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Sedimentology And Pleistocene History Of Lake Tahoe, California - Nevada

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Sedimentology And Pleistocene History Of Lake Tahoe, California - Nevada

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Content 70-23,161 HYNE Jr. , Norman John, 1939- SEDIMENTOLOGY AND PLEISTOCENE HISTORY OF LAKE TAHOE, CALIFORNIA-NEVADA. University of Southern California, Ph.D., 1969 Geology University M icrofilm s, A XERQKCompany, Ann Arbor, Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED SEDIMENTOLOGY AND PLEISTOCENE HISTORY OP LAKE TAHOE, CALIFORNIA-NEVADA fey Norman John Hyne A Dissertation Presented to the FACULTY OP THE GRADUATE SCHOOL UNIVERSITY OP SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OP PHILOSOPHY (Geological Sciences) August 1969 U NIVER SITY O F S O U TH E R N C A LIFO R N IA T H E G R A D U A TE SCHO OL U N IV E R S IT Y PARK LOS A N G E LE S . C A LI FO RNIA 9 0 0 0 7 This dissertation, written by JSQRMAIL.JQHN.-KYJNE.............. under the direction of h.is.... Dissertation Com mittee, and approved by a ll its members, has been presented to and accepted by The Gradu ate School, in partial fulfillm ent of require ments of the degree of D O C T O R O F P H IL O S O P H Y O f)ean DateJ^l11 ? . ? . ? . FRONTISPIECE Oblique aerial photo of Lake Tahoe from over Carson Valley, Nevada. View is westward over the Carson Range, southern Lake Tahoe and the crest of the Sierra Nevada. ii iil |C , f ABSTRACT Lake Tahoe, California-Nevada, occupies a large north-south graben on the crest of the Sierra Nevada. Closure is completed on the south by a glacial outwash- deltaic mass and in the north by complicated structure and volcanlcs. The graben is floored with at least 0.5 km of sediment, derived from the glaciated southern and western drainage basins. Relic glacial outwash-deltaic structures are common only along these shores. During glaciations, ice dams formed in the Truckee Canyon outlet causing elevated lake levels. Joklhlaupe (floods through breached ice dams) rapidly lowered elevated lake levels to the present lava-darn level. Glacial outwash-deltaic structures prograded during elevated lake levels experienced massive slumping during i * Joklhlaups creating a Jumbled slump topography at the base of the western side-wall as well as layers of slump sedi ments out in the central lake basin. The slump layers are composed of unsorted glacial sediments with no ordered internal reflections. These chaotic layers are character istically overlain and underlain with well-layered turbidites of Recent and interglacial ages. Three periods of glacial progradations and sub sequent slumping and chaotic layers are recognized: Hobart, iv Donner Lake and Tahoe. A more recent glaciation, Tioga, resulted in limited progradation with no slump redistribu tion. Extrapolated radiocarbon sedimentation rates sup port this chronology. Turbidity currents are active and subaqueous gullies are being eroded in some modern Lake Tahoe environ ments. Lake sediments range from fine-grained, central basin turbldites to coarse, well-sorted, reworked deltaic sands. Organic carbon and nitrogen are high in surface areas of active, fine-grained sedimentation but are rapidly oxidized and removed with time. No carbonates are present in the sediments. Clay mineralogies are apparently con trolled by weathering intensities rather than source rock lithologies. Illite is the most abundant clay mineral with lesser amounts of montmorillonite and a trace of kaolinlte. Ice-rafted sand grains and authigenic vivianite spherules commonly create an unusual texture in the fine grained sediments. v CONTENTS Chapter Page ABSTRACT...................................... iv CONTENTS...................................... Vi ILLUSTRATIONS................................. ix PLATES........................................ xi TABLES........................................ Xi ACKNOWLEDGMENTS............................... xli INTRODUCTION ................................. 1 Purpose ................................... 1 Geography ................................. 1 Location ............................. 1 Dimensions........................... 4 - Climate ............................... 5 Previous work . « . ....................... 6 GEOLOGY OP LAKE TAHOE AND ITS BASIN.......... 9 The Lake Tahoe b a s i n ..................... 9 Structure ..... ................... 9 A g e.................................... 10 Llthologies........................... 12 Bathymetry and physiography of Lake Tahoe . 16 Bathymetry........................... 16 Natural physiographic provinces .... 17 vi Chapter Page Eastern side-wall ................. 17 Southern delta ..... ........ 24 Western side-wall ................. "24 Northern closure ................. 28 Central hasln ..................... 28 Seismic characteristics of Lake Tahoe . . . 29 Natural provinces ..................... 30 Source of the sediments............... 47 Correlation of drainage basin events with lake sediments................. 48 Pleistocene events in the Lake Tahoe drainage b a s i n.......... 48 Pleistocene events In Lake Tahoe sediments....................... 55 Central basin scarplet .............. 69 Central basin mounds ................. 72 Summary of geophysical characteristics 73 Sedimentary characteristics of Lake Tahoe . 75 Textures............................. 75 Geochemistry ......................... 94 Carbonates....................... 94 Organic carbon ................... 95 Nitrogen......................... 98 Chapter Page Clay mineralogy................... 101 Authlgenic vivianite ............. 102 Summary of sedimentary characteristics 103 Absolute age dating of events............ 104 CONCLUSIONS.................................... 109 REFERENCES . ............................... 115 viii ILLUSTRATIONS Figure Page 1. Location and control map . * ............. 2 2. Generalized geology of the Lake Tahoe drainage basin ........................... 1? 3. Bathymetry and piston coring map of Lake Tahoe...................................... 13 4. Natural provinces of Lake Tahoe........... 20 5. Eastern side-wall and southern delta profiles...................... 22 6. Subaqueous gullies, western side-wall, northern closure and central basin profiles 25 7. Seismic profile M-N....................... 31 8. Seismic profile P-Q-R ..................... 34 9. Seismic profile I-,T....................... 36 10. Seismic profile K-L....................... 38 11. Seismic profile D-E....................... 40 12. Seismic profile G-H....................... 42 13. Seismic profile C-B-C ..................... 45 14. Glaciations in Truckee Canyon ............. 51 15. Seismic profile A-B ................. 57 16. Seismic profile R-S....................... 59 17. Seismic profile 0-N....................... 62 18. Seismic profile H-I....................... 65 19. Seismic profile F-G....................... 67 ix Figure Page 20. Seismic profile B-BC..................... 70 21. Mean grain size and sorting distribution . 76 22. Volcanic ash distribution.............. 87 23* Organic carbon and nitrogen distribution . 96 24. Organic carbon distribution with depth in the sediment........................ 99 x Plates Page FRONTISPIECE .................................... II I. Structures In sediments close to source . 79 II. Structures In sediments distant from source.................................... 81 III. Sediment Inclusions ...................... 84 IV. Deltaic s a n d s.......... ................. 90 Tables Page 1. Lake Tahoe parameters ................... 4 2. South Lake Tahoe climate ................. 5 xi ACKNOWLEDGMENTS Tlie execution of the Lake Tahoe study is a unique and entertaining story in itself. It encompasses a broad spectrum starting in the early "lean" days of the fall of 1968 when equipment and a working platform were obtained "at cost" and "on credit" as finances were, at that time, nonexistent. After the initial investigation disclosed the spectacular but completely unexpected results, the equipment, cooperation and finances were more easily ob tained. Many people contributed to the success of the study. Dr. D. S. Gorsllne provided encouragement and direction which made the Lake Tahoe study possible. Thanks also are due to Drs. Richard 0. Stone, Richard H. Merrlam and Gerald Bakus of the University of Southern Califoraia for their time, advice and interest In various aspects of the problem and for critical reading of the manuscript. Dr. Charles R. Goldman of the University of California at Davis was especially generous with ship time on the San Gulseppi at Lake Tahoe. Special gratitude is extended to two people whose personal interest in the project greatly contributed to the success of the study. Paul Chelminski of Bolt Associates, Norwalk, Connecticut, lent an air gun and his personal assistance to the geophysical investigation. Glenn Amand- xii son, a South Lake Tahoe contractor, contributed consider able personal time and engineering innovations to the dif ficult but successful piston coring operation. The financial assistance obtained through two National Science Foundation grants (GA 10686 and GA 13082) and a Sigma Xi Grant-in-aid-of-Research is sincerely ap preciated. xiii 1 INTRODUCTION Purpose lake Tahoe is In an area that was sensitive to Pleistocene climatic changes. Glaciations in the drainage basin are well documented. Problems of eroded sections and biogenic mixing, common In marine strata, are not en countered in Lake Tahoe sediments. The lake is an ef fective sediment trap; therefore, all drainage basin events are recorded in the sediments. An investigation of the lake utilizing marine techniques and equipment should produce data for an excellent chronology of Pleistocene, Sierran events. It is surprising that essentially no previous work has been done. Geography Location Lake Tahoe is located on the crest of the Sierra Nevada, the largest range In California (see frontis piece). San Francisco is about 320 kms (200 miles) to the west and Los Angeles is 800 kms (500 miles) to the Bouth, of the lake. The 120th Meridian which closely parallels the Californla-Nevada border almost bisects the lake placing about one-third of the lake in Nevada (Pig. 1). Figure 1. Location and Control Map. Geographic names and the location of seismic profiles and precision depth recording profiles are shown. 2 | LAKE TAHOE s t a t e I in e p o in t SAND POINT TRUCKEE RIVER DOLLAR .POINT CARSON RANGE T A H O E .C ITY IEVADA CALIFORNIA OEAl C/taho^Tnes >UGAR PINE POINT P.D.R. EMERALD BAYI SOUTH LAKE TAHOE CASCADE LAKE MEYERS GRADE UPPER TRUCKEE RIVER Dimensions 4 Lake Tahoe is the largest Alpine lake on the North American continent. TABLE 1 LAKE TAHOE PARAMETERS Elevation Maximum Length Maximum Width Depth Area 1897-1899 m 34 kms 17 kms maximum lake (6223-6229 ft) (21 miles) (13 miles) 494 m 500 sq.kms average drainage basin 249 m 1300 sq kms The maximum depth of Lake Tahoe, tenth deepest lake in the world (Hutchinson, 1957), is listed as 501 m (1645 ft). On the North American continent only Crater Lake in Oregon and Great Slave Lake in Canada are deeper. Charles Burckhalter of the United States Naval Observatory measured the maximum depth in the 1870's using a champagne bottle and a rope (Hinkle and Hinkle, 194-9)* and this is published on United States Coast and Geodetic Survey chart 5001. With the electronic apparatus used in this study, it was determined that the lake is no deeper than 494 m (1620 ft). This maximum depth makes Lake Tahoe only the eleventh deepest lake in the world immediately behind Lake Sarez (500 m) in the Soviet Union. The great depths in Lake Tahoe create an exceptionally large reservoir of water; larger than the ten largest man-made reservoirs in the United States combined (Hinkle and Hinkle, 1949). The drainage basin is nearly symmetrical about the lake with no drainage streams more than 5 kms in length ex cept for the Upper Truckee River at the south end of the lake. The Truckee River, Lake Tahoe's outlet, originates near Tahoe City on the northwest shore of the lake and flows in a broad arc for 160 kms past Truckee, California and Reno, Nevada terminating at Pyramid Lake, Nevada. A dam at Tahoe City regulates the flow of this water. Climate Weather conditions vary considerably from season to season at Lake Tahoe. TABLE 2 SOUTH LAKE TAHOE CLIMATE# January July average mean precipi average mean precipi temnerature tation temnerature tation -3°C (27°F) 15-5 cms 16°C (6l0P) 0.8 cms (6 ins) (31 ins) * Data from anonymous, 1967 Precipitation is concentrated in the months from November to March (United States Department of Commerce, 1959). Although long periods of subfreezing weather are common to Lake Tahoe, only Emerald Bay is known to oc casionally freeze. Previous Work Despite an abundance of information on the Lake Tahoe basin, little has been published concerning geology below the lake surface. Generalized bathymetry is pre sented on United States Coast and Geodetic Survey chart 5001 (1923) . The soundings were obtained by lead line. Owing to the wide spacings of the soundings much detail is missing. Goldman and Court (1968) described a “lake mount" which rises over 125 m from the central floor of the lake for which a possible intrusive volcanic origin was suggested. Le Conte (1883) was one of the first to attempt scientific measurements at Lake Tahoe during the summer of 1873. He measured temperature at depth and obtained a secchi disc depth of 33 m with a dinner plate. He also explained why the lake never freezes, why it is so blue, and why drowned bodies don't reappear. Although he dis cussed seiches in European lakes, he found no evidence for their occurrence in Lake Tahoe. More accurate recent work is reported by Goldman and Carter (1965). A summer thermocline develops at 20 to 30 m separating surface waters, which may range up to 17°0, and bottom water near maximum density (4°C). Stability is lost during the winter months as temperatures approach maximum density throughout the water column, and complete overturn occurs principally during February and March. Goldman and Carter (1965) also reported secchi disc depths ranging between 30 to 40 m with no significant differences between the depths recorded now and those observed by Le Conte. This is a remarkable value for transparency since maximum readings are only about 20 m in the clearest waters off southern California. Leggett and McLaren (1968) noted that a hydrophotometer was used to measure light penetra tion exceeding 120 m in the lake. These transparency values are related to the water's low biomass and low suspended sediment content. Goldman and Court (1968) reported seiches of 12-24 hour periods and amplitudes of a few millimeters in the lake. Matthews (1968) commented on a possible seiche produced by the 1966 earthquake centered 20 miles north east of Tahoe City. Crippen (1968) calculated the flush ing time of Lake Tahoe to be about 1000 years. Considerably more is known about the biology than any other aspect of the lake. Juday (1906) investigated the lake in relation to its trout population. Kremmerer, Bovard and Boorman (1923) studied the plankton noting their small biomass and diurnal migration of the copepods. Hubbs and Miller (1948) identified the fish fauna as Lahontan. Frantz and Cordone (1967) located bottom plants at depths deeper than 120 m. Goldman and Carter (1965) determined the net primary production of the lake to be 2 36.22 g c/yr/m . This productivity averages 1.65 mg ■ X c/day/m . Owing to this extremely low value, the lake is classified as ultra-oligotrophic. A nitrogen deficiency is suggested as the cause of this situation (Goldman, 1967). 9 GEOLOGY OP LAKE TAHOE AND ITS BASIN The Lake Tahoe Basin Structure The Carson Range is immediately east of the lake and the Sierra Nevada to the west, with the floor of the Carson Valley to the east In the Basin Ranges (see frontispiece). Le Conte (1875) was the first to recognize the graben structure. Lindgren (1911) extended the Lake Tahoe graben boundary faults northward Into the Truckee basin and Sierra Valley. The western boundary fault system was mapped as passing through the west end of Don- ner Lake, the Squaw Valley ski area and Meyers grade (Hudson, 1948; and Louderback, 1924) along with Rubicon Point and Sugar Pine Point in Lake Tahoe itself (Burnett, 1968). The eastern boundary fault system has not been located outside the Lake Tahoe basin in which It parallels the northeastern shore south to Deadman's Point. The faults bounding the lake are probably active as the fault which defines the eastern front of the Carson Range is obviously active and defined by hot springs and scarplets (Lawson, 1912). Hot springs are located in Lake Tahoe along a fault contact between granltics and vol- canlcs near Stateline Point but are limited in volume (Waring, 1915). Earthquake epicenters are not common in 10 the immediate vicinity of Lake Tahoe. Only one (4.8 on the Richter scale) has occurred since 1934 (Wolfe, 1968). Most earthquakes felt in the Lake Tahoe area occur either in the Basin-Ranges to the east or along the northern ex tension of the Lake Tahoe graben structure in the Truckee area. The extent of faulting in the formation of the Lake Tahoe basin is a subject of controversy. Lindgren (1911) who first recognized the continuity of the structures between Lake Tahoe and Truckee basins and Sierra Valley proposed that these were partitioned into distinct basins by andesitic eruptions in the single, continuous trough. Louderback (1911) suggested that Lake Tahoe was due exclusively to faulting. Blackwelder (1933) proposed that a combination of warping and faulting created the basin. The dam which closes the Tahoe basin is com posed of andesitic mudflow breccia with no granitic rocks exposed in that area so that it cannot be determined with any certainty if volcanics alone or volcanics overlying up-faulted basement rock are responsible for the closure along the north shore (Burnett, 1968). Birkeland (1963) believes an unexposed granitic block completed the damming. Age Recent work has considerably narrowed the range of times considered In the formation of the Lake Tahoe basin. The lake long has been recognized as being relatively old (Russell, 1885). Traditionally linked to the uplift of the Sierra Nevada, the Lake Tahoe graben formation was originally described as Mlo-Pliocene followed by a period of quiescence and then continued Pllo-Pleistocene de formation (Hinds, 1952). Thompson and White (1964) demonstrated that the middle Pliocene Truckee Formation was being deposited along the flanks of the Oarson Range during the initial uplift of that range. Axelrod (1962) utilized reconstructed stream profiles and paleobotanical evidence to indicate a Kansas age for the uplift. M,eyers grade, where the easternmost headwaters of the South Fork of the American River was beheaded by the Upper Truckee River during the formation of the Lake Tahoe graben, is overlain by undisturbed moraines of Tahoe glaciation age, post dating the deformation (Burnett, 1968). Blrkeland (1963) concluded that most major landforms in the north Lake Tahoe and Truckee basins were formed by faulting and warping after andesitic volcanlsm and deformation took place in the Late Pliocene or Early Pleistocene. These deformations ceased before the youngest of the two pre- Wisconsin glaciations (Donner Lake). The age of the volcanics is supported by K-Ar age dates reported by Wahrhaftig, Morrison and Birkeland (1965). In the Donner Pass area, north of Lake Tahoe, the volcanics range in age 12 from 2.6 to 7.4 m. y. At Carnelian Bay, 3ust north, of Dollar Point, Lake Tahoe, they date Between 2.2 and 2.5 m. y. In a gravel pit near Tahoe City, Lousetown volcanics which overlie the oldest lake deposits and are tilted 10 to 16° to the east (Birkeland, 1965) are dated at 1.9 t 0.1 m. y. Lousetown volcanics also crop out in Truckee Canyon where they were deposited after the canyon had been cut to within 60 to 90 m of its present depth. Volcanism In the immediate area of the lake appears to have been con centrated during this period from 2 to 1 m. y. Llthologles Llthologies in the Lake Tahoe basin can convenient ly be divided Into four categories: metamorphics, granitics, volcanics and Quaternary deposits (Pig. 2). The metamorphics are both metasediments and metavolcanics scattered as roof pendants on the granitics. Little is known about these in the lake area except for the Mesozoic section described by Loomis (I960) in the southwest corner of the basin. Granodiorite of the Sierra Nevada batho- 11th is the most abundant bedrock in the basin. Small bodies of quartz dlorite, diorite and gabbro are located primarily near the metamorphics (Burnett, 1968). The volcanics, principally Pliocene in age, are located in the northwestern portion of the basin and along the Lower Truckee River. These consist mainly of thin- Figure 2. Generalized geology of the Lake Tahoe drainage basin. The drainage basin bedrock is predominantly granitic except along the volcanic, northern portion. Glacial deposits are located only along the southern and western shores of the lake. 13 14 ^ g L A K E BEDS 1 | GLACIAL H m ! VOLCANIC i l l METAMORPHIC ■ ■ g r a n it ic i p i w % t I N I 1 0 15 bedded mudflow breccias. The vents, centered near Mounts Pluto and Watson, never have been located. Gave Rock on the eastern shore and Eagle Rock on the western shore are eroded, intrusive plugs. Stevens peak at the southern most part of the basin is also a Tertiary volcanic ac cumulation. Well-preserved cinder cones in the vicinity of Tahoe City are evidence of limited, continued volcanism into the Quaternary. The most interesting deposits are those of Quaternary age. Because of its elevation, the hake Tahoe basin experienced extensive Pleistocene glaciation. This glaciation occurred largely along the western, Sierra Nevada side of the basin. Only extremely limited glaciers developed on the northern slopes of the highest peaks of the Carson Range to the east. Valley glacier development probably responded to both a lowering of temperatures and an increase in precipitation. As the eastern Carson Range has elevations several hundred meters higher than the western Sierra Nevada, development of the glaciers ap pears to have been controlled by the amount of precipita tion. At present, the western crests average 127 cms of precipitation whereas the eastern crests average only 51 cms per year (Burnett, 1968). Pleistocene moralnal sedi ments blanket large areas of the southern and western drainage basins. A large outwash-deltaic structure formed by the confluence of valley glaciers is located along the 16 southern shores of the lake. The Sierra side of the lake still retains its angular Alpine topography with paternoster lakes (Cascade and Fallen Leaf) and a fjord (Emerald Bay), whereas the Carson Range is characterized by gently rolling relief. Granodiorite is the most easily eroded lithology as it rapidly responds to mechanical weathering. Volcanic mud flows in the northern part of the basin are also easily weathered as glass matrix alters to clay and a thick soil is rapidly developed. Least weathered are the metamorphics and lava flows. Landslides are common along some of the steep slopes (Matthews, 1968) and several relatively large shoreline deposits have developed during the Recent lake level stand. Bathymetry and Physiography of Lake Tahoe Bathymetry A precision depth recording program (Fig. 1) was initiated at Lake Tahoe when it became apparent that United States Coast and Geodetic Survey chart 5001 (Lake Tahoe) lacked sufficient detail to accurately describe the lake. A Gifft recorder was coupled to an EDO transducer aboard the San GuIbbppI, a 10 meter, former salmon trawler. Ship speed was 6 knots and navigation was accomplished with sextant bearings throughout the field studies. 17 Considerable geology was Interpreted from the detailed bathymetric map (Pig. 3). The graben origin of the lake basin is apparent from the steep, north-south, fault slopes leading down tc the relatively flat central lake surface. Because of this geometry, 35 percent of the lake's 500 sq kms are below the 450 ra contour. Natural Physiographic Provinces On the basis of bathymetry, the lake basin is divided into five, distinct, natural provinces (Pig. 4), namely: the eastern slde-wall, southern delta, western slde-wall, northern closure and central basin. Eastern Side-Wall The eastern side-wall is characterized by steeply dipping slopes along its entire length (Pig. 5a) which can readily be interpreted as a fault scarp. This slope reaches a maximum dip of 24° in the vicinity of Deadman's Point (Pig. 5a, profile C). Some slopes dip without any apparent change below the flat lake floor (Pig. 5a, pro file D), whereas in others the side-wall to central basin transition is obscured by small mounds up to 20 m in height (Pig. 5a, profile A) or large unattached blocks (Pig. 5a, profile C) at the base of the slopes. Struc tural noses are apparent on some of the profiles (Pig. 5a, profile B). Figure 3* Bathymetry and Piston Coring Map of Lake Tahoe. Steep, fault scarps are seen along the western and eastern shores and jutting out into the lake from the northern shore. Jumbled, slump topography is located along the western side-wall. The central basin mounds are located far from the side-walls. 18 1 1 0 I t o LAKE TAHOE C O N T O U R INTERVAL 5 0 M 14* t r u c k e e r iv e r MARLETTE LAKE 13* O O ° 5Dm EMERALD BAY CASCADE LAKE, UPPER TRUCKEE RIVER • p i s t o n c o r e N A U T I C A L M I L E S FALLEN LEAF LAKE « Figure 4. Natural provinces of Lake Tahoe. 20 21 NORTHERN CLOSURE WESTERN SIDE-WALL CENTRAL BASIN ° Oo EASTERN ' SIDE-WALL SOUTHERN DELTA LAKE TAHOE Figure 5» Eastern side-wall and southern delta profiles. a. Eastern side-wall profiles. Slump mounds are located at the base of this fault scarp. b. Southern delta profiles. Mound and gully topography is prominent along the delta face. 22 5 a e a s te rn s id e -w a ll Zfllj m * M i 5 b s o u th e rn d e lta 24 Southern Delta The relatively flat topography both above and below lake level at the southern shore of the lake Is known to be a Pleistocene delta formed by the confluence of valley glaciers (Pig. 5t>). Today, the Upper Truckee River and the smaller Taylor Creek debouch into marshy areas at the southern shore. These streams are much too small to significantly effect the delta. A shallow shelf less than 5 m deep has developed for a distance of at least 1 km be yond the shore. Prom the 50 m contour to the central basin the slopes are a relatively gentle 9°. The most noticeable features on the delta are the numerous gullies and ridges (Pig. 5"b, profiles E and P) which run down slope. Even though some are situated off subaerial drainage mouths (Pig. 6a, profile U), there is no connection between the features across the wide, shallow shelf. Because of this, it is believed that the offshore gullies are not Influenced by the present sub aerial drainage. The transition between the deeper deltaic slopes and the flat central basin is characterized by shallow slopes and small mounds (Pig. 5t, profile G). Yfestern Slde-Wall The western side-wall displays considerably more varied topography (Pig. 6b) than the eastern side-wall. Only off Rubicon Point, along the southernmost length of the slde-wall, do slopes reach 39° and the slope dips un- Figure 6. Subaqueous gullies, western side-wall, northern closure and central basin profiles. a. Subaqueous gullies. Gullies in pro files V and W are actively being maintained. b. Western side-wall profiles. Large slump mounds are located at the base of these slopes. c. Northern closure profiles. The steeply dipping Stateline Point fault is illustrated. d* Central basin profiles. Central basin mounds are located on all the profiles. 25 w G a s u b a q u e o u s gullies 6d CENTRAL BABIN w a S' w -T * I- ' Gb w e s te rn s id e -w a ll 500 at L. — K 6c NORTHERN CLOSURE M- TJ ZM - M 27 interruptedly beneath the central basin floor (Fig, 6b, profile L). Further north, off Sugar Pine Point, slopes approach 34*° but small, broad mounds and a single, large detached block form the base of the side-wall and its transition to the central basin (Fig. 6b, profiles J and K). A wide, relatively shallow shelf rimming this point has developed. In the large embayment between Sugar Pine Point and Dollar Point, the bathymetry is characterized by a jumbled, chaotic topography of gullies and large mounds (Fig. 6b, profile I). Some of these gullies can be traced up the slopes, across the shallow shelves and Into the mouths of subaerial streams such as McKinney, Blackwood and Ward Creeks (Fig. 6a, profiles V and W). Because of this con tinuity, it is believed that these gullies are actively maintained and possibly undergoing erosion. On the deeper portions of the side-wall, the slope descends in a series of steps with flat-lying sediments apparently ponded land ward of a series of deeper and deeper mounds. South of Dollar Point a wide, shallow shelf has developed behind the 50 m contour which ranges out to 3 kms offshore. This structure appears to be deltaic but is developed off the Truckee River, the lake's outlet. There are no gullies issuing from the Truckee River and crossing the shallow shelf. East of Dollar Point, the steep shelf is Interrupted by a gentle slope break between 200 and 300 28 m. Relatively few mounds are found on the base of this slope (Fig. 6b, profile H). Northern Closure Off Stateline Point, a fault scarp is inferred from the steeply dipping (45°) slopes trending northeast- southwest (Pig. 6c, profiles M and N). The fault slope dips below the flat central basin without any apparent change. West of the scarp, a broad, shallow embayment is developed. Several gullies which may be related to sub aerial drainage transverse the slope (Pig. 6a, profile X). Bast of Stateline Point a distinct nose juts 4 kms out into the lake (Pig. 6c, profiles 0 and P). Small mounds between the nose and the eastern side-wall suggest sediment ponding. Several gullies originating near the shore line cannot be traced any deeper than 150 m. One of these, over 50 m deep with a distinct V-shaped cross- section, crosses the shallow shelf to the mouth of Incline Creek (Pig. 6a, profile Y). Subaerial erosion and sub sequent submergence by tectonics is suggested by the limited depth (less than 150 m) of these gullies and their location shoreward of an apparent graben structure. Central Basin The central lake basin is characterized by a flat, relatively monotonous surface. Where relief exists, it takes the form of apparently isolated mounds which reach 29 a maximum height of over 120 m above the flat floor with slopes up to 17° (Fig. 14). These mounds are located far from the side-walls and are localized along a north-south projection of Stateline Point down three-quarters of the length of the lake. A small scarplet of several meters height crosses several of the profiles (Fig. 14, profile Q). This scarp let is interpreted as an extension of.the large Stateline Point fault scarp with Its upthrown block to the north west. Seismic Characteristics of Lake Tahoe A continuous seismic reflection survey of Lake Tahoe (Fig. 1) was accomplished using a Bolt Associates air gun aboard the San Guiseppi. The air gun was a PAR model 600 with a chamber volume of 1 cu in. firing at 1400 lbs psi. A Bolt model 1011 hydrophone streamer and Bolt model PA-7 preamp/filter were used. The trace was printed by an Ocean Sonics recorder set at 0.5 second sweeps. On all profiles crossing the central basin it is apparent that the lake is floored with a considerable thickness of sediments. Only along the bedrock noses which jut out into the lake from the northern closure and along the steep, side-wall slopes is the seismically "opaque*1 bedrock reflected. In all other areas, the geo- 30 physical system was not powerful enough to penetrate the complete sedimentary section. Figure 7 is a typical profile across the central basin. Sediments are at least ^ second in travel time (approximately 370 m) thick. Sediments buttressed against the eastern side-wall dip westward below the penetrating thickness of the geophysical system indicating that sig nificantly more sediments lie below this depth. The thickness of sediments is surprising in view of the lake's relatively young age (late Pliocene) and small drainage area (Fig. 2). It is apparent that the drainage basin of Lake Tahoe has experienced a period or periods of significantly accelerated erosion in the past to create this large volume of sediments. Natural Provinces Granitic bedrock, locally covered by a thin blanket of sediments, is apparent on all eastern side-wall pro files. The steeply dipping fault which separates layered sediments from "opaque" bedrock can be traced below the sediment surface to beyond the limit of seismic penetra tion (Fig. 7). Small mounds at the base of the slopes are . sedimentary and appear to have slumped off the steep side wall. Along the western side-wall granitic bedrock is characteristically covered by a considerable thickness of Figure 7. Seismic profile M-N. Multiple echoes have been removed from all seismic profiles. 31 M L A I C t l E V E l LAKE (TAHOE SE C,= A P K O X . 373 GEOPHYSICAL PROFILE N \ i SEC 2 i NAUTICAL m i l e s ro sediments. Steep, bedrock surfaces are exposed only along the southernmost length of this section (Pig. 8). Despite the great thickness of sediments, granitic side-wall and the fault surface is indicated on many of the profiles (Pig. 9). Well-layered deltaic-outwash sediments are ap parent on the Sugar Pine Point (Pig. 10) and Dollar Point (Pig. 11) structures. Inside the embayment, between these two structures, the slope is covered with a thick volume of sediments displaying a chaotic internal structure with no horizontal reflectors and is expressed as a jumbled mound topography on the surface (Pig. 9). The southern delta reflects a well-layered deltaic structure (Pig. 8) in the profiles* Small mounds at the base of the structure are sedimentary and appear to be slump deposits. No bedrock reflections are recorded under the delta. Beneath the deltaic surface, three excellent reflecting horizons occur at depths of 24, 135 and 265 m. Because of their shape, position and Internal reflections, these are interpreted as ancient deltaic surfaces. The spectacular tectonics Inferred from the northern closure bathymetry is confirmed by the geophysical records. The hanging wall of the large, southwest-northeast trend ing Stateline Point fault is composed of bedrock with con siderable sediment ''ponded1 1 to the west behind the hanging wall (Pig. 12). A bedrock nose jutting out into the lake from the eastern side of the northern closure completes the Figure 8. Seismic profile E-Q-R. Three buried, deltaic surfaces are located beneath the southern delta. Two basins are located in Emerald Bay. The westernmost is the catch basin for turbidites originating in the bay. 34 TERMINAL MORAINE EMEtALD MT / ■ U 1 R IE O D E LTAIC J U IF A C H POST GLACIAL SEDIMENTS SEC N M V J 1 Figure 9. Seismic profile I-J. The extent of the chaotic layer which crosses the central basin and is traced into the fumbled slump topo graphy at the base of the western side-wall is shown. 36 SLUMP M OUND b e d r o c k LAKE TAHOE —■ 3 1 Figure 10. Seismic profile K-L. This profile crosses the Sugar Pine Point deltalc-outwash structure. 38 K SUGAR PINE POINT L LAKE TAHOf N.M V j J VO Figure 11. Seismic profile D-E. Four periods of progradations are delineated in the Dollar Point deltaic-outwash structure. Gullies which originate in the mouths of subaerial drainage are seen cutting the most recent sediments. 40 d o l l a r p o i n t CANYONS LAKE TAHOE SLUMP MOUND H tioga 0TAHOE F^Idomivieh lake E 3 H O B A H T 9 1 H .M .. J \ H Figure 12. Seismic profile G-H. Four periods of progradations are delineated In the northern closure deltaics. 42 H LAKE t t V f l LAKE TAHOE ^ / / ____ H' Q n D T | o c 3 A (tataItaHOE Q doimner lake [h hTHOBART N auTiCai M*it5 ‘i SECONDS -o 44 sediment-filled graben structure which is walled by the two bedrock noses (horsts). A thick volume of sediments was ponded behind the easternmost nose. The Stateline fault nose supports a thick section (125 m) of sediments in an area of limited fluvial drainage. Easily weathered volcanic mudflows which were rapidly eroded from the sur rounding areas (Pig. 2) are the source of these sediments. The central basin is covered by a characteristical ly well-layered but thin (less than 40 m) sedimentary section (Pig. 13). Beneath this surface layer is a thicker strata which displays hyperbolic reflecting points characteristic of a chaotic internal structure. This chaotic layer locally projects through the surface layer to form the central basin mounds (Pig. 9). Large mounds at the base of the eastern side-wall reflect well- developed layering similar to the deltaic-outwash structures of the western side, but have roots in the chaotic layer (Pig. 9). The chaotic layer can be traced into the jumbled topography of the western side-wall em- bayment (Pig. 9) which also displays the same character istic chaotic internal structure. Below the chaotic layer in the central basin are thick, well-layered strata similar to the surface strata. The central basin stratigraphy can be characterized by well-layered, undisturbed sedimentary strata overlying and underlying a disturbed sedimentary layer of chaotic internal structure. Figure 13. Seismic profile C-BC. Well-layered Recent turbidltes overlie the chaotic layer. 45 TILTED SLUMP BLOCK LAKE TAHOE WELL LAYERED RECENT s e d im e n t s I Source of the Sediments 47 Source for the great volume of basin sediments is along the southern and western side-walls. This is evident from the great masses of sediments in these areas and the pinching out of sedimentary layers toward the east (Pig. 9). This sediment thinning with increasing distance from the source relationship is well-documented in sedi mentary basins (Gorsllne and Emery, 1959)* The present Lake Tahoe fluvial drainage is relatively symmetrical about the lake, favoring neither side-wall (Pig. 2). The signi ficant difference in source area erosional histories is the Pleistocene glaciations which occurred only along the southern and western portions of the basin. Large sedi mentary structures at Dollar Point and Sugar Pine Point along with the Jumbled sediments covering the intervening slopes can be traced up into the great volume of glacial moraines and outwash on the lake's shores (Pig. 2). Well-layered sediments in the central basin are ponded behind mounds along the western side-wall (Pig. 9). The topography produced is analogous to abyssal plains and hills topography of the oceans (Menard, 1956). Sediment - laden southern, western and northern shores are the source of the turbidites flowing out into the central basin. Canyons along the western side-wall embayment transect the uppermost well-layered sediments (Pig. 11) and can be traced up-slope into the mouths of subaerial streams. Draining from an area of high precipitation and uncon solidated glacial sediments (Fig. 2), heavily laden western basin streams at flood stage debouch along the steep west ern side-wall slopes where the turbid waters are trans formed into turbidity currents. Density currents and related channels are well-documented in lakes (Forel, 1885 and Gould, 1950). There is considerable but inconclusive evidence that turbidity currents are able to maintain and erode canyons in an oceanic environment (Shepard and Dill, 1966). Turbidity currents may also be triggered by seismic events along the sediment covered, steep slopes or by shoreline erosion during storms. The thick mass of well- layered sediments underlying the chaotic sediments in the central basin is acoustically the same as the surface turbidites. Correlation of Drainage Basin Events with Lake Sediments Pleistocene Events In the Lake Tahoe Drainage Basin Russell (1883) recognized the extent of glaciation surrounding Lake Tahoe and Le Conte (1883) went so far as to speculate that the entire basin had been filled by ice. Hubbs and Miller (1948) utilizing fish fauna, connected Lake Tahoe with the Pleistocene Lahontan system. In his classic work on Sierran glaciations Blackwelder (1931) named one of the stages after the lake. Birman (1964) re defined the Sierran system to include three groups. These are: (Matthes Group III (Recess Peak (Hilgard (Tioga Group II (Tenaya (Tahoe Group I Sherwin Group III is Recent and consist of limited glacial ad vances and cirque glaciers. Group II is Wisconsan in age whereas Group I is reportedly Illinoisian. Blackwelder (1931) included a McGee of possible Kansan age in his stratigraphy. Birkeland (1964) completed a detailed glacial stratigraphy of the area north of Lake Tahoe. Birkeland was unable to establish any conslstancy in boulder- weathering ratios criteria and was, therefore, unable to correlate directly with Birman's stratigraphy. Birke land *s sequence is: Prog Lake Tioga Tahoe Bonner Lake Hobart The Prog Lake deposits are limited to cirques and correlate with Birman's group III. Hobart till, the oldest, is deeply weathered and exposed in only a few locations (Fig. 14). Glaciation of Donner Lake age was at least as extensive as that of Hobart. Donner Lake glaciers merged in the Upper Truckee Canyon forming a compound valley glacier 35 kms long with a maximum thickness of 256 m. Tahoe age glaciers were less extensive forming a compound valley glacier only 110 m thick. Tioga age glaciers were even less extensive, barely forming a trunk glacier in the Upper Truckee Canyon. Many absolute age dates are re ported for these glaciation (Birkeland, 1964 and 1965). Hobart and Donner Lake tills are pre-Wlsconsan and Birke land (1964) estimated the Donner Lake glaciation to have been initiated about 400 to 600 thousand years ago. Tahoe and Tioga glaciations are Wlsconsan and directly cor relate with Blackwelder1s stratigraphy. Birkeland failed to recognize a Tenaya stage (Birman, 1964) in the Truckee area. The upper limit of Tioga glaciation was established by radiocarbon as 9900 t 800 B. P. on the south shores of Lake Tahoe (Haynes, Damon and Gray, 1966). An interesting effect of the Upper Truckee Canyon valley glaciers was the damming of Lake Tahoe's outlet causing a higher lake level stand during Pleistocene glaciations. Lake Tahoe first occupied a higher stand soon after deformation formed the basin and Late Pliocene Big Chief basalt flows dammed the lake to an elevation of 2073 m (Birkeland, 1963). After this outlet had been con siderably lowered by erosion, the Tahoe City flows were Figure 14. Glaciations in Truckee Canyon, Hobart, Donner Lake and Tahoe were extensive enough to block Lake Tahoe's outlet. 51 52 S Q U A W CR EXTEN T OF G L A C IA T IO N T IO G A t a h o E BEAR CR TRUCKEE RIVER PROSSER CR ?!*■**•• F*trSlK?R«* * ALDER CR ^ • n T iilT l f R n t T TRUCKEE C O L D CR. DEEP CR P O LE CR D O L L A R P O IN T D O N N E R LAKE X HOBART TILL 0 S kmi 10 A F T E R B I R K E L A N D . 1 96 4 53 extruded and the lake level rose to 2134 m. During the Donner Lake glaciation, the ice elevation was enough to dam the lake to 2085 m, a rise of 183 m. A bench Is evident around the lake between elevations of 2073 and 2042 m (Burnett, 1968) but It is not certain whether this shore line is related to the volcanic or to the glacial damming (Birkeland, 1963). Lindgren (1896) and Jones (1929) mapped lacustrine sediments at about that elevation in the southern portion of the basin. Lake sediments overlain by- possible Donner Lake drift at an elevation of 1939 and 1926 m occur near Tahoe City and pre-Wisconsan (Donner Lake?) deltaic deposits crop out at 1951 m southwest of Tahoe City (Birkeland, 1963)* Ice-rafted metamorphic and granitic erratics are found at 1951 m in the volcanic ter rain Immediately north of Dollar Point. On the southwest side of the basin (at Lonely Gulch) brownish lake sedi ments are exposed at an elevation of 76 m above lake level (McAllister, 1936 and Wahrhaftig, 1965). Cave rock and Eagle rock, two volcanic plugs, appear to have been wave- planed to about this elevation (Evans and Matthews, 1968). Tahoe glaciation could have dammed Lake Tahoe to an elevation of 1963 m, a rise of 64 m, however, no Tahoe age lacustrine deposits have been found at this elevation. Lake deposits of this age have been reported at 1917 m and 1914 m near Tahoe City and at 1926 m at the south end of the lake. Lindgren (1897) noted an abundance of 12 and 24 m terraces. Eagle Rock, near Tahoe Pines, is wave-cut at an elevation of 27 m. It appears that the Tahoe glacia tion was responsible for a rise of no more than 27 m in the lake (Birkeland, 1964). Tioga glaciation could not have dammed the lake to more than 3 m, however, Birkeland (1964) was unable to locate any lake deposits of Tioga age at that elevation. Lindgren (1897) described Ice-rafted metamorphic erratics along the south shore In terraces at 4 to 6 m. There is well-documented evidence that the lake level fell from these elevated, Pleistocene stands at a catastrophic rate. When the lake level rose above nine- tenths of the ice dam elevation, glaciers were buoyed up and water allowed to flow out underneath the ice. The l i resulting flood is termed a jokulhlaup (Thorarinsson, 1939» Marcus, I960 and Stone, 1963). Evidence for the Pleisto cene (Hobart, Bonner Lake and Tahoe age) lake Tahoe- Truckee Canyon JJokulhlaups occurs at Verdi, Nevada where enormous boulders up to 10 m in diameter are located and where giant gravel bars rise 12 m above the adjacent ter race surfaces (Birkeland, 1965). These floor deposits Interbed and overlie the Pleistocene Lake Lahontan sedi ments at Mustang, Nevada. Lake levels at Lake Tahoe re spond to long-term, Recent climatic events (Harding, 1948), but the fall from the Pleistocene ice-dam lake level to the 1 1 lava dam lake level during the jokulhlaups must have been 55 spectacular. Pleistocene Events in Lake Tahoe Sediments Three glaciations, Hobart, Donner Lake and Tahoe, created ice dams along the Truckee Canyon outlet causing elevated lake levels. Deltaic-outwash structures which had prograded into the lake during these elevated lake levels experienced an essentially instantaneous loss of support as lake levels fell during the Joklhlaups. This must have triggered slumping on a catastrophic scale along the steep slopes of the lake. The large volume of chaotic sediments and Jumbled topography at the base of the slump slopes (western side wall) are the deposits formed by this slumping (Pig. 9). This mass is traced into the chaotic layer which thins toward the eastern side-wall as it crosses the central basin. Near the eastern side-wall, several large deltaic masses are located off Deadman's Point (Pig. 9). Deadman's Point is the only area along this side-wall with signifi cant, Recent sedimentary progradation at the modern lake level (Pig. 2). The fluvial deltaics prograded at elevated lake levels near Deadman's Point slumped as a single co hesive structure to the base of this slope during the lake lowering as evidenced by the position of the roots of these blocks in the chaotic layer. Purther evidence for massive slumping is found be- neath the chaotic sediments covering the western side-wall. Figure 15 outlines a large, well-layered block tilted to ward the west. This block is 0.8 km long, 0.5 km wide and 70 m high. The block is obviously allogenic because its excellently layered reflections are surrounded by chaotic sediments with no ordered, internal reflections. This block is strikingly similar to the slump blocks off the Grand Banks (Heezen and Brake, 1964). Occurrences of large-scale slumping of this magnitude are well-documented in the marine environment (Menard, 1964). The lake Tahoe block is a large deltaic-outwash structure which Blumped as a cohesive mass during a lake lowering. Ages of the central basin stratigraphy can be inter preted from relationships in the southern delta. Three distinct deltaic surfaces are reflected below the present delta (Figs. 8 and 16). The origin of the delta as an outwash plain formed by the confluence of valley glaciers is well-documented. Three burled, deltaic surfaces are in a stratigraphlc position which correlates them with Hobart, Bonner Lake and Tahoe drainage basin glaciations and the present deltaic surface is correlative with the youngest glaciation, Tioga. This relates the least extensive glaciation (Tioga) to the southern delta’s smallest volu metric unit (the uppermost). Large deltaic structures prograded at elevated lake levels must have slumped into the central basin leaving the base of the deltaics at the Figure 15. Seismic profile A-B. A large, tilted slump block Is buried below the Jumbled slump topography at the base of the western side-wall. 57 A * ; TILTED SLUMP BLOCK LAKE TAHOE nautical miles U1 oo Figure 16. Seismic profile R-S. Four periods of progradations are delineated in the southern delta. The two oldest periods dip below the central basin strata. 59 SLUMP LAKE itahoe C f \ o lower, lava dam, level where they were wave-planed during the interglacials. Effective wave-plan at ion is evident from the wide, shallow shelf (Pig. 3) developed on the present delta. Subsequent deltaic progradation during later glaciations resulted in compaction and lowering of buried deltaic surfaces to their present levels. Compac tion in modem deltaic environments is common (Scruton, I960). Along the southern length of the eastern side-wall (Pig. 17) where strata dip steeply toward the west, the Donner Lake and Hobart glacial deltaics approach the sur face. The two oldest (and deepest) deltaic surfaces can be traced out into the central basin where they dip below the central basin sequence and to below the depth of recorded seismic penetration thickness (Pig. 16). Lipping beds in the Lonner Lake delta give the impression of fore set beds. Tahoe deltaic sediments can be traced out into the large mounds which display distinct horizontal re flections and beyond into the central basin and the chaotic layer. During Tioga glaciation, limited erosion and rela tively stable lake levels altered the southern delta to a significantly smaller degree. Because of this, the mound and gully topography on the delta face must have formed during the delta adjustment from the elevated Tahoe glacia' tion lake level by slumping and wave planation to present Figure 17. Seismic profile 0-N. Sediments steeply dip toward the western side-wall. 62 ROCHE MONTONNEE LAKE TAHOE 1 N.M ON 64 day lava dam level. The well-layered sediments surrounding the chaotic layer rest on the deltaic surfaces of the southern delta indicating that they icorrelate primarily with Recent and Interglacial periods. This Is also shown along the western side-wall and central basin where the well-layered sediments (Recent) pond landward of the chaotic layer (glacial). Further evidence for the Pleistocene slumping and lake stratigraphy is located in the other deltaic-outwash structures of the lake. Three distinct surfaces are pre sent in the large Dollar Point sedimentary structure (Pigs. 11 and 18). Sedimentary progradation along a bed rock core on this structure must have occurred during reversed Truckee River flow when ice dams occupied Truckee Oanyon. Large boulders on Dollar Point shores and along the shallow shelf were ice-rafted and deposited in their present location when icebergs grounded in the shallow water off the outlet. The three burled surfaces are slump scars which correspond to events following Hobart, Donner Lake and Tahoe elevated lake levels. Some slump mounds displaying foreset beds are located both north and south of the points. On Figure 18 the older age of the Hobart and Donner Lake slump scars which dip below the central basin strata can be seen. Figure 19 connects the Dollar Point Figure 18. Seismic profile H-I. Four periods of progradations are outlined. The Bcarplet cutting central basin sediments is 12 m high. 65 d o l l a r p o in t H fau lt LAKE TAHOE Q hobart Figure 19* Seismic profile F-G. This profile correlates the Stateline Point and Dollar Point deltaics. Both show four periods of progradation. The sediment-filled graben is located to the east of the deltaics. 67 CANYONS R ECENT a BEDROCK NO SE LAKE TAHOE F^itioba ] doi\ iiuer lake {HOBART TA TA SEDIMENT FILLED GRABEN deltaic surfaces with the eroded volcanic mudflow breccia surfaces along the Stateline Point bedrock nose. Three prominent burled slump scars are evident in this structure which was prograded during glacial periods of rapid erosion and elevated lake levels. Chaotic layer sediments are present in both the graben strata and ponded behind the bedrock nose adjacent to the eastern side-wall testify ing to the extent of the glacial slumping event over the entire central basin floor. Slump scars are not recog nized in the Sugar Pine Point structure but slump struc tures (Tahoe age) occur at the base of the slope and ex tend into the central basin chaotic layer. Along the eastern side-wall, where sediment sources are most distant and strata correspondingly thinner, is a compressed central basin stratigraphy. Figure 20 shows at least one, and possibly two, more chaotic layers wedging out against the side-wall below the Tahoe glacial chaotic layer. These two deeper layers correspond to Donner Lake and Hobart glaciations. Central Basin Scarplet The fault creating the central basin scarplet (Pig. 17) located off Dollar Point displays increasing displace ment of central basin strata with depth and there is a freBh scarplet 12 m high on the surface. These relation ships show the fault to have been active continuously dur- Figure 20. Seismic profile B-BC. Three "buried chaotic layers are outlined in the compressed section along the eastern side-wall. 70 B-C B IEDROCK LAKE TAHOE -0 72 ing the period of strata deposition and that it is active today. Particularly significant is that the upthrown side is toward the west, similar to the large Stateline Point fault. Its position is far from the side-wall fault and the upthrown side (to the west where the glacial sediment accumulations are located) negates any isostatic compensa tion origin. In view of its location on the projection of the Stateline fault in an area where the spectacular northern closure tectonics abruptly disappear, the fault is interpreted as the continuation of the Stateline Point fault. The presence of active faulting suggests some side-wall slumping and turbidity currents could be selsmi- cally triggered. Central Basin Mounds In all central basin profiles displaying mounds, the mounds have their roots in the chaotic layer (Pig. 9). Except for the well-layered blocks off the deltas and the eastern side-wall, the mounds display the same character istic, hyperbolic reflecting, disoriented internal struc ture as the chaotic layer. In many profiles the well- layered older sediments pass unaffected below the mounds (Pig. 17). No thickening or thinning of the chaotic layer occurs near the mounds. A possible origin for the mounds is the glacial slumping of large masses of ice-cemented sediments off the 73 side-walls. Because of Ice buoyancy, tbe blocks are characterized by a relatively low specific gravity. Dur ing the massive slumping, they would tend to slide to the deepest levels of the lake bottom resting on the chaotic layer and upon finally melting, release the large volume of sediments. A diaplrlc origin was rejected owing to the lack of thickening or thinning of beds and the steep sides of the •** • mounts. A Varian Rubidium magnetometer was towed over the central basin mounds in an attempt to either confirm or negate a volcanic origin. No change in the magnetic field over the mounds was observed indicating a non-volcanic origin. Strong magnetic readings were recorded near Cave Rock, a volcanic plug. Summary of Geophysical Characteristics The great thickness of sediments flooring lake Tahoe was derived from the glaciations occurring along the southern and western side of the basin. During the most extensive glaciations (Hobart, Donner Lake and Tahoe) ice dams formed In Truckee Canyon creating elevated lake levels. Deltaic-outwash structures prograded out along steep slopes at elevated lake levels and then slumped Into the lake at a catastrophic rate when the ice dams broke. Three buried deltaic surfaces and three slump scars in the western side-wall deltaic-outwash structures cor 74 relate with the events. Slump deposits include the large jumbled sediments in the western side-wall embayment, mounds at the base of the sedimentary structures, mound and gully topography on the southern delta face and three chaotic layers with hyperbolic reflection points wedging out across the central basin toward the eastern side-wall. Tioga glaciation was characterized by minor progradation and relatively stable lake levels. Evidence for slumping or large scale changes during this interval is scarce. Thin Tioga deposits cover most present day sedimentary structure surfaces. The central basin chaotic layers are surrounded by well-layered sediments attributed primarily to turbidity currents. Western, northern and southern sources for these turbidites are indicated in the topography which is similar to oceanic abyssal hills and plains. Active can yons which can be traced into subaerial stream mouths occur along the western side-wall. Floods, seismic events and shore erosion during storms are suggested as triggers for the turbidity currents along the steep, sediment covered slopes. Seismic and magnetic evidence negate either a dia- plric or volcanic origin for the central basin mounds. Their chaotic internal structure and roots in the chaotic layer indicate that the sediments may have been deposited as ice-cemented masses contemporaneously with chaotic layer 75 sediments* Subsequent melting of the mass formed the sedi ment mounds. Sedimentary Characteristics of Lake Tahoe Textures Twenty piston coring sites were selected from the geophysical profiles (Jig. 5). The piston coring was ac complished using a Kullenberg type piston corer from a large pile-driving Large. The lithology of the piston core from station 11 was correlated with its trigger core at 29 cms. As much as 30 cms of surface sediments may have been lost during the piston coring operations. Representative mean grain sizes effectively de lineate the sedimentary environments of Lake Tahoe (Pig. 21). These sizes are derived from either surface samples or the samples most representative of the bulk lithology. The natural bathymetric provinces of the lake essentially coincide with the general sedimentary regimens except for samples located in the greatly restricted environment of Emerald Bay. Central basin sediments are characterized by very fine-grained, moderately sorted sediments. Mean grain sizes primarily range from 0.009 to 0,006 mm (fine silt- size). Diatom frustules constitute a significant portion of these sediments and may, therefore, effectively limit Figure 21. Mean grain size and sorting distri bution. Sediments are coarse in the slump and deltaic deposits. 76 LAKE TAHOE 0 0 0 5 . 3 . 2 \ 0 . 0 0 5 m m / 0 . 0 0 7 S ™ 0 . 0 5 TX 0 . 0 0 3 12~ 0 . 0 1 mm 0 . 0 0 7 1.0 1. 0 O .O O S m m I.O m m 0 . 0 0 7 1 .7 0 .3 m 0 . 0 0 7 0 . 0 0 7 ■ — 0.01 mm"* OOOB. 0 . 0 0 6 0. 2; 0 . 9 g r a d e d 61 o . l M E A N G R A IN SIZE 0 . 7 S O R T IN G NAUTICAL MILES 78 mean grain size variations. The finest grained sediments (0.002 mm) blanket the central basin mounds indicating the effectiveness of these as topographic barriers to turbldite deposition. The central basin sediments are typified by moderate sorting values (1.7 to 2.0) •with the poorest sort ing on the central basin mounds (2.3). Numerous diatom frustules may also control the sorting values in this environment. Radiographs of sediments on the periphery of the central basin (cores 1 and 14) are characterized by numerous graded sandy layers (PI. I). Sediments in the interior of the central basin (cores 10, 11, 12, and 13) display numerous fine laminations but no graded coarse layers (PI. II). This decrease in the coarsest grain sizes along the path of a turbidite may be used as an index to the proximity of the sediment sources (Bouraa, 1962) . Thin laminations also occur on the 120 m high cen tral basin mounds (core 12) suggesting that the lamina tions are due to sedimentation variations in the water column caused by such events as floods and dry seasons in the drainage basin. Sediment plumes in surface waters are often observed originating off stream mouths during flood ing. Because of the absence of significant biological activity in Lake Tahoe, an important consideration in the formation of varves in other lake sediments, the correla- Plate I. Structures in sediments close to source. 79 STRUCTURES OF SEDIMENTS CLOSE TO SOURCE THICK GRADED LAYERS THICK g r a d e d : LAYERS Plate II. Structures in sediments distant from source. 81 STRUCTURES 05= SEDIMENTS DISTANT FROM SOURCE WIDELY SPACED LAMINATIONS 83 tion of the thin laminations with annual events is tenuous. Pine laminations in the central basin sediments are related to varying proportions of sediment contributions by distal turbidite and drainage basin events. There is no significant increase in sand content with depth in the sediments which is interpreted to indi cate that no changes in the turbidite patterns occurred. At the base of the central basin mounds (core 11), how ever, there is a marked increase in sand content and coarser graded structures near the surface signifying a possible decrease in slope stability and increase in turbidite activity as the sedimentary blanket thickens on the central basin mounds. A common occurrence in central basin sediments is individual sand grains in a fine-grained matrix (PI. III). Ice, which often forms in Emerald Bay and along re stricted segments of the shoreline during the winter, in corporates shoreline sediments which are subsequently rafted into the lake. After the ice melts, the grains probably settle to the bottom to produce this unusual sedi mentary texture. During Pleistocene glaciations, a considerable number of icebergs must have formed by calving of lake front glaciers. Large volumes of sediment were discharged into the lake upon melting at a time approximately con temporaneous with the massive slumping. Both slump and Plate III. Sediment inclusions* SEDIMENT INCLUSIONS CORE “ 13 CORE1« ICE RAFTED SAND GRAINS CDRE IB VIVIANITE CD VJ1 86 Ice-rafted sediments would produce identical seismic re flections. The chaotic layer, therefore, may "be composed of varying components of these sediment types. Seismic reflections strongly suggest the central basin mounds were formed contemporaneously with the chaotic layer and are composed of the same material as the chaotic layer (unsorted, unstructured, glacially-derived slump sediments). Core 12 confirms this. After penetrat ing 31 oms of thinly laminated, fine-grained sediments, the piston core bottomed in 11 cms of unsorted, clay to cobble-sized sediments. The core-catcher was clogged with a cobble-sized basalt fragment. Also present in the sedi ments were large fragments of granodiorite and consolidat ed sediments. The variety of lithologies coupled with the poor sorting of the sediment strongly support the ice rafting-slumping origin of the chaotic layer. Two volcanic ash layers are Included in the central basin stratigraphy (Fig. 22). These 1- to 2-cm thick, light gray layers are characterized by angular glass shards. No ash layers are present around the base of the mound (core 11) or on top of the mound (core 12), further supporting the proposed higher sedimentation rate around the base and instability on the top of the mounds. Southern delta sediments also display a great variety of textures. Sediments on the delta face (cores 6 and 7) are characterized by coarse (mean grain size, very Figure 22. Volcanic ash distribution. Rela tive depths to the ash layers are an indica tion of sedimentation rates. 87 17 16 17 C O R E 14 13 10 100 200 300 400 C M S WESTERN S ID E - W A L L N O R T H E R N C LO S U R E C E N T R A L B A S IN S O U T H E R N D E L T A EMERALD BAY VOLCANIC ASH DISTRIBUTION CO CD coarse sand), well-sorted sands. The lithology is strik ingly similar to grus, reflecting the predominately granitic bedrock drainage basin. The light color of the deltaic sediments easily distinguish them from the darker deltaics of the northern volcanic areas (PI. IV). The clean, well-sorted sands confirm that the originally poorly sorted deltaics were wave planed during the inter- glacial stages of lower lake levels. Blanketing the light sands is a thin (5 to 10 cms), fine sand layer which represents the Recent contribution to the southern delta by the Upper Truckee River. Seismic reflections on the ridge and gully topo graphy of the southern delta reveal the ridges to be relic deltaic slump structures (Pig. 16) of Tahoe glaciation age after which the gullies became inactive. Limited Tioga deltaics did not prograde out into this area. Sedi ments from the top of these ridges (core 8) support the predicted old age. Pine-grained sediments (mean, 0.016 mm) displaying thin laminations and ice-rafted sand grains similar to central basin sediments blanket these struc tures. Sediments on the deepestsections of the delta face (cores 5 and 9) represent the transition between the deltaic and central basin environments. The phi mean grain sizes (0.008 mm) and sorting (1.7) of these sedi ments are more closely representative of the central basin. Plate IV. Deltaic sediments* Deltaics from a granitic source (cores 6 and 7) are noticeably lighter than those from a predominately volcanic source (core 18). 90 91 \ iifrr/li *■. f ; : 0s F j i U . Vu^v a.^ i m---. * i n IC T iA - '- ^ '^ x ..'- .v » < .> . . . *— -t . f , J %'«' 1 t ^ >f^ t l L .—»<J , . ^ ' * ’ i j» iil I m m ), I I - - 1 1 5 » - - ^ ii P M h p i p 92 Graded laminations becoming finer with distance from the delta are common. Coarse sand layers (PI. I) also are present. No major drainage is located along the northern closure indicating that the thick deltaics consist of easily weathered volcanic mudflow breccias stripped off the adjacent terrain. Northern closure sediments (cores 15 and 16) are characterized by the finest grain sizes (0.005 mm) and poorest sorting (3.2). Laminations are extremely thin, blending into an almost homogenous struc ture (PI. II). The depth to the volcanic ash layers in these sediments is almost twice as deep as in the central basin indicating a significantly higher sedimentation rate (Pig. 22). The high sedimentation rate coupled with the fine laminations characterize an environment of small but frequent sedimentary events. Sediments along the western side-wall are coarse. Off Dollar Point (core 18) a coarse, unsorted layer (mean diameter 0.2 mm, sorting 3.6) of 23 cms thickness covers a large graded sand structure of at least 200 cms thickness. The turbidlte origin of this structure Is supported by the clay clasts present in the sequence. The dusky brown color, 5YR 2/2 (Geological Society of America, 1963) of these deltaic sands from a predominately volcanic source readily separates them from the lighter colored deltaics from granitic sources (PI. IV). On the southern face of the 93 Dollar Point delta (core 19) the coarse deltaic sands showing no structures except large pebbles (ice-rafted?) are covered with a thin, 8 cms-thick blanket of dark yellowish brown sand. This characteristic sand blanket also occurs on the large deltaic slump (core 18) north of Dollar Point. Below this layer lies the volcanic ash at 8 to 9 cms indicating a Recent period of erosion or non deposition. Below the ash the-finely laminated sediments with a large sand content (68.percent) suggest a flood water or turbidite source in the vicinity. In the western side-wall embayment (core 20) ex tremely poorly sorted (pebbles to clay-sized) sediments were encountered. It appears that this sample was ob tained from a topographic high composed of chaotic layer sediments rather than turbidltes ponded behind the barriers (Pig. 15)• Geophysical profiles show Emerald Bay to consist of two basins (Pig. 8) containing a considerable thickness of sediments (up to 40 m). It’ ’ ~is assumed that these sediments II are all post Tioga as that glaciation occupied the fjord. The basin closest to the cirque is a catch basin for most turbidltes originating in the bay. Sediments from this basin (core 2) contain large graded structures start ing with fine sands and silts at the top and granitic pebbles at the base. Between turbidite events, sections up to 130 cms thick of thinly laminated, fine-grained sedi- 94 meats were deposited. Although present in the adjacent basin, volcanic ash is not encountered in this basin indi cating its rapid sedimentation rates. Sediments in the outer basin (cores 3 and 4) are relatively homogenous and fine-grained. Mean grain sizes range from 0.0032 to 0.0072 mm whereas sorting is a poor 1.7 to 2.3. A few micaceous laminations are present. Frequent large rock fragments testify to the effectiveness of ice-rafting in Emerald Bay. The sediments of core 9 (southern delta-central basin) and core 12 (central basin mound) were texturally analyzed in detail. The lack of significant changes in sediment size distributions in these samples suggest that there have been no significant changes in the sedimentary regimens during this time. Geochemistry Total carbon and carbonate analyses were accomplish ed in a LECO carbon analyzer as described by Kolpack and Bell (1968). The sedimentary regimens and geochemical parameters of Lake Tahoe produce several unusual geo chemical environments. Lake Tahoe sediments definitely cannot be used to typify all lake sediments. Carbonates Fifty-six samples were analyzed and none contained 95 any carbonates. This deficiency is a product of the ultra- oligotrophic level of biological activity. Calcium ions average only 9.0 mg/L and alkalinity is 41 mg/L in the water (Goldman and Carter, 1965). More Important, however, is the nitrogen deficiency which prevents the growth of calcium carbonate-secreting planktonic and benthic com munities. Organic Carbon Owing to the absence of carbonates, total carbon determinations are essentially organic carbon determina tions. The surface distribution of organic carbon (Pig. 23) effectively delineates areas of active, fine-grained sedimentation, 1 percent, from relic or coarse-grained sediments, < 1 percent. Because of the absence of a benthic community, the source of the organic carbon is primarily diatoms from the "planktonic rain." In areas of rapid sedimentation, deltaic and turbidite regions, this "rain" is diluted by sediments. Areas of rapid sedimentation such as the inner turbidite catch basin in Emerald Bay are low in initial organic carbon whereas the adjacent outer basin in Emerald Bay is richer in initial organic carbon by a factor of about 500. Lake Tahoe waters are saturated with respect to oxygen throughout the water column. Coarse-grained sedi ments permit interstitial water circulation which ac- Figure 23. Organic carbon and nitrogen distri bution. Organic carbon and nitrogen is characteristically low in areas of relic or coarse sediments. 96 97 LAKE TAHOE SIl II) 1 11 I.M 1 .1 1 • II N 113 III II OROANIC CARBON III) NITROQCN NAUTICAL MILEB 95 celerates oxidation and the removal of organic carbon. Fine-grained sediments restrict interstitial water circula tion and, therefore, decrease the rate of organic carbon oxidation producing a characteristic slowly decreasing organic carbon distribution with depth in the sediment. Areas of relic sediments or rapid fine-grained deposition such as the top of the mounds are low in carbon as it has long since been oxidized and removed by interstitial water circulation. Organic carbon was determined in detail for all samples. In all sediments, except for those initially impoverished in organic carbon, the amount rapidly de creased with depth in the sediment. No major organic carbon fluctuations occur in the central basin sediments with depth indicating no major changes in the sedimentary regimens of the sediments. Figure 24 presents a detailed profile of the organic carbon content as it decreases with depth in the sediments due to oxidation. Nitrogen A modified Kjeldahl method (Bradstreet, 1965) was used for ammonoid determinations.- She-nitrogen content of the sediments generally reflects the organic carbon dis tribution (Fig. 24). Yalues range as high as 0.51 percent nitrogen in surface sediments with high organic carbon content (Emerald Bay) and as low as the values of reagent Figure 24. Organic carbon distribution with depth in the sediments. This profile is typical of Recent, fine-grained sediments as the initial organic carbon is oxidized and removed with time (depth in sediment). 99 100 % ORGANIC CARBON 10 2.0 30 40 100 200 300 400 500- i 1 / CMS DEPTH IN SEDIMENT CORE 1 C J - CENTRAL BASIN 101 blanks (0.04 percent) In areas of relic sediments (western side-wall). Organic nitrogen is less than 0.05 N/l in the Lake Tahoe water column (Goldman and Garter, 1965). Original carbon to nitrogen ratios in diatoms are about 5 (Vinogradov, 1953). Organic carbon to nitrogen ratios range from 6 to 16 in surface sediments of high nitrogen content, indicating that the nitrogen is being oxidized and removed faster than the organic carbon. Nitrogen is completely removed at a depth of 160 cms in core 9. Olay Mineralogy Oriented clay slides were made from the clay-sized fractions. These were X-rayed on a Norelco diffracto meter after air drying, saturation with ethylene glycol and heat treatment at 550°C. Relative abundances of the clay minerals were estimated using the ratios of the peak areas, as determined by planimeter, under the 7-15 ^ (kaolinite), 10 A (illite) and 14 A (montmorillonite) peaks after air-drying. The use of compensating factors to compute absolute clay mineral abundances (Weaver, 1958) was avoided due to their inherent ambiguities. Clay minerals are predominantly illite (45 to 80 percent) with lesser amounts of montmorillonite (5 to 52 percent) and traces of kaolinite (0 to 15 percent). Despite obvious lithological differences in the northern 102 and southern drainage basins, no regional differences in clay mineralogy were found. Weathering intensity rather than source rock lithology must be the controlling factor in the formation of clay minerals in the Lake Tahoe drain age basin. The ineffectiveness of chemical weathering in the drainage basin is reflected in the sparse kaolinite development. A central basin-southern delta core (9) displays no change in clay mineralogy with depth. The complete absence of kaolinite is characteristic of the chaotic layer suggesting an intensification of mechanical ' “erosion during that period. Authigenlc Vivianite Characteristic of the fine-grained sediments from the central basin and the northern closure deltaics is the occurrence of the authigenlc mineral vivianite (Pe^PgOQ . 8h20)» PI. III. Originally earthy in color, upon recovery it rapidly oxidizes to an irridescent blue-green. The mineral which occurs as sand-sized spherules or bladed spherules is present in the sediments in thin layers of a few cms thickness starting from 80 to 260 cms from the surface. In several instances fecal pellets were ob served that are being replaced by vivianite which explains the source of the phosphate. Iron is apparently carried In the interstitial water of the sediments from a source such as biotlte to the fecal pellet nucleation centers. 103 The strong concentration of iron in the interstitial water is shown by the rapid color changes in the water and sedi ments upon exposure to air. The clay colors are normally olive grays, 5Y4/1 (Geological Society of America, 1963) but rapidly develop an oxidation rim of yellowish brown, 10YR5/4 Summary of Sedimentary Characteristics Sediment textures and chemistry are significantly different between contrasting sedimentary environments in Lake Tahoe. TJnsorted, glacially derived sediments of heterogeneous lithologies compose the chaotic layer which forms the central mounds and occurs along the western side-wall. Coarse, well-sorted sediments characterize the southern delta and the Dollar Point structure. Pine- grained, thinly laminated sediments blanket the central basin and relic structures along the northern closure, western side-wall and southern delta. Variations in the coarsest grain sizes in graded layers and thickness of laminations indicate the proximity of turbidite sources. Ice-rafted grains and authigenic spherules of vivianite are common in the fine-grained sediments. Diatom frustules may strongly influence mean grain sizes and sorting in the fine-grained sediments. The surface distribution of organic carbon (derived from the diatom "rain") is high in fine-grained, Recent 104 sediments but is extremely low in coarse or relic sedi ments. Organic carbon rapidly oxidizes and is removed with time (depth) in the fine-grained sediments. Nitrogen, originally present in one-fifth the quantity of organic carbon, is more rapidly depleted with depth in the sedi ments. No carbonates are present in the sediments. Clay minerals are predominantly illite with lesser quantities of montmorillonite and traces of kaolinite. No regional trends are apparent in the clay mineralogies. Absolute Age Dating of Events Obtaining absolute age dates on events in Lake Tahoe is difficult as most horizons are burled beneath a great thickness of sediments. Material for the dating is scarce. Organic carbon content rapidly falls off with depth in the sediments and the volcanic ash samples are too young to be dated by K-Ar methods. Radiocarbon dates, however, were obtained from the Age Determination Labora tory of Isotopes, Inc. at Westwood, New Jersey for sedi ments bracketing the volcanic ashes in order to utilize these as time horizons. The effects of compaction on the sedimentation rate were determined with dry weight density measurements on core 9 sediments. Originally 0.5 gms/cm^ at 50 cms, the sediments are compacted to a maximum density of 1.5 gms/cm^ at 250 cms after which no increases in density are 105 noted. Radiocarbon ages of sediments bracketing the ash layers at 90 and 120 cms in core 10 (Rig. 22) are 5750, 64-70 and 7880 B. P. Ages of the volcanic ashes, there fore, are approximately 6110 and 7175 B. P. The top 25 cms of the core radiometrlcally dates at 2060 B. P. This partially is due to loss of surface sediment during piston coring. Thirty cms of sediment lie between the ashes corresponding to a sedimentation rate of 28 cms/1000 yrs. Eighty cms of sediment lie between the surface sample and the uppermost ash indicating a sedimentation rate of 20 cms/1000 yrs. Assuming a 24 cms/1000 yrs average and allowing for a compaction factor of at least two, sedi mentation rates must average about 12 cms/1000 yrs in the vicinity of core 10 (central basin). Core 13, also in the central basin, contains two volcanic ash layers at 155 and 235 cms. Utilizing the sediment interval between ashes, the sedimentation rate is 75 cms/1000 yrs. This value appears high as the sedi mentation rate to the first ash layer (compensating for a loss of 30 cms of surface sediments during piston coring) is 30 cms/1000 yrs. Averaging the values and assuming a compaction factor of two, the rate is 25 cms/1000 yrs at the site of core 13. At core 10, the chaotic layer is overlain by 12 m of well-layered sediments (Pig. 15). Extrapolating the 106 sedimentation rates for core 10, approximately 100,000 years of deposition occurred since the deposition of the chaotic layer. Thirty-four m of well-layered sediments overlie the chaotic layer at the location of core 13 (Pig. 18). Extrapolating core 13 sedimentation rates, 130,000 years of sedimentation have occurred since the chaotic layer formation. The chaotic layer was predicted to be Tahoe glaciation age from the geophysical studies. Two volcanic ashes also occur in the deltaic sedi ments of the northern closure (cores 15 and 16). Assuming these ashes to correlate with those dated in the central "basin, sedimentation rates can be computed. Using the same methods as in the central basin sediments to compute sedimentation rates, core 15 has a 20 cms/1000 yrs rate whereas core 16 averages 35 cms/1000 yrs. Overlying the Tioga deltaics on the geophysical profiles is a thin layer which is also draped over the gullies cutting the Tioga depo.sits (Fig. 19). These relationships identify the deposits as post-Tioga (Recent) sediments. These are 4 m thick in the area of core 15 (Pig. 12), whereas they are 5 m thick at the location of core 16 (Pig. 27). Utilizing the radiocarbon date of the deepest volcanic ash and extrapolating the sedimentation rates to the underlying sediments, 12,000 years of sedi mentation followed the Tioga deposits in the area of core 15t whereas 11,000 years followed at the location of 107 core 16. Too many variables reduce the accuracy of extrapo lating the sedimentation rates to underlying strata. Al though not quantitatively accurate, these estimates are significant qualitatively. Tioga glaciation terminated at Lake Tahoe approximately 10,000 B. P. (Haynes, Damon and Gray, 1966) which agrees well with the ages extra polated from northern closure cores. In the central basin, geophysical profiles showed Tioga glaciation to have resulted In no massive slumping. The uppermost chaotic layer was identified as Tahoe age. Extrapolated sedi mentation rates from central basin cores strongly support this Tahoe glaciation age. Dalrymple (1964) on the basis of K-Ar dates, estimated that Tahoe glaciation occurred less than 100,000 B. P. The central basin extrapolated radiocarbon dates also agree well with the Early Wisconsan ages obtained by Broecker, et^ al.(1968) on Barbados Is- 230 land using Th -growth method. Differences in sedimentation rates computed for sediments between ashes and those computed from surface to first ash sediments suggest these rates vary and are significant only as averages. It is assumed that no sedi ments are older than 10,000 B. P. in Emerald Bay as the basin was occupied by Tioga glaciation. Extrapolated sedimentation rates, however, indicate the sediments to be significantly older than 10,000 B. P. This is due to the 108 increased turbidite sedimentation immediately following Tioga glaciation and during Recent smaller glaciations (Birman, 1964). Extrapolated sedimentation rates, there fore, cannot he used in relatively restricted environ ments. In the central basin, turbidite sedimentation was accelerated during Tioga glaciation, resulting in greater apparent ages for the extrapolated sediment deposition times. A rhyollte pebble, sheared off during the coring, overlies the upper volcanic layer in the northern closure sediments (core 15) indicating that the source of vol- canics Is amongst the Recent volcanic structures along the northern shore of Lake Tahoe. The radiocarbon age for the second ash layer (between 6470-7880 B. P.) Indicates it may correlate with Mount Mazama ash (6600 B. P.) from Crater Lake, Oregon, although the Mount Mazama ash Is not known to have extended this far south (Pryxell, 1965). Radiocarbon dates and their extrapolations qualita tively confirm the glacial periods of progradations and slumping identified in the geophysical profiles of Lake Tahoe. 109 CONCLUSIONS Previous work at Lake Tahoe, represented almost exclusively by lead line soundings, gave little indication of the record of a unique sequence of geological events contained In the sediments accumulated in the bottom of the lake. The sedimentary environments are as complex as any encountered and the stratigraphy as varied and thick as the most interesting continental or marine sequences. The application of marine investigation techniques to the continental environment of Lake Tahoe have demon strated that: 1. Lake Tahoe occupies a large graben formed by major, north-south faults. The eastern fault is exposed along the steep eastern side-wall, whereas the western fault is characteristically covered by a thick mass of sediments. The present-day closure is completed by large glacial outwash-deltaic structures on the south and com plex structure and volcanics on the north. Two fault bounded bedrock noses which define a smaller graben Jut out into the lake from the northern closure. Active faulting in Lake Tahoe is shown by previously recorded epicenters, along with fault displacement in the most re cent sediments on the lake floor. 2. Lake Tahoe is floored by a great thickness of 110 glacially-derived sediments. Seismic reflections record at least 0.5 km of sediments on the bottom of the lake. Nowhere is bedrock encountered except along the steeply dipping granitic side-walls and the bedrock noses of the northern closure. Sediment source is primarily the glacially eroded southern and western portions of the basin causing a thinning and pinching out of strata to ward the eastern side-wall. Large glacial outwash-deltaic structures have prograded out along the western side-wall and southern shores. 3. Central lake basin stratigraphy is character ized by a unique, alternating sequence of distorted, chaotic sediments and well-layered strata. The chaotic layer sediments which display a completely disordered Internal structure are massive slump sediments deposited during glaciations. Large glacial outwash-deltaic structures prograded out into the ice-dammed, elevated I P lake. Truckee Canyon joklhlaups rapidly lowered lake levels to the lava dam level triggering massive slumping down the steep slopes. Slump deposits are present in the jumbled- topography at the base of the western side-wall and out into the lake in the chaotic layer. Chaotic layer deposits have extremely poor sorting and a variety of lithologies. The well-layered sediments overlying and underlying the chaotic layer are comprised principally of Recent and interglacial fine-grained turbidites with some Ill continual suspensate "rain*1 resulting in thin lamination texture. 4. Pour glacial periods of progradations and three glacial periods of massive slumping are recognized in Lake Tahoe. Three chaotic layers are indicated along the eastern side-wall where the central basin sediments pinch out. Three buried deltaic surfaces are delineated beneath the southern delta and three slump surfaces are present in the glacial outwash-deltaic structures along the western side-wall. The uppermost buried deltaic surface is contemporaneous with the uppermost chaotic layer. A more recent period of glacial progradation gives the out- wash-deltaic surfaces their present morphology. The three oldest periods of progradation and subsequent slumping are correlated with the three drainage basin glaciations which created ice-dammed, elevated lake levels. These are from oldest to youngest: Hobart, Bonner Lake and Tahoe. The most recent glaciation, Tioga, was less extensive causing no ice damming and is correlated with the youngest period of progradation. 5. Two layers of volcanic ash blanket the central basin. Radiocarbon dates on sediments bracketing the ashes show the ashes to be deposited about 6110 and 7175 B. P. The uppermost ash appears to have erupted locally, whereas the deepest ash may correlate with the Mount Mazama ash of Crater Lake, Oregon. Extrapolated sedi 112 mentation rates Indicate 100,000 years of well-layered sediment deposition since Tahoe glaciation chaotic layer formation and 11,000 years since Tioga glaciation pro- gradations. These extrapolated sedimentation rates qualitatively confirm the glacial stratigraphy. 6. Sedimentary textures, both relic and Recent, directly reflect the sedimentary environments which have affected them. Coarse, well-sorted sands are characteris tic of reworked deltaic deposits. Poorly sorted, glacial sediments typify the slump deposits. Pine-grained, moderately sorted sediments are turbidlte and suspensate deposits on the central basin. Proximity to turbidite source is indicated by the maximum grain sizes in graded layers and the thickness of laminations. Ice-rafted sand grains and authigenlc vivianite spherules impart an un usual texture to the fine-grained sediments whose mean grain size and sorting is primarily controlled by diatom frustules. 7. Surface organic carbon and nitrogen effectively delineate areas of slow, fine-grained sedimentation from relic or coarse sediments. Surface organic carbon derived primarily from the constant diatom "rain," ranges up to 6 percent in areas of active, fine-grained sedimentation. Oxidation and removal rapidly depletes the organic carbon with time (depth in the sediment), leaving no organic car bon In relic sediments. Nitrogen parallels organic carbon 113 in both surface and depth in sediment distribution but is originally present in smaller quantities and subsequently is more rapidly oxidized. No carbonates are present. 8. No regional trends in clay mineralogies are apparent. Olay minerals are predominantly illite with lesser amounts of montmorillonite and traces of kaolinite. No significant variations in relative proportions are ap parent despite the lithologic difference between the volcanic northern drainage basin and the granitic southern drainage basin. Intensity of weathering is the apparent controlling factor. 9. Turbidity currents are active and subaqueous gullies are being eroded in the present Lake Tahoe environ ment. The occurrence of turbidity currents which originate along the steep sediment blanketed side-walls is evident in the Recent well-layered sediments ponded in steps be hind topographic barriers out into the central basin. Chaotic layer mounds protruding through the turbidites present a lacustrine topography suggestive of the abyssal hills and plains of oceanic archipelago aprons. Trigger ing events include seismic shocks, shoreline storm erosion and drainage basin flooding. Several gullies are located along the southern delta, western side-wall and northern closure but only those along the western side-wall are active and can be traced up-slope into subaerial drainage mouths. These gullies cut the most recent sediments. 114 Higher precipitation and a blanket of unconsolidated glacial deposits in the western side of the drainage basin contribute to the formation of turbidity currents and gully cutting. Relatively few lacustrine deposits have been recog nized in the geological record and it is generally con sidered that little information can be obtained from their short histories of limited environments. In the case of Lake Tahoe a thick sedimentary sequence has accumulated which contains a distinct record of its unusual environ ments even though the lake itself is still in a biological ly and chemically immature state. 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Creator Hyne, Norman John, Jr. (author)

Core Title Sedimentology And Pleistocene History Of Lake Tahoe, California - Nevada

Degree Doctor of Philosophy

Degree Program Geological Sciences

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 Gorsline, Donn S. (committee chair), Stone, Richard O. (committee member), Zimmer, Russel L. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c18-415672

Unique identifier UC11361055

Identifier 7023161.pdf (filename),usctheses-c18-415672 (legacy record id)

Legacy Identifier 7023161.pdf

Dmrecord 415672

Document Type Dissertation

Rights Hyne, Norman John, Jr.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA