with gravity transport. Hollister and Albee (1966) made a thorough study of minor structures exposed on the broken surface of a house-size block of glaucophane schist near Solvang, California. Based on relationships of internal structure to external form of the block, together with preferred orientation of glaucophane needles, the authors concluded that deforma tion and metamorphism of the block accompanied tectonic transport. Bailey and others (1964, p. 105) pointed out that many tectonic blocks of high grade metamorphic rock exhibit shells of chlorite-actinolite schist in which mineral orientation is parallel to tha margins of the block. Where tectonic blocks are common they are scattered through broad linear belts believed to be major shear zones. It is doubtful that gravity sliding could have fostered the stresses and pressure-temperature fields which are recorded in the sheared and metamorphosed peri pheries of tectonic blocks. In summary, the chaotic structural style of the Franciscan group in the Stanley Mountain area may in part be due to depositional disturbances such as submarine slumping, but the processes of penetrative shearing, syn kinematic blueschist metamorphism, and emplacement of exotic blocks of amphibolite and coarse-grained glauco phane schist cannot be attributed to gravitational move ments of the argille scagliose type. 206 The writer believes that penetrative shearing and blueschist metamorphism accompanied downwarping of Franciscan trench sediments. The shearing produced a chaotic mixture of tectonic inclusions: re-orientation of shear surfaces during subsequent periods of high-angle faulting and folding contributed to the chaotic structural style. Extensional features of tectonic inclusions, a record of synkinematic blueschist metamorphism, and scarcity of fold hinges indicate deep-seated deformation of Franciscan rocks along subparallel shear surfaces. Shearing of Franciscan sediments at the continental margin was probably initiated by underthrusting of the Pacific Ocean basin. The writer believes that westward thrusting of the Great Valley sequence in early Tertiary time was also related to underthrusting of the continent by the ocean basin. Franciscan deformation and thrusting of the Great Valley sequence, however, were discrete tectonic events separated in geologic time by periods of high-angle faulting and folding recorded in Franciscan rocks. The idea of underthrusting at continental margins is not original with the writer. Stille (1955* P» 184) concluded that the geographic distribution of intermediate- and deep-focus earthquakes in the western Pacific defines a zone of underthrusting which comprises movements along many individual surfaces. He believed that oceanic trenches mark the intersection of the thrust zone with the 207 ocean floor. Dietz (1963, p. 332) proposed that down- welling sectors of thermal convection cells initiated underthrusting at the trench axis. He did not believe that collapsing of filled-up trenches was a basic mechanism of continental accretion. The writer disagrees and contends that blueschist belts along the Pacific margin furnish evidence for a mechanism of accretion in which downwarping of trench sediments played a significant role. Prior to, or during, shearing and metamorphism of Franciscan rocks, blocks of amphibolite were emplaced up ward along high angle faults and underwent retrograde metamorphism to the blueschist facies. To date, two periods of blueschist metamorphism have been determined in Franciscan rocks of the California Coast Ranges; latest Jurassic or earliest Cretaceous (Lee and others, 1964, p. 105); and Late Cretaceous (Blake and others, 1967, p. 6). Periods of folding and high-angle faulting, which intervened between penetrative shearing of Franciscan rocks and thrusting of the Great Valley sequence, caused re-orientation of shear surfaces. A poorly-defined regional fold axis, b, plunging approximately 30 degrees N40W can be inferred from a TT-diagram of Franciscan foliation (Fig. 10a); the geometry of the fold(s) cannot be reconstructed. Axes of rare mesoscopic folds plunge in different directions. Folding of the shear surfaces with respect to b was followed by emplacement of serpentinite along faults dipping vertically or steeply to the north east; serpentinite lenses locally truncate foliation. After development of these high-angle faults, thrusting of the Great Valley sequence in post-Cretaceous pre- Oligo'cene time caused some brecciation and tectonic transport in Franciscan rocks immediately beneath the thrust surface, and emplaced thrust slices of serpentinite. Thrusting of the Great Valley sequence, however, did not cause additional penetrative shearing of Franciscan rocks. The thrust fault was subsequently warped into the broad northwest-trending Stanley Mountain antiform. Pre-Oligocene Deformation and Diagenesis of the Great Valley Sequence Formations of the Great Valley sequence have an aggregate thickness of 96OO feet in the Stanley Mountain area (Plate 3). The top of the Carrie Creek Formation of Upper Cretaceous age is not exposed. Unconformities mark the base of the Jollo and Carrie Creek formations, indicating tectonic disturbances at the end of Late Jurassic and Early Cretaceous time. Taliaferro (1944, p. 519) referred to these tectonic events as the Diablan orogeny and mid-Cretaceous disturbance respectively. Graywackes and greenstones of the Knoxville Forma- 209 tion in the Stanley Mountain area contain mineral as semblages diagnostic of the prehnite-pumpellyite facies, which Coombs (i960, p. 339) considered to bridge the gap between zeolite facies and greenschist facies. According to Coombs and others (1959) P* 59) pumpellyite and pre- hnite are associated in volcanic graywackes and tuffs of New Zealand at minimum stratigraphic depths of 23)000 feet. At this depth detrital plagioclase is completely albitized. Since the top of the Cretaceous is exposed in neither the Stanley Mountain area nor Nipomo quadrangle (Hall and Corbato, 1967) p. 565)) the question arises as to whether or not Cretaceous rocks were sufficiently thick to provide the load pressures necessary for the growth of prehnite-pumpellyite facies minerals. Page (1966, p. 261) mentioned a 28,000-foot Upper Cretaceous section exposed west of Merced in the southern Coast Ranges. He noted that Upper Cretaceous rocks form a nearly continuous belt southward as far as Kern County. Load pressures alone, therefore, could have accounted for diagenetic reactions which produced mineral assemblages of the prehnite- pumpellyite facies in Knoxville rocks of the Stanley Mountain area. The age of these reactions can only be approximated; the unconformity between Cretaceous and younger rocks is at the base of the Sespe Formation of Oligocene age. Minerals of the prehnite-pumpellyite facies 210 could have formed during Late Cretaceous time and/or dur ing the early Tertiary prior to stripping of overburden by erosion. Reactions in the Knoxville Formation could have taken place before, during, or after thrusting of the Great Valley sequence. Thrust Faulting Since the Sespe Formation of Oligocene age rests unconformably on allochthonous Carrie Creek Formation in the Stanley Mountain area and on autochthonous Franciscan group to the southwest (Woodring and Bramlette, 1950, p. 13)» thrust faulting must have taken place in post- Cretaceous (Campanian) pre-Oligocene time. Elsewhere in the southern Coast Ranges, thrusting of the Great Valley sequence has been interpreted as a Late Cretaceous or early Tertiary event. Rage (1966, p. 272) concluded that thrust displacement on the Tesla-Ortigalita fault in the Diablo Range took place in the Paleocene and/or Early Eocene. He based his conclusion on the fact that Middle Eocene sediments, which unconformably overlie alloch thonous Upper Cretaceous rocks, contain Franciscan detritus. Also in the Diablo Range, Dickinson (1966a, p. 463) considered thrusting in the Table Mountain area to be a Late Cretaceous and/or Paleocene event. In the Yolla Bolly area of the northern Coast Ranges, Blake and others 211 (1967, p. 6) dated a schist believed to have been formed contemporaneously with thrusting of the Great Valley sequence. Rubidium-strontium isotopic analysis yielded an early Late Cretaceous age. Late Tertiary and Quaternary History Formations of Oligocene and Miocene age rest un- conformably on Mesozoic rocks in the Stanley Mountain area. Most Tertiary strata are confined to the Huasna syncline southwest of the Bast Huasna fault, and a few outliers occur to the northeast of this fault. According to Hall / and Corbato (1967, p. 578), the Nipomo-Santa Maria region was a positive land-mass during most of early Tertiary time. In late Oligocene and early Miocene time the region submerged beneath a shallow sea in which sediments of the Sespe and Vaqueros formations were deposited„ Crandall (1961) suggested that outcrops of Sespe Forma tion represent outliers on a Cretaceous surface, which was subjected to erosion prior to Vaqueros deposition. Local ly, however, the Vaqueros formation rests concordantly upon the Sespe. According to Taliaferro (1943c, p. 444) the Vaqueros sea spread over an area of considerable topo graphic relief. Over most of the southern Coast Ranges, the Miocene began with comparatively uniform sinking, which in the middle Miocene gave way to downwarping of 212 basins and accumulation of great thicknesses of Miocene sediments (Taliaferro, 1943b, p. 155 )• Deposition ap pears to have been continuous within the Huasna basin throughout the Miocene. The Stanley Mountain antiform, Huasna syncline, and East Huasna fault are products of late Pliocene deformation. Compton (1966, p. 1378) con cluded that deformation was deep-seated, perhaps due to northeast to southwest flowage under the crust. In the Stanley Mountain area, as elsewhere in the southern Coast Ranges, elevated Pleistocene and younger stream terraces indicate vertical movement of crustal blocks. Christensen (1965, p. 1113) suggested that these movements represent the last phases of an orogeny in which a period of stress relaxation, intrusion at depth, and uplift follows hori zontal compression. THE FRANCISCAN-SIERRAN BOUNDARY Though the basal portion of the Franciscan group has not been recognized in the Coast Ranges, the presence of tectonic inclusions of serpentine and peridotite sug gests that the Franciscan was deposited directly upon an ultramafic oceanic crust. The Sierran basement complex of metamorphic and granitic rocks is characteristic of continental crust. The contact between Franciscan and Sierran basement, therefore, represents an important boundary between rocks of oceanic and continental affinity. An understanding of the nature of the Franciscan- Sierran boundary provides a basis for interpreting the Mesozoic and Cenozoic tectonic history of western Cali fornia. In the southern Coast Ranges, and in the southern part of the northern Coast Ranges, the Franciscan-Sierran boundary Is buried beneath the Great Valley sequence. Here, the nature of the Franciscan-Sierran boundary must be inferred from contact relationships between Franciscan rocks and Great Valley sequence. In northern California, the Franciscan-Sierran boundary emerges from beneath a cover of Great Valley rocks and becomes the contact be tween Franciscan group of the northern Coast Ranges and crystalline rocks of the Klamath Mountains (Fig. 15). Contact Between Franciscan Group and Great Valley Sequence Geologists have recently interpreted the contact between Franciscan rocks and Great Valley sequence as a large-scale thrust fault or zone which in places has been folded and cut by high-angle faults (Dickinson, 1966a, p. 271). In some instances the thrust fault or zone has been named, for example the Stony Creek fault zone in northern California, whereas in other cases the thrust is unnamed and must be designated by the area in which it was OREGON CALI FORMA Cope Mendocino Son Francisco m . 0 0 M .E3 Los Angeles Y///A Area of exposed or inferred Sierron Y//AA basement {metamorphic-granitic) | Area of exposed or inferred Franciscan basement • • • • • Interred position of Sierran- Franciscan boundary Known thrust contacts between Franciscan group and overlying Great Valley Sequence ® Oso Canyon area © Stanley Mountain area (3) Table Mountain area ® Tesla-Ortigalita fault (5) Stony Creek foult Thrust contact between Franciscan group and Sierran-type basement (6 ) South Fork Mountain fautt ( j ) Sur fault P l g u r e 1 5 . T h r u s t c o n t a c t s b e t w e e n F r a n c i s c a n g r o u p a n d G r e a t V a l l e y s e q u e n c e , a n d b e t w e e n F r a n c i s c a n g r o u p a n d 3 1 e r r a n - t y p e b a e e m e n t i n t h e C o a s t R a n g e s . 215 described. Prom south, to north (Pig. 15)> these faults or areas are: (1) Oso Canyon near the Santa Ynez River, 10 miles north of Santa Barbara; (2) the Stanley Mountain area; (5) the Table Mountain area near Parkfield in the southern Diablo Range; (4) the Tesla-Ortigalita fault in the northern Diablo Range; and (5) the Stony Creek fault zone in the northern Coast Ranges. The Oso Canyon area lies athwart the boundary be tween the east-west trending Santa Ynez Mountains and the northwest trending San Rafael Mountains. Marrow blocks of Franciscan rocks have moved upward along high-angle reverse faults which parallel the east-west trending Santa Ynez fault. Page and others (1951» P* 1778) believed that the Franciscan rocks represent cores of piercement folds highly modified by faulting. In the Oso Canyon area the Espada Formation of Late Jurassic and Early Cretaceous age rests upon unfossiliferous Franciscan rocks. Dibblee (1966, p. 13) interpreted the contact as an unconformity, but George Meyer and William Dickinson (oral commun., 1968) have since re-interpreted the contact as a thrust fault. The base of the Espada is sheared and tightly folded at the contact, and intensity of deformation diminishes up sectiono Conglomerates in the Espada Forma tion lack Franciscan debris, but conglomerates in the Sierra Blanca Limestone of early Eocene age contain abundant Franciscan clasts. Thrust faulting must have 216 taken place in post-Early Cretaceous pre-Eocene time. In the Table Mountain area of the southern Diablo Range, Dickinson (1966a, p. 465) mapped a folded thrust surface along which Panoche strata of Upper Cretaceous age, subparallel to the thrust, were emplaced tectonically above the Franciscan group. Pliocene-Pleistocene reverse and strike-slip faults have displaced segments of the thrust. The Tesla-Ortigalita fault along the east flank of the northern Diablo Range has been interpreted as a large- scale folded thrust which separates Franciscan rocks in the core of the Diablo antiform from peripheral Great Valley rocks (Page, 1966, p. 271). Diapiric movement of Franciscan rocks during Plio-Pleistocene orogeny caused local overturning and fracturing of the thrust surface; rapid lateral and vertical variation in the attitude of the thrust is pronounced (Briggs, 1953, p. 52). According to Page (1966, p. 271), the main Franciscan terrane of the Diablo Range is a fenster encircled by the Tesla- Ortigalita fault (Fig. 15). Minimum displacement of Great Valley rocks is 15 miles. The Stony Creek fault extends north-south along the east flank of the Coast Ranges near the west side of the Sacramento Valley and forms the boundary between Franciscan group and Great Valley sequence (Fig. 15). Slickensided lenses of serpentinite and greenstone occupy the fault zone within a belt 0.5-2.0 miles wide and more than 70 miles long (Page, 1966, p. 270). Strata of the Great Valley sequence steepen toward the belt of ultramafic rocks, which in places encloses small lenses of sedimentary rock that appear to be fault slices (Irwin, I960, p. 62). Brown (1964, p. 9) interpreted the Stony Creek fault to be a thrust fault along which the Great Valley sequence over rode the Franciscan group. Interposed between a klippe of volcanic rock of Late Jurassic-Early Cretaceous age and relatively undeformed Franciscan graywacke is a nearly flat-lying friction carpet of chaotically deformed Francis can rocks. Irwin (1964, p. 7) concluded that structural relations between Franciscan group and Great Valley sequence demand 50 miles of relative westward displacement of miogeosynclinal over eugeosynclinal rocks. Ultramafic rock may represent dislocated parts of a single sheet emplaced along a subhorizontal fault between the two facies. South of Clear Lake, Swe (1968) reported an allochthon of Great Valley rocks composed of three suc cessive thrust plates, which are probably a westward extension of the Stony Creek fault. Contact between Franciscan Group and Sierran-type Basement A granitic-metamorphic basement, herein referred to 218 as Sierran-type basement, is exposed in the Klamath Mountains province of northern California and southwestern Oregon. The boundary between Sierran-type basement of the Klamath Mountains province and Franciscan rocks of the northern Coast Ranges has long been considered a thrust fault (Irwin, I960, p. 59). According to Blake and others (1967* P* 3)» rocks of the Klamath Mountains have been thrust westward, virtually as a single plate, over the Franciscan of the northern Coast Ranges along the South Fork Mountain fault. They noted that blueschist metamorphism of Franciscan rocks increases upward toward the sole of the thrust and attributed the metamorphism to tectonic overpressures associated with Late Cretaceous thrusting. In the southern Coast Ranges, the Sur fault separates Sierran-type basement of the Salinian block from Franciscan rocks of the Macimiento block (Fig. 15). Page (1966, p. 268) has interpreted this boundary as a northeast-dipping thrust fault. In summary, during Late Cretaceous and/or early Tertiary time, continental basement and associated shelf facies rocks (Great Valley sequence) were thrust relative ly westward over Franciscan rocks deposited directly upon oceanic crust (Fig. 16). The root of the thrust, exposed as the boundary between Klamath Mountains and Coast Range provinces, extends southward beneath the Sacramento and Coast Ranges Great Valley r Inferred position of San Andreas Fault (displacement restored) present sea level G R E A T V A L L E Y S E Q U E N C E N ~ ~ \ FRANCISCAN GROUP MANTLE Serpentinite 10 SCALE 20 3 0 Miles Horizontal and vertical Figure 16. Idealized vertical section across western California showing relative westward thrusting of Great Valley sequence over Franciscan group in early Tertiary time (modified after Bailey and others, 1964, p. 164). 2 1 9 220 San Joaquin valleys. The imaginary line defined by the Klamath-Coast Range boundary and its southward extension may represent the position of a Mesozoic continental margin. Eastward underthrusting of the continent by the Pacific Ocean basin, as envisioned by Dietz (1963), may have initiated tectonic dislocation at the margin. Under thrusting by the ocean basin would be a mechanism of continental accretion by which Mesozoic oceanic crust was incorporated into the Cenozoic continental mass. Offset of Franciscan-Sierran Boundary by San Andreas Fault Hill and Dibblee (1953, p. 449), Page (1966, p. 268), and others have pointed out that the Nacimiento-Sur fault zone and Franciscan-Sierran boundary are analogous, inasmuch as both separate granitic-metamorphic basement rocks on the northeast from Franciscan rocks on the south west. The Kacimiento-Sur fault zone appears to be a northeast dipping thrust where best exposed along the coast. Perhaps the thrust fault mapped in the Stanley Mountain area is part of this thrust zone. If so, the Salinian block must be interpreted as a thrust plate truncated on the northeast by the San Andreas fault. Hill and Dibblee (1953, p. 449) suggested that right lateral movement along the San Andreas fault duplicated the boundary between Franciscan basement and metamorphic- 221 granitic basement in California. Hill and Hobson (1968, p. 126) suggested that duplication involved approximately 450 miles of apparent right-lateral slip. They projected the Sierran-Franciscan boundary from the western margin of the Klamath Mountains southward under the Sacramento and San Joaquin Valleys to the San Andreas near Maricopa; and projected the Nacimiento-Sur fault zone northward under the continental shelf to the San Andreas near Cape Mendo cino (Pig. 15). COHCLUSIOHS The Franciscan group and Knoxville Formation in the Stanley Mountain area are separate and distinct mappable units which do not grade into one another. Textural and mineralogic differences are clear-cut. Franciscan meta- sedimentary rocks have been penetratively sheared and partially recrystallized, whereas Knoxville sedimentary rocks are well bedded and laminated. Franciscan greenstone occurs as discontinuous lenticular bodies in which igneous textures have been altered or destroyed. The Knoxville greenstone is a tabular body of areal extent consisting of layered flows, pillow lavas, and tuff. Mineralogy of the Franciscan group is diagnostic of blueschist facies meta morphism, whereas mineralogy of the Knoxville Formation indicates that Knoxville rocks recrystallized in the 222 prehnite-pumpellyite facies under conditions of diagenesis or low grade regional metamorphism. The tectonic style of the Franciscan group contrasts with the tectonic style of the Knoxville Formation. Rocks of the Franciscan group have "been deformed by multiple tectonic events; contacts between discontinuous lenses (tectonic inclusions) are shear surfaces which may or may not parallel original bedding. Structure of the Knoxville Formation is relatively simple. Distinctive stratigraphic units, which can be traced for miles, are exposed in a north-dipping homocline gently warped into open subsidiary folds. The contact between Franciscan group and overlying rocks of the Great Valley sequence in the Stanley Mountain area is interpreted to be a thrust fault. Formations in thrust contact with the Franciscan group are: the Knox ville Formation of Late Jurassic age; the Jollo Formation of Early Cretaceous age; and the Carrie Creek Formation of Late Cretaceous age. Folding and. erosion of allochthonous Great Valley rocks has exposed the autochthonous Francis can group as a fenster in the core of the,abroad northwest- southeast trending Stanley Mountain antiform. Thrusting caused: (1) tectonic thinning of the Knoxville shale unit and Jollo Formation; (2) pronounced brecciation of Great Valley rocks; (3) incorporation of quartz-mica schist into a discontinuous friction carpet of sheared Franciscan rocks 223 beneath the thrust; and (4) emplacement of serpentinite (most of which has been subsequently altered to silica- carbonate rock) as thrust slices. The thrust surface transects Knoxville strata at a low angle and intersects unconformities within the Great Valley sequence, bringing Jollo Formation and Carrie Creek Formation into contact with the Franciscan group. Relative westward thrusting of the Great Valley sequence took place in post-Cretaceous (Campanian) pre-Oligocene time. The Franciscan depositional environment is inter preted as an oceanic trench which persisted from Late Jurassic to Late Cretaceous time; Great Valley sediments accumulated in a more stable environment on the continental flank of the trench. In the Great Valley sequence, un conformities at the base of the Jollo and Carrie Creek formations indicate tectonic disturbances at the end of Late Jurassic and Early Cretaceous time. The writer be lieves that downwarping, penetrative shearing, and syn kinematic blueschist metamorphism of Franciscan rocks at the continental margin occurred in response to under thrusting by the Pacific Ocean basin. Penetrative shear ing produced a chaotic intermixing of Franciscan rock types. After high-angle faulting and folding had re oriented shear surfaces and contributed to the chaotic nature of the Franciscan terrane, the Great Valley sequence was thrust relatively westward over the Franciscan group \ ‘ 224 in early Tertiary time. This thrusting is also considered to be related to underthrusting of the continent by the ocean basin. Sediments of Oligocene and Miocene age were deposited upon eroded Great Valley sequence in the Stanley Mountain area. Late Pliocene deformation produced the Stanley Mountain antiform, Huasna syncline, and East Huasna fault. The contact between Franciscan and Sierran-type basement represents an important boundary between rocks of oceanic and continental affinity. In the northern Ooast Ranges and in the Diablo Range of the southern Coast Ranges, geologists have reported thrust relationships be tween Franciscan group and Sierran-type basement and be tween Franciscan group and Great Valley rocks which were deposited upon Sierran-type basement. Mapping of a thrust contact between Franciscan group and Great Valley sequence in the Stanley Mountain area extends these relationships to the Hacimiento block. The present study provides additional evidence that westward thrusting of continental rocks relative to oceanic rocks was a tectonic event of ma^or significance during early Tertiary time. REFERENCES CITED REFERENCES CITED Albee, A. 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W., 1962, Theoretical petrology: New York, John Wiley and Sons, Inc., 416 p. Blake, M* C., 1965, Structure and petrology of low- grade metamorphic rocks, blueschist facies, Yollo Bolly area, northern California: Stanford Univ., Stanford, Calif., Ph.D. thesis. Blake, M. C., Irwin, W. P., and Coleman, R. G., 1967, Upside-down metamorphic zonation, blueschist facies, along a regional thrust in California and Oregon: U. S. Geol. Survey: Prof. Paper 575-C, p. 1-9. Borg, I. Y., 1956, Glaucophane schists and eclogites near Healdsburg, California: Geol. Soc. America Bull., v. 67, n. 12, p. 1563-1583. Briggs, L. I., 1953, Geology of the Ortigallta Peak quadrangle, California: Calif, Div, Mines Bull. 167, 61 p. 227 Brown, R. D., 1964, Thrust-fault relations in the northern Coast Ranges, California: U. S. Geol. Survey Prof. Paper 475-D, p. 7-13. Brown, W. H., Pyfe, W. S., and Turner, P. J., 1962, Aragonite in California glaucophane schists, and the kinetics of the aragonlte-calcite transformation: Jour. Petrology, v. 3, p. 566-582. Canfield, C. R., 1939, Subsurface stratigraphy of Santa Maria Valley oil field and adjacent parts of Santa Maria Valley, California: Am. Assoc. Petroleum Geologists Bull., v. 23, n. 1, p. 45-81. Christensen, M. N., 1965, Late Cenozoic deformation in the central Coast Ranges of California: Geol. Soc. America Bull., v. 76, p. 1105-1124. Coleman, R. G., and Clark, J. R., 1968, Pyroxenes in the blueschist facies of California: Am. Jour. Sci., v. 266, n. 1, p. 43-59. Coleman, R. G., and Lee, D. E., 1963, Glaucophane- bearing metamorphic rock types of the Cazadero area, California: Jour. Petrology, v. 4, n. 2, p. 260-301. Compton, R. R., 1966, Analyses of Pliocene-Pleistocene deformation and stresses in northern Santa Lucia Range, California: Geol. Soc. America Bull., v. 7Z» n. 12, p. 1361-1380. Coombs, D. S., 1953, The pumpellyite mineral series: Mineral. Mag., v. 30, n. 221, p. 113-135* _______ , I960, Lower grade mineral facies in New Zealand: Int. Geol. Congress, 21st Session, Pt. 13, P* 339-351. Coombs, D. S., Ellis, A. J., Pyfe, W. S., and Taylor, A. M., 1959, The zeolite facies, with comments on the interpretation of hydrothermal syntheses: Geochim. and Cosmochim. Acta, v. 17, p. 53-107. Crandall, B. G., 1961, The - stratigraphy of the Buckhorn Sandstone, Santa Barbara, San Luis Obispo counties, California: California Univ., Los Angeles, M. A. thesis. Crawford, ¥. A., and Pyfe, W. S., 1965, Lawsonite equilibria: Am. Jour. Sci., v. 263, p. 262-270. "" 2281 i Crittenden, M. D., 1951, Geology of the San Jose-Mount Hamilton area, California: Calif* Div. Mines Bull. 157, 74 P* Cummins, W. A., 1962, The graywacke problem: Liverpool i Manchester Geol. Jour., v. 3, p. 51-72. Davis, E. P., 1918, The Franciscan Sandstone: Calif. ' Univ., Dept. Geology Bull., v. 11, p. 1-144. Dibblee, T. W., Jr., 1966, Geology of the central Santa Ynez Mountains, Santa Barbara County, California: Calif.’Div. Mines Bull. 186, 99 p. Dickinson, W. R., 1966a, Table Mountain serpentlnite extrusion in California Coast Ranges: Geol. Soc. America Bull., v. 77, n. 5, p. 451-472. _______ , 1966b, Structural relationships of San Andreas fault system, Cholame Valley and Castle Mountain Range, California: Geol. Soc. America Bull., v. 77, n. 7, p. 707-726. Dietz, R. S., 1963, Collapsing continental rises: an actualistic concept of geosynclines and mountain building: Jour. Geology, v. 71, n. 3, p. 314-333. Drake, C. S., Ewing, M., and Sutton, G. H., 1959, Continental margins and geosynclines, p. 110-196, in The east coast of Horth. America north of Cape Hatteras: Physics and Chemistry of the Earth, v. 3. Easton, W. H., and Imlay, R. W., 1955, Upper Jurassic fossil localities in Franciscan and Knoxville forma tions in southern California: Amer. Assoc. Petroleum Geologists Bull., v. 39, P* 2336-2340. Elter, P., Giglia, G., Nardi, R., Tongiorgi, M., and Trevisan, L., 1964, General description of the Apuane, p. 1-4, Section VIII, in Maxwell, J. C., Editor, Guidebook, International Field Institute, Italy: Washington, D. C., American Geol. Inst. Ernst, W. G., 1963a, Petrogenesis of glaucophane schists: Jour. Petrology, v. 4, n. 1, p. 1-30. _, 1963b, Significance of phengitic micas from low- grade schists: Am. Mineralogist, v. 48, p. 1357-1373. _, 1965, Mineral parageneses in Franciscan meta- ' morphic rocks, Panoche Pass, California:? Geol. Soc. America Bull., v. 76, p. 879-914. Essene, E. J., Fyfe, W. S., and Turner, F. J., 1965, Petrogenesis of Franciscan glaucophane schists and associated metamorphic rocks, California: Beitr. Mineral, u. Petrog. II, p. 695-704. Evans, J. 0., 1966, Structural analysis and movements of the San Andreas fault zone: University of California, Los Angeles, Ph.D. thesis. Fairbanks, H. ¥., 1904, Description of the San Luis quadrangle: U. S. Geol. Survey Geological Atlas, Folio 101. Fyfe, W. S., Turner, F. J., and Verhoogen, J., 1958, Metamorphic reactions and metamorphic facies: Geol. Soc. America Memoir 73, 259 p. Fyfe, W. S., and Zardini, R., 1967* Metaconglomerate in the Franciscan formation near Pacheco Pass, Cali fornia: Am. Jour. Sci., v. 265, n. 9, p. 819-830. Ghent, B. D., 1963, Fossil evidence for maximum age of metamorphism in part of the Franciscan Formation, northern Coast Ranges, California: Calif. Div. Mines Sp. Rept. 82, p. 41. _______ , 1964, Petrology and structure of the Black Butte area, Hull Mountain and Anthony Peak quadrangles, northern Coast Ranges, California: California Univ., Berkeley, Ph.D. thesis. _______ , 1965, Glaucophane-schist facies metamorphism in the Black Butte area, northern Coast Ranges, Cali fornia: Am. Jour. Sci., v. 263, n. 5, p. 385-400. Gluskoter, H. J., 1964, Orthoclase distribution and authigenesis in the Franciscan Formation in a portion of western Marin County, California: Jour. Sed, Petrology, v. 34, p. 335-343. Graham, J. J., 1962, A review of planktonic foraminifera from the Upper Cretaceous of California: Cushman Found. Foram. Res. Contributions, v. 13, Pt. 3, p. 100-109. Hall, C. A., and Corbato, C. E., 1967, Stratigraphy and structure of Mesozoic and Cenozoic rocks, Nipomo quadrangle, southern Coast Ranges, California: Geol. Soc. America Bull., v. 78, n. 5, p. 559-582. 230 Heard, H. C., and Rubey, W. W., 1966, Tectonic implica tions of gypsum dehydration: Geol. Soc. America Bull., v. 77, n. 7, p. 741-760. Hill, M. L., and Dibblee, T. W., Jr., 1953, San Andreas, Garlock, and Big Pine faults, Oalifornia— a study of the character, history, and tectonic significance of their displacements: Geol. Soc. America Bull., v. 64, n. 4, p. 443-458. Hill, M. L., and Hobson, H. D., 1968, Possible post- Cretaceous slip on the San Andreas fault zone. p. 123-129 in Dickinson, ¥. R., and Grantz, A., Editors, Proceedings of conference on geologic problems of San Andreas fault system: Stanford Univ. Publ. Geol. Sci., v. XI, 375 p. Hills, E. S., 1963, Elements of structural geology: Hew York, J"ohn Wiley and Sons, Inc., 483 P* Hollister, L. S., and Albee, A. L., 1966, Tectonic em placement of a glaucophane schist block, Santa Barbara County, California, in The Geological Society of America, Abstracts for 1 9& 5: Geol. Soc. America Special Paper 87. Hsu, K. J., 1966, Franciscan rocks of the Santa Lucia Range, California, and the ''Argille Scagliose" of the Apennines, Italy: A comparison in style of deforma tion, in The Geological Society of America, Abstracts for 1965: Geol. Soc. America Special Paper 87. , 1967, Mesozoic geology of the California Coast Ranges— a new working hypothesis, p. 279-296, in Etages Tectoniques: Neuchatel, les Editions de la Baconnifere. Hutton, C. 0., 1938, The stilpnomelane group of minerals: Mineral. Mag., v. 25, n. 163, p. 172-206. Irvine, T. U., 1965, Sedimentary structures in igneous Intrusions with particular reference to the Duke Island ultramafic complex: Soc. Econ. Paleontologists and Mineralogists, Special Paper 12, p. 220-232. Irwin, W. P., 1957, Franciscan group in Coast Ranges and its equivalent in the Sacramento Valley, California: Am. Assoc. Petroleum Geologists Bull., v. 41, n. 10, p. 2284-2297. 251 Irwin, W. P., I960, Geologic reconnaissance of the northern Coast Ranges and Klamath Mountains, California, with a summary of the mineral resources: Calif. Div. Mines Bull. 179» 80 p. _______ , 1964, Late Mesozoic orogenies In the ultramafic "belts of northwestern California and southwestern Oregon: U. S. Geol. Survey Prof. Paper 501-C, p. 1-9. James, H. L.f 1955, Zones of regional metamorphism in the Precambrian of northern Michigan: Geol. Soc. America Bull., v. 66, p. 1455-1488. Jennings, C. W., 1958, Geologic map of California, Olaf P. Jenkins Edition. San Luis Obispo sheet: Calif. Div. Mines. _______ , 1959, Geologic map of California, Olaf P. Jenkins Edition, Santa Maria sheet: Calif. Div. Mines. Kerr, P. P., 1959, Optical mineralogy: New York: McGraw- Hill Book Company, Inc., 442 p. Korn, H., and Martin, H., 1959> Gravity tectonics in the Naukluft Mountains of South West Africa: Geol. Soc. America Bull., v. 70, n. 8, p. 1047-1078. Lee, D. E., Coleman, R. G., Bastron, H., and Smith, V. C., 1966, A two-amphibole glaucophane schist in the Eranciscan Formation, Cazadero area, Sonoma County, California: U. S. Geol. Survey Prof. Paper 550-C, p. 148-157. Lee, D. E., Thomas, H. H., Marvin, R. P., and Coleman, R. G., 1964, Isotopic ages of glaucophane schists from the area of Cazadero, California: U. S. Geol. Survey Prof. Paper 475-D, p. 105-107. Marchetti, M. P., 1956, The occurrence of slide and flow- age materials (olistostromes) in the Tertiary Series of Sicily: Int. Geol. Congress XX Session (Mexico City), Section V, p. 209-225, Maxwell, J. C., 1959, Turbidite, tectonic and gravity transport, northern Apennine Mountains, Italy: Am. Assoc. Petroleum Geologists Bull., v. 43, n. 11, p. 2701-2719. 232 Maxwell, J. 0., 1964, Torriglia-Ottone area, p. 1-8, Section XI, in Maxwell. J. C., Editor, Guidebook, International Field Institute, Italy: Washington, D. C., American Geol. Inst. McKee, B., 1962, Widespread occurrence of jadeite, lawsonite, and glaucophane in central California: Am. Jour. Sci., v. 260, p. 596-610, _______ , 1966, Knoxville-Franciscan contact near Paskenta, western Sacramento Valley, California, in The Geological Society of America, Abstracts for 1965: Geol. Soc. America Special Paper 87. Misch, P., 1966, Tectonic evolution of the northern Cascades of Washington State, p. 101-148, in Symposium on tectonic history and mineral deposits of the western Cordillera in British Columbia and neighbor ing parts of the United States: Can. Inst. Mining and Metallurgy Special Volume 8. Moore, J. G., 1965, Petrology of deep-sea basalt near Hawaii: Am. Jour. Sci., v. 263, n. 1, p. 40-52. Newton, R. C., and Kennedy, G. C., 1963, Some equilibrium reactions in the join CaAlgSIoOs-HgO: Jour. Geophys. Research, v. 68, n. 10, p. 2967-2983. Oakeshott, G. B., 1929, The petrography of the Stanley Mountain Franciscan of the Nipomo quadrangle: California Univ., Berkeley, M. S. thesis. Packham, G. H., and Crook, K. A. W., i960, The principle of diagenetic facies and some of its implications: Jour. Geology, v. 68, n. 4, p. 392-407. Page, B. Mo, 1963, Gravity tectonics near Passo Della Cisa, northern Apennines, Italy: Geol. Soc. America Bull., v. 74, p. 655-672. _______ , 1966, Geology of the Coast Ranges of California, p. 255-276 in Geology of northern California: Calif. Div. Mines and Geol. Bull. 190, 508 p. Page, B. M.s Marks, J. G., and Walker, G. W., 1951» Stratigraphy and structure of mountains northeast of Santa Barbara, California: Am. Assoc. Petroleum Geologists Bull., v. 35, n. 8, p. 1727-1780. Raleigh, G. B., and Paterson, M. S., 1965, Experimental deformation of serpentinite and its tectonic implica tions: Jour. Geophys. Research, v. 70, n. 16, p. 3965-3985. Ramberg, H., 1955* Natural and experimental boudinage and pinch-and-swell structures: Jour. Geology, v. 63, n. 6, p. 512-526. Rast, N., 1956, The origin and significance of boudinage: Geol. Mag., v. 93* n. 5» P- 401-408. Reed, R. D., 1933» Geology of California: Am. Assoc. Petroleum Geologists, Tulsa, Oklahoma, 355 p. Reinhard, M., and Wenk, E., 1951> Geology of the Colony of North Borneo: British Terr, in Borneo Geol. Survey Dept., Bull. 1, 160 p. de Righl, M. R., and Cortesini, A., 1964, Gravity tectonics in foothills structure belt of southeast Turkey: Am. Assoc. Petroleum Geologists Bull., v. 48, n. 12, p. 1911-1937. Savage, D. E., Downs, T., and Poe, 0. J., 1954, Cenozoic land life of southern California, p. 43-58 in Jahns, R. H., Editor, Geology of southern California: Calif. Div. Mines Bull. 170. Siegel, S., 1956, Nonparametric statistics for the behavioral sciences: New York, McGraw-Hill Book Company, 312 p. Slemmons, D. B., 1962, Determination of volcanic and plutonic plagioclases using a three- or four-axis universal stage: Geol. Soc. America Special Paper 69, 64 p. Stille, H., 1955» Recent deformations of the earth's crust in the light of those of earlier epochs, p. 171-192 in Poldervaart, A., Editor, Crust of the Earth: Geol. Soc. America Special Paper 62. Swe, W., 1968, Stratigraphy and structure of late Mesozoic rocks south and southeast of Clear Lake, California, in The Geological Society of America, Abstracts for 1968, Geol. Soc. America Special Paper. 234 Taliaferro, N. L., 1943a, Franciscan-Knoxville problem: Am. Assoc. Petroleum Geologists Bull., v. 27, n. 2, p. 109-219. ________, 1943b, Geologic history and structure of the central Coast Ranges of California, p. 119-163 in Geologic formations and economic development of the oil and gas fields of California: Calif. Div. Mines Bull. 118, 773 P. , 1943c, Geology of the Huasna area, p. 443-447 in' Geologic formations and economic development of the oil and gas fields of California: Calif. Div. Mines Bull. 118, 773 p. ________, 1944, Cretaceous and Paleocene of Santa Lucia Range, California: Am. Assoc. Petroleum Geologists Bull., v. 28, n. 4, p. 449-519. Turner, P. J., 1938, Progressive regional metamorphism in southern New Zealand: Geol. Mag., v. 75, n. 886, p. 160-174. ________, 1948, Mineralogical and structural evolution of the metamorphic rocks: Geol. Soc. America Memoir 30, 342 p. Turner, P. J., and Verhoogen, J., i960, Igneous and meta morphic petrology: New York, McGraw-Hill Book Co., Inc., 694 p. Williams, H., Turner, P. J., and Gilbert, C. M., 1954, Petrography: San Francisco, VI. H. Freeman and Co., 406 p. Woodring, W. P., and Bramlette, M. N., 1950, Geology and paleontology of the Santa Maria district, California: U. S. Geol. Survey Prof. Paper 222, I85 p. Yates, R. G., and Hilpert, L. S., 1946, Quicksilver deposits of eastern Mayacmas district, Lake and Napa counties, California: Calif. Jour. Mines and Geology, v. 42, n. 3, p. 231-286. oVi 91--' m 8fc . k m •UJW A9IUD4-S ▼ * 1 * m W ' a jkc m m +36i:v.\' I K j l x Jk9w%l3i*i + Tmsm + +Tmsm + Shell Pk +50 + T . :OUS T E R T I A R Y QUATERNARY Upper Oligocene Miocene Recent r~ Lower Middle Upper OUS TERTIARY QUATERNARY EXPLANATION c CD O < d a. Qal Alluvial deposits a > Q. D a > ■o TJ CD C CD O O CD * O _l CD C CD O O • < CD dTmsmp I 1 : I I ! ~ r Santa Margarita Formation Soft white coarse-grained sandstone, fossil beds, and white calcareous si/tstone. ITmm, i -i—I . Monterey Formation Well-bedded white silts tone, tight brown fine- to medium-grained sandstone, and cherty or porce/aneous shale. -Tmps Point Sal Formation Light brown soft diatomaceous, tuffaceous or siliceous silt stone and fine-grained white sandstone. Rincon Form ation Light brown and white conchoidally fractured soft c/aysfone. \ \ \ 'T \ Jo-m va. \\\\N I I Vaqueros Formation Massive light brown and white calcareous coarse grained sandstone and soft brown si/tstone. y DC < H a: u i- m sr S ilica-carb o n ate rock it • * ^ .Tos * • > • - f t S esp e Formation Conglomerate inter bedded with red and green coarse-grained sandstone. J iL CD o-< a Z) Kucc Carrie Creek Formation Massive and thick-bedded light brown medium- to coarse-grained biotite sandstone, well-bedded d) D O L d Ml STRUCTURAL SYMBOLS ---------- Contact (dashed: located within tOO ft. dotted: concealedJ -U 9 | ° - -- .» High-angle fault showing dip (dashed: located within 100 ft. ; dotted: concealed) 30 A t A nX. .A-.. Thrust fault, showing dip (dashed: located within 100 ft ; dotted: concealed) > 1 5 Mesoscopic fold, showing shape in profile and plunge Anticline, showing trace of axial surface j — > Stanley Mountain Antiform, f showing inferred trace of axial surface S tr ik e and Dip Inclined beds / Vertical beds 50 y Inclined primary layers in Knoxville greenstone y Vertical layers 5 0 y Inclined foliation in Franciscan group y Vertical foliation 50 y Franciscan bedded metachert: inclined beds y Vertical beds A ' L ine of section 12000 Feet 8000 10000 6000 4000 Contours at 1000ft, 1800ft., 2600ft. Geology by J. “ T7 i l l l ; | : i l l l i l l j l l l l l l l l l t a ttllilWBiiplsilB ^ p j l p j ^ /O n A/iso " C ree k y • 48 Kucc 68 — Qal ^ Geology by J. A. Brown, Jr. 1968 ^io^Jkgs 33/ ^ . Upper Lower Upper coarse-grained sandstone. Kucc Carrie Creek Formation Massive and thick-bedded light brown medium- to coarse-grained biotite sandstone, welt-bedded olive drab to black micaceous si/tstone, and cobble conglomerate. TTTT Jollo Formation Dark green to black mudstone, yellow-brown coarse grained sandstone, and thick conglomerate lenses. ---Jksh-I- (0 D O Lj J O ? u i DC o L. ■V c a a U ) c n < DC. D ! . u m Franciscan group f , undifferentiated greenish gray m etagraywacke, metacongiomerate, dark gray argillite, greenstone, irregularly-bedded light-colored metachert, and glaucophane schist; fa, amphibolite; um, serpentinized ultramafic rocks. Knoxville Formation Jksh, mainly dark gray shale with interbedded nodular limestone, well-bedded brown fossiliferous arkosic gray wacke, and intraformotiona! conglomerate; Jkgs, greenstone (andesitic and basaltic flows, pillow lava, and tuff); Jkc, weII-bedded dull brown and black to greenish black chert; Jkgw, mainly dark gray arkosic graywocke with interbbedded black shale and dark green tuff in lower part. GEOLOGIC MAP OF STANLEY MOUNTAIN AREA, CALIFORNIA ( f l f t EAST HUASNA FAULT STA N LE Y MOUNTAIN A N T IF O R M i \ \ ^ STA N LEY MOUNTAIN u \ N a n t i f o r m XI in f t □ EAST HUASNA FAULT \\\ W \ STANLEY MOUNTAIN A N TIFO R M z LU UJ 4 0 0 0 ' CO O — 3 0 0 0 — 2000' Jkgs Jksh' Kucc . ^ . J k g w seo level § ° Ll. - 3 0 0 0 Jkgs Jksh iooo' sea level mte Interpretation 0 “ s ° s U- - I — j z d < UJ Kucc Jkg w Jksh Jkgs FISH CREEK A' r -4 0 0 0 ' -3 00 0 ' -2000 I 0 0 0 1 N Kucc , ^ sea level L ii UJ t r . o B ‘ >- DC < t- oc UJ 3 0 0 0 Kucc 2000' '^\Jkgw. iooo1 sea level a) c a o o a> q . ^ Q . 3 0) ■a *o a> 5 < o _l Q> C CD O o < ! cn Santa Marga Soft white coarse-g beds, and white cat Monterey Welt- bedded white s to medium-grained or porcetaneous sh W Point Sa Light brown soft dk or siliceous si/tstor white sandstone. Rincon Light brown and whit soft day stone. Vaqueros Massive light brown at grained sandstone a, Sespe Conglomerate interb coarse-grained san E X P L A N A T I O N Tmsm inta Margarita Formation 'hite coarse-grained sandstone, fossil and white calcareous si/tstone. Monterey Formation edded white si/tstone, tight brown fine- Hum-grained sandstone, and cherty ceianeous shale. rTmqs r! Point Sal Formation 'jrown soft diatomaceous, tuffaceous ceous si/tstone and fine-grained sandstone. Rincon Formation irown and white conchoidally fractured fay stone. >~ a: < h- os L lI Silica-carbonate rock To-rnva- Vaqueros Formation e tight brown and white calcareous coarse- d sandstone and soft brown si/tstone. irMosrir! Sespe Formation » merate inter bedded with red and green grained sandstone. _Kucc _ 2000 0 2000 4000 STANLEY MOUNTAIN ANTiFORM sea level STANLEY MOUNTAIN A N TIFO R M STANLEY MOUNTAIN ANTIFORM STANLEY MOUNTAIN A N TIFO R M • / / CUYAMA RIVER CUYAMA RIVER i f ^ , — 1000* - sec level Kucc • — 3 0 0 0 - 2 0 0 0 ' 1000' sea level r r CO 3 O UJ o < I — UJ t r o o co co < tr 3 Q > Cl 3 a> o 3 < u 3 coarse - grained sandstone. Kucc Carrie Creek For Massive end thick-bedded A to coarse-grained biotite sar, olive drab to black micace cobble conglomerate. Jollo Forme Dork green to black mudstone, y grained sandstone, and thick t -Jkgw 7 v V V V 1 Jkgs v v - ' v v -Jksh' Knoxville Forrr Jksh, mainly dark gray shale nodular limestone, well-bedded arkosic gray wacke, and it conglomerate; Jkgs, green and basaltic flows, pillow It Jkc, well-bedded dull brow greenish black chert; Jkgw, gray arkosic gray wacke wit block shale and dark green tut GEOLOGIC STRUCTURE STANLEY MOUNTAIN AREA, S0UT Geology by J. A bed sandstone. ■ Kucc : b Creek Formation 1 thick-bedded tight brown medium- ained biotite sandstone, well-bedded o black micaceous si/tstone, and glomerate. r.Kii, Jollo Formation black mudstone yellow-brown coarse- 'tone, and thick conglomerate lenses. -Jkgw 7 T'JC-X-T v v v v v Jkgs V V v'v v -~Jksh~ co ZD o UJ (J iS UJ cc o •a c= a o CO CO < cr 3 ~D Franciscan group (Structure not shown) f, undifferentiated greenish gray metagraywach. metacongiomerate, dark gray argillite, greenstone, irregularly-bedded light-colored metachert, ana glaucophane schist; um, serpentinized ultram afic rocks. >xville Formation dark gray shale with interbedded One, well-bedded brown fossi/iferous ywacke, and intraformationa! e ; Jkgs, greenstone (andesitic flows, pillow lava, and tu ff); ided dull brown and black to 'k chert; Jkgw, mainly dark ■ graywacke with interbbedded 7 d dark green tuff in tower part. I TURE SECTIONS ACROSS THE SOUTHERN COASTER A NG ES, CALIFORNIA (ogy by J.A. Brown, Jr. 1968

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Creator Brown, James Alexander, Jr. (author)

Core Title Thrust Contact Between Franciscan Group And Great Valley Sequence Northeast Of Santa Maria, California

Degree Doctor of Philosophy

Degree Program Geology

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Davis, Gregory A. (committee chair), Reaves, Gibson (committee member), Taylor, Vernon A. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c18-642278

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