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Sand transport and petrofacies of the Lake Tahoe littoral zone
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Sand transport and petrofacies of the Lake Tahoe littoral zone
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SAND TRANSPORT AND PETROFACIES / OF THE LAKE TAHOE LITTO R AL ZONE, by James Michael Waldron A Thesis Presented to the FAC U LTY OF THE GRADUATE SCHOOL UNIVERSITY OE SOUTHERN C ALIFO R N IA In P arital F u lfillm e n t o f the Requirements fo r the Degree MASTER OE SCIENCE (Geological Sciences) December 1982 UMI Number: EP58723 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. OissgrtaHbn Publishing UMI EP58723 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 UNIVERSITY OF SOUTHERN CALIFORNIA T H E G RA DU A TE SCHO O L U N IV E R S IT Y PARK LOS A N G ELES. C A LI FO R N IA 9 0 0 0 7 This thesis, written by James Michael Waldron under the direction of h.\?....Thesis Committee, and approved by a ll its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillm ent of the requirements fo r the degree of Master of Science Dean D ate..... THESIS COMMITTEE Chairman CONTENTS Page ABSTRACT ..................................................................................................... V . IN T R O D U C T IO N ..................................................................................................... I Purpose ..................................................................................................... I Previous w o r k ............................................................................................... I G eography....................................................................................................... 2 L o c a tio n .................................................................................. 2 C lim a te ..................................................................................... 3 Wind c irc u la tio n ..................................................................... 6 Lake basin geom etry............................................................. 6 GENERAL GEOLOGY OF THE LAKE TAHOE B A S IN ................................ 14 PROCEDURES .....................................................................................................2 0 Sampling .....................................................................................................2 0 Textural p ro ce d u re s.....................................................................................2 1 Pétrographie procedures.............................................................................2 2 RESULTS AND DISCUSSION OF RESULTS..................................................... 2 4 Pétrographie results and d iscu ssio n ........................................................2 4 C om position.............................................................................2 4 S h a p e ........................................................................................3 3 D iscu ssio n ................................................................................36 Textural results and discussion ...............................................................5 8 D iscu ssion ................................................................................6 6 CONCLUSIONS .....................................................................................................7 7 ACKNO W LEDGM ENTS.......................................................................................... 7 9 REFERENCES .....................................................................................................8 0 . , Î i ILLUSTRATIONS Figu re Page I. Map showing study area and sample loca tio ns..................................... .................. ■ 5 2. Map showing bathym etry o f Lake T a h o e .................................................................. 3. Schematic cross section o f Lake Tahoe showing graben s tr u c tu r e ..................................... 4. Subaerial distribution o f rock ty p e s ........................... 5. Sediment color d is trib u tio n ........................................... é. P lutonic-volcanic-m etram orphic ternary d ia g ra m ........................ ....................................... 7. Q uartz-rock fragm ents-feldspar te rnary d ia g ra m ............................................. .................. 8. Q uartz-rock fragm ents-feldspar group d is trib u tio n ........................................................................ 9. Shoreline sediment te x tu r e s ........................................ 10. Q-mode cluster a n a ly s is ................................................ il. D istribu tio n o f clusters 1 through 6 .......................................................... 12. B ivariate scatter plot-m ean grain diam eter vs. phi skewness............................................. 13. B ivariate scatter plot-phi kurtosis vs. phi standard d e v ia tio n ............................................. ...................... 54 14. Grain size distribution o f the Lake Tahoe litto ra l and stream s e d im e n ts ........................ 15. Summary o f transport d ire c tio n s ..................... 1 i I Table TABLES Page I. Frequency o f wind d ir e c tio n ..................................... .............. 7 2. Frequency o f wind speeds, 0700-0800 PST ............................................................... ..................................... 8 3. Frequency o f wind speeds, 1400 PST........................ ..................................... 9 4. Physcial properties o f Lake T a h o e ........................... 5. Pétrographie data description fo r medium sand fra ctio n o f all s a m p le s ..................... 6, Rock fragm ent pétrographie d a t a ........................... 7. F -m a trix and classification m atrix, quartz-rock-fragm ents-feldspar g ro u p s ................ ..................................... 48 8. F -m a trix and classification m atrix. com puter groupings ..................................................... ..................................... 56 9. Summary o f grain size s ta tis tic s ............................. 10. Summary o f fa cto r v a ria n c e ..................................... II. Rotated fa cto r lo a d in g s ............................................. i V ABSTRACT Conventional textural and pétrographie techniques were used to estim ate net litto ra l sand transport, depict textural variations, and compositional petrofacies at Lake Tahoe, C alifornia-N evada. A set o f 106 offshore, 106 foreshore, 106 bockshore and 31 stream samples were collected during September, 1978. T exturolly, the mean moment values for the entire sample population are 0.84mm fo r grain size, 0.82 fo r phi standard deviation, 0.16 for phi skewness, and 4.37 fo r phi kurtosis. An R-mode fa cto r analysis of the textural data demonstrates th a t two factors explain 80 percent of the to ta l lake sample variance. Factor I, which represents 42 percent of the total variance, depicts offshore fining. Factor 2 represents the flu via l sediment contribution to the sample population. The offshore-fining trend suggests a net offshore sand transport. Results o f the pétrographie analysis indicate that the litto ra l sand is com positionally described by five groups on a standard quartz - rock fragm ents - feldspur ternary diagram, or six clusters using m u tiva ria te grouping techniques. Compositional p lo ttin g o f each sample in its respective position around the lake perim eter shows th at the longshore d rift o f the litto ra l sand is confined to specific shoreline segments. The direction o f net transport depends solely on the prevalent wind direction. Fluvial transport and c lif f erosion are im portant in supplying sand to the litto ra l zone. The data strongly suggests that the beach and nearshore sediments ore d ire ctly derived from the rocks on the adjacent shoreline. INTRODUCTION Purpose In order to better assess the effects of shoreline urbanization in the Lake Tahoe area, a complete environmental study including physical, chemical, and biological conditions within the littoral zone is required. One of the important aspects concerns the transport and mineral composition of sand in the littoral environment. Inasmuch as major recreational beaches are covered with a veneer of sand, it is essential to better understand littoral sediment movement in order to effectively preserve these areas. In this pilot investigation, the author has used textural and pétrographie techniques to estimate net littoral sand movement and compositional petrofacies. This investigation, at the University of Southern California, is part of a larger study under the direction of D r. R.H. Osborne. Previous Work A number of scientific workers hove examined the area in and around the Lake Tahoe basin. One of the earliest workers. Le Conte (1875, 1883) carried out scientific measurements in Lake Tahoe, along with a study of the suboeriol geology. The scientific measurements included temperature at different depths, lake water clarity, and Interesting explanations of why the lake does not freeze, why it is so blue, and seiches (for which he found no evidence). Lindgren (1896, 1897, I9II) and Louderbock (I9II, 1923) examined the subaerial geology of the area. More recently, Birkeland (1963) examined the effect of Pleistocence volcan ism in the Truckee River canyon and the changing of lake water levels as a result of glaciation (1964). Loomis (1964) has mapped the Fallen Leaf Lake quadrangle, which includes the southern port of the Tahoe basin. The California Division of Mines and Geology (Burnett, 1967; Matthews, 1968) carried out a very comprehensive mapping study of the Lake Tahoe basin, and surrounding areas. The Quarternary history, os well as an examination of the surficiol bottom sediment, is summarized by Hyne and others (1972) and Court and others (1972), respectively. Engstrom (1978) prepared an analysis of the dynamics of Lake Tahoe shoreline processes for an environmental impact report presented by Phillips, Brandt, Reddick, McDonald and Grefe, Incorporated (1978). Geography Location Lake Tahoe is located high in the Sierra Nevada in a valley between the Carson Range on the east, and the Sierra Nevada on the west (Fig. I). The city of San Francisco is about 320 kms (200 miles) to the west and Los Angeles is approximately 800 kms (500 miles) to the southwest of the lake. The Lake is nearly bisected by the 120th Meridian, with about one-third of Lake Tahoe located in Nevada. Climate In a general discussion of the climate of the Lake Tahoe basin, it is necessary to note that there are factors which effect the entire basin, as well as clim atic variations between the eastern and western shores, and between high and low elevations. Topography exerts a strong influence on the climate of this region. From 55 to 70 percent of the average annual precipitation falls as either rain or snowfall, depending on elevation, from December through March. The annual precipitation, predominantly in the form of snow, has a normal range of approximately 127 cm (50 inches) along the western edge of the basin, to a low of 38.1 cm (15 inches) in the central portion of the lake. In terms of snowpack, the normal range is from 40.6 cm (16 inches) to 558.8 cm (220 inches) (Tahoe Regional Planning Agency). Summers are relatively long and cool, with an average temperature of about 25.6° C. Winter months are seldom severely cold, averaging l.7-4.4° C at lower elevations during the daytime, and from -10° C to -5 ° during the night. These mild temperatures, unusual for on elevation of more than 1,830 m (6000 feet), can be explained by noting (I) the high amount of winter sunshine; (2) the relatively mild Pacific air mass that affects the area during the winter; (3) the insulation provided by the clean air; (4) the "heat reservoir" characteristics of the lake, i.e., once the water has been warmed by the sun, the water tends to retain this heat for a considerable time period, thus warming the isurrounding area; and (5) a night-time, low-level temperature inversion. (Lake Tahoe Regional Planning Agency, 1971). - Location and sample location maps. Figure ST-27 CRYSTAL BAY ST-28 ST-25 ST-29 L A K E TAHOE STUDY A R EA ST-24 AGATE BAY FftANOSCO ST-23 ST-30 LOS A M C LE S TAHOE CITY r^UÇKEE R ST-2 ST-22 ST-21 S T -3 DEADMAN PT GLENBROOK BAY TAHOE PINES ST-20 SUGA ST-7 ST-9 ST-10 ZEPHYR COVE RUBICON ST-18 EMERALD BAY S T -12 ST-13 ST-17 FALLEN LEAF LAKE NAUTICAL MILES Wind Circulation Just as the climate of the basin is affected by its topography, so too is the circulation of wind. Observations mode at the U.S. Coast Guard Station in Tahoe C ity are summarized in Table I. As can be noted, wind directions during the summer, fall, and winter are slightly west of north, and account for 12 percent, 37.7 percent, and 43.0 percent, respectively (Tahoe Regional Planning Agency). Wind from the southwest account for 16.1 percent of the observed frequency during summer, and 24.1 percent during the fall. Topographic sheltering of wind from the northeast, east, and southeast is evident by noting the low frequencies of these wind directions. Wind speeds vary greatly between the morning and afternoon hours at Lake Tahoe. As can be seen in Tables 2 and 3, the morning hours are predominantly calm (wind speeds less than 6 knots), whereas the afternoon hours show a marked increase in wind speed, especially during the summer and fall months. There is a 50 percent chance of wind speed increase during the afternoons of the spring and winter months. Lake Basin Geometry Table 4 shows the various physical parameters of Lake Tahoe. Hutchinson (1957) records a maximum depth of 501 m (1645 ft.) whereas Hyne (1969) records a maximum depth of 494 m (1620 ft.). Depending on who is correct, the lake is either the tenth or eleventh deepest lake in the world. Regardless of this discrepancy, Lake Tahoe is volumetrically the largest Alpine lake in North America. Drainage is very nearly symmetrical about the lake. Except for the Upper Truckee River at the south end of the lake, no drainage stream is more than > . « w ü V o û c © lij oc o ^ ^ ^ m o • • I I * • f O — e v i r o f o » o o ^ ^ O O 0 0 ^ CM 0> K > — O C M 0> lO — C M ^ 0 0 C M h » id 0> 0) — lO — (j> c o © o - © o > i o I 1 l o 00 N m o h » 00 0 ) 0 C M tf) ro M * O — K) 00 |g) lo ^ C M fO in C M — co (OO 3<D CTO) l i Q ) © « a ^ co I CN ° ® •»- G) O t- © ± >* - o 3 C C — < 0 ^-0 o c o “ 1 ©co 1 1 ^ c a c3 o H ü < u . © oc tj © oc L U ® O z oc C L C O ■a c ^ 2 o oc tJ C D C o o © h* C O Q _ O O 0 0 O I o o N O c d lo c o c o > M * k. Q ) C M C O _a O lo O m o C M o m co o i r lo — fO 0) o > lO — C M — C M C M — — h » — » C M “ • C M co < J > \ 0) o> — C J > en en fO en 1 - co C L O M * < n O C D a i c o en tf> lO T- c o c o h" c — C M o > k. 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O > * O C < D C D < C D C C c c o © c o C D < D O C © O j C c a H E o C ( f jH C D o z c o c o w — O) O) ca O C ■ * “ T ^. C D O l C O C O © HOC O C H C O C D —^ Uj C O C O C O ® _ - 2^ C D C D C D O © 2 HOC 3 C O _J H C D O) — gÇ _J C O C O C O ® <CD 0)0) Q © U . K - O C OCHCDO) — ^ LU C O C O C O ® H I I I "5 © Z © h - C O . q ; — C O C O C O ^ 0)<3)0) 5 km long. By noting that the entire Lake Tahoe basin Is only 1300 km^ (500 mi^) and that the surface area of the lake itself is 5(X) km^ (ISO mi^), one can see that the entire drainage area is only 1.6 times the area of the lake! It is because of this limited subaerial drainage area, and the resulting low stream sediment and nutrient load introduced into the lake, that the lake maintains its clarity. The lake basin proper, is steep-sided on the east and west and flat-bottomed for the most part (Fig. 2). A t Rubicon Point the sub lacustrine slope is more than 40° (Court and others, 1972), whereas at the north and south ends of the lake, the slopes are less than 1° . Seismic profiling has shown that there are numerous spurs above the lake floor and abundant slump structures along the edge (Hyne, 1972). Geophysical profiling shows at least 400 m (1300 ft.) of layered sediment above the lake bedrock (Court and others, 1972.). A sediment rate of 7.6 cm per 1,000 years in inferred from this data when one considers that the lake is PIio-PIeistocene in age. Recent evidence shows that the rate of sedimentation has increased proportionately with the rate of urbanization (Tahoe Regional Planning Agency, 1971). Figure 2. - Bathymetry of Lake Tahoe (after Hyne, 1972) CONTOUR INTERVAL 5 0 METERS STATELINE POINT, '50, 1 0 0 ^ 150 250 450 450 TAHOE PINES CAVE ROCK RUBICON POINT EMERALD B A Y y UPPER TRUCKEE RIVER Table 4. - Physical properties o f Lake Tahoe Surface area o f Tahoe Watershed - 500 square miles Surface area o f Lake Tahoe - 193 square miles Length o f Lake Tahoe - 22 m iles Width o f Lake Tahoe - 1 2 miles Length o f Shoreline - 7 1 miles Maximum depth o f Lake - 1645 feet Average depth o f Lake - 989 fe e t Volume o f water in Lake - 122 m illion acre-feet C la rity o f Lake Waters - Such th a t light penetrates to a depth exceeding 400 fe e t 1 3 GENERAL GEOLOGY OF THE LAKE TAHOE BASIN Lake Tahoe occupies one of the most southern basins of a series of tectonic depressions, that form a north northwest-trending graben complex that extends from Mexico into Washington State (Durrell, 1956; Bateman and Wahrahaftig, 1966). Although the basin traditionally has been assigned to the Sierra Nevada province (Jenkins, 1938), the basin is actually part of the Basin and Range province, A single crest divides the drainage throughout most of the Sierra Nevada, with a gentle western slope leading to California's Great Valley, and a steep eastern slope leading to the interior of the Basin Ranges. This crest divides at the southernmost origin of the Truckee River into the north-westward trending Sierra Nevada and the north trending Carson Range. Lake Tahoe is located in the graben (Fig. 3) between these two crests. The floor of this basin is 1,433 m (4700 feet) above sea level, which is the same as the floor of the Carson Valley to the east. The first to perceive this graben structure (Fig. 3) was Le Conte (1875). The boundary faults were later extended northward into the Truckee basin and Sierra Valley by Lindgren (1911). The western system of graben boundary faults that pass through the western end of Donner Lake, the Squaw Valley ski area, and Meyers Grade at the southern shore of the lake were defined by Louderback (1924) and Hudson (1948, 1951), Burnett (1968) shows the fault passing through Rubicon Point and Sugar Pine Point. An eastern boundary fault system has been pinpointed outside of the Lake Tahoe basin where it parallels the northeastern shoreline south to Dead man's Point. 1 4 Figure 3. - Schematic cross section of Lake Tahoe showing graben structure. (after Burnett, 1967) 1 5 o UJ g u 2 o . oc w oc < C Û a. % META A ' v K T ' - , \ ' r SIERRA C ‘ 'N E V A D A GRANITE LAKE TAHO 5 'VALLEY F ILL % c ;v 16 Although earthquake activity in the area has been moderate during historic time (Wolfe, 1968), active hot springs and scarplets show that the faults are probably still active. Work compiled by Birkeland (1963) shows that most of the major I and forms in the north Lake Tahoe and Truckee basins were shaped after andesitic VO I can ism and deformation which occurred in late Pliocene or early Pleistocene tim e. Deformation of the rocks ceased just before the onset of the youngest of the pre-Wisconsin glaciations (Donner Lake) (Birekeland, 1964). K -A r age dates (Wahrhaftig and others, 1965) of the volcanic rocks support this hypothesis. The Lake Tahoe basin is composed of four major lithologies: metamorphic, granitic and volcanic rocks, and Quaternary sediment (Burnett, 1968). Figure 4 shows the siÆ>aerial distribution of the lithologies within the Lake Tahoe drainage basin. Both metased imen tar y and met avo lean ic rocks occur as scattered roof pendants in the granitic rocks, but very little information is available concerning these rocks. The most abundant bedrock found in the basin consists of intrusive granodiorite associated with the Sierra Nevada batholith. Gabbro, diorite, and quartz diorite occur as small rock bodies, mostly adjacent to metamorphic rocks (Burnett, 1968). K -A r dating of biotites in the granodiorite, yield an age determination of I06j^ 2 million years (Burnett, 1968). Pliocene volcanic rocks (lahars) are predominantly located in the northwestern part of the Tahoe Basin and along the Lower Truckee River. 1 7 Figure 4. - Subaerial distribution of rock types (after Hyne, 1972). BLACKW OOI CREEK Ï ROCK TYPES ^THIRO CREEK IN C L IN E CREEK WATSON CREEK S TA TE LIN E PO INT TAHOE TAHOE ________V I f'N E S A c GLENBROOK BAY G'.ENBROOK I RUBICON PO INT UPPER .TRUCKEE 6 0 0 - ^ -RIVER GRANITIC E p VOLCANIC E ] GLACIAL DEPOSITS METAMORPHIC C I Z 3 LAKE BEDS Q ! ? 3 4 5 Miles 012 34 5 Kilometers CONTOUR INTERVAL, 6 0 0 FT. BOUNDARY OF LAKE TAHOE DRAINAGE BASIN 19 The labors effectively dam the north portion of the lake. Due to extensive Pleistocene glaciation, which occurred mostly along the western Sierra Nevada side of the basin, morainal blankets of sediment cover large areas of the southern and western drainage basins. The southern shore of Lake Tahoe, is in fact, a large outwash-deita formed by the convergence of valley glaciers (Burnett, 1968). There is, however, some evidence of limited glaciation on the northern slopes of the highest peaks of the Carson Range. A detailed description of Pleistocene glaciation can be found in Birkeiand (1968). PROCEDURES Sampling The sampling plan used in this study was designed to show long-shore and offshore variations in grain size parameters and lithologie composition. The hypothesis was that meaningful information regarding sand sources and the net littoral movement of sediment could be attained using a systematic sampling plan (Krumbein and Graybiii, 1965). Accordingly, a sample set spacing of 1.8 km (I nautical mile) was used around the perimeter of Lake Tahoe. In addition, all major fluvial sources also were sampled to determine their respective contribution to the total littoral sand budget. A total of 106 locations were sampled around the lake perimeter, and 3 1 fluvial samples were collected (Fig. I). A t each lake sample location, a bockshore, foreshore and offshore sand sample was taken. Grab samples of approximately 700 grams were extracted from all locations, including fluvial 20 sample sites. A 7.6 L (2 gallon) bucket, weighted on one side, was used to dredge offshore samples, at an approximate depth of 6 meters (20 feet), unless otherwise specified. The entire suite of 349 samples was collected between September 27 and September 30, 1978. Textural Procedures Since approximately two-thirds of the samples were collected wet (offshore and foreshore), it was necessary to dry the samples before splitting. A fter recording the gross color of the wet samples, using the Geological Society of America color chart, each sample was oven-dried (100° C). A fter drying, the collected sand samples were then split to approximately 70-100 grams using a mechanical sample splitter and microsplitter. From previous textural studies of the same nature (Folk, 1974), it has been determined that this is an optimum weight fraction for analysis. Clogging of the sieve screens by using too much sample is a frequent cause of poor results (Folk, 1 974). If the sample contained mostly gravel and cobbles, a quartering technique was used to split the sample. The remainder of the sample was bagged, numbered, and stored for future reference. An 8 percent heated solution of hydrogen peroxide was used to dissolve any organic tissue present in the sample. In some instances, the procedure had to be repeated several times to ensure complete digestion. Care must be taken in that as the H 2 O 2 solution plus sample is heated (40° C), it tends to boil over, thus losing part of the split sample, which would bias later results. All samples were again oven-dried. A fter thorough drying, each sample was shaken for 15 minutes on a Fisher-Wheeler sieve shaker using 21 sieves at one-half phi Wentworth-Udden size intervals. The sieve grades ranged from 4 mm (-2.0 phi) to 0.063 mm (4.0 phi). Each pan fraction was weighed to the nearest 0.01 gram and recorded. Sand samples in the medium grain size range (0.5 mm to 0.35 mm) were separated after weighing to be used for pétrographie analysis. The data obtained was processed using a Fortran IV computer program which calculates weight percent, cumulative weight percent, and standard moment measures including mean grain size (mm and phi), phi standard deviation, phi skewness, and phi kurtosis. A histogram showing groin size distribution for each samples is also plotted. Pétrographie Procedures A total of 86 thin sections were prepared for pétrographie analysis, A majority of the samples picked for pétrographie analysis were taken from the foreshore zone. Previous studies have shown that this is where most sediment transport occurs (Komar, 1976), so it was thought that this zone would be best suited for petrologic analysis in order to determine net sediment transport directions. Bockshore and offshore samples also were studied in some areas to document minera logical changes from bockshore to offshore. Samples selected for pétrographie anyalysis were taken at equal intervals of I nautical mile (1.8 km) around the lake, so as to give good coverage in the least number of points. Although it is best to study all size fractions of a particular sediment sample to obtain the most complete information (Folk, 1974), it seemed most efficient to examine the medium sand range, 0.35 mm (1.5 phi) to 0.50 mm (1.0 phi), since all samples included a sand size component in this range. 22 All of the slides were stained with sodium cobalt-nitrate to facilitate feldspar identification. Sand composition was determined by counting at least 300 points using the Glagolev-Chayes method as described by Galehouse (1971). By counting at least 300 points per section, it is statistically estimated that the correct volume of each component lies within 6% of the obtained volume, at the 95% confidence level (Van der PI as and Tobi, 1965). A spacing of 0.50 mm was used to assure that the spacing interval was greater than or equal to the largest particle size diameter encountered in the section. The compositional variables distinguished are monocrystalline, straight extinction quartz; monocrystalline, undulose extinction quartz; plagioclase; potassium feldspar; magnetite-ilmenite (opaque minerals); hornblende; mica; sphene; polycrystalline quartz (2-3 crystals); polycrystalline quartz ( 3 crystals); unstable quartz; volcanic rock fragments; plutonic rock fragments; metamorphic rock fragments; clastic fragments (sandstone, siltstone, shale); accessory minerals (epidote and tourmaline); and chert. Sphericity (Folk, 1955) and roundness (Rittenhouse, 1943) also were determined by counting 100 grains per thin section. 23 RESULTS AND DISCUSSION OF RESULTS Pétrographie Results and Discussion Composition The obtained compositional data are summarized in Table 5. Quartz The classification used in this study follows Basu and others (1975). Accordingly, monocrystalline quartz may be either non-undulatory or undulatory, depending on whether the flat stage rotation necessary for extinction is less than or greater than five degrees, respectively. Polycrystalline quartz is divided into two groups: those grains with two to three crystals and those grains with more than three cyrstals per grain. As noted in Table 5, the Lake Tahoe littoral sand samples have very little monocrystalline straight extinction quartz, on the average, only 0.06 percent with a standard deviation (s.d.) of 0.19. Polycrystalline quartz also is scarce. Quartz grains with from two to three crystals show on average abundance on only I.OI percent (s.d. = 4.04), and grains with greater than three crystals per grain show an average abundance of 0.75 percent (s.d. = 2.74). By far, the most abundant quartz type is monocrystalline and undulose. This component shows an abundance of 20.57 percent (s.d. = 8.09). There is also quite a large range for variance, from 0.30 to 46.10 percent. This large range can be attributed directly to source rock type and location. In all cases, the amount of quartz in any one sample is inversely proportional to the amount of volcanic rock fragments in the same sample, i.e., where the 2 4 3 C S üJ q o q O O O O O O O O o X , o — lo o o ^ m m m i o c n c o o ) < < — < D S > ^ ^ C M 0> K <0 O « C M fO fO - , C M C T > — « _ ^ r O O C D O O O O « « C M N* ^ C D O 0 > O O O — 2 - 0 » lO o » K c o ® a E C O C D "5 S £ 2 ^ Ü | o S S o o o o o o o o o • • # • • • • • • • « • — j O O — o o o o o o o o o 5 < 2 > O O Ô O O O o o 6 6 d 6 o o d d c o ■ o c ca C D O 00 00 O ) O » ^ O l O _ CMh-CM00 j _ — m ( T > O » C M C D C 0 — ^ C D C M O < O c M d t o i o t o d t o d i o i o ^ m O — lO 0 0 < o o» C D o CD 00 C M C M ^ O » O O — C M c A C M O » d ■ D 0 E Z (O < o iD LU - - ^ o o O D C D ^ C M C M C M (T > N - l O O ^ CD CD CD — O » O » t O — K > C M — C D c O O * “ — C M O C M ® tf>00<**0®K>CM f^Nooh-m^y^cM d d c D — d d o o o a > 0 ■o e a X J o j= a « a o» 0 0 CL 1 id 0 .a 0 N #- D C < 3 O UJ (0 O _j 3 O z 3 Z O z UJ UJ z z N H- O C < 3 O U J co o 3 O z 3 < < L U H- I - C O C O C O < >- > O C oc ü o o o z z o o 2 2 L I J .H Ul I U J H H U J 1 o 1 1 1 LU 1 < < œ < < 1 1 1 O 1 3 3 2 1 1 1 Z 1 O O g : C O : 1 1 UJ _J 1 1 LU UJ c o o _j — 1 < 1 1 1 1 1 1 f f l z 1 1 1 1 Z _l Z _ l LU o c 1 1 œ 1 1 _l — 1 UL lU LU O 1 < < z UJ O X H # - 2 LU Z ü H 1 1 C O c o 3 c o UJ Z UJ o Q Q UJ — 1 1 1 LU > c c > o c c o 3 X O Q — 1 Z ü ü < O o X Z < < UJ >- > - H < o c Q . o c C O ü X _ j -1 o o _ > 2 o < û . o o û . O û . < X m 2 c o C L û . 1 1 1 1 1 1 1 1 1 1 1 1 t 1 1 1 1 co 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 < I I I 1 1 1 J— 1 I 1 1 1 1 1 1 1 c o > 1 1 1 o c 1 1 1 1 i 1 o 1 1 ! co 1 1 t I w 1 1 T |_ 1 1 1 1 ( ' ;z 1 C M NI 1 N CO O C O C < c o > o c 0 — co z < 1 I - LU o c o I co <0 I o <5 -Il 5 ^ ^ iii 9 g OQ z z .co - I 3 Z O -I 3 - > CL LU 2 CO O H £ Î «. 0 s w ü g 2 1 O > o o co 2 H W I - < C O UJ ce I — < ü LU -I ü X ü < ü UJ ’ ■ 2 .3 shoreline I i the logy is predominantly volcanic, the occurrence of quartz, except chert, is low. Chert (microcrystalline quartz) abundance is directly proportional to the abundance of volcanic rock fragments. These sharp cut offs are very helpful in postulating littoral transport directions and petro facies. None of the quartz types show much alteration, weathering, or embayments. Feldspar The feldspars, as a group, are the most common constitutents found in the littoral sand of Lake Tahoe. Of the two types, plagioclase predominates with an average abundance of 28.92 percent (s.d. = 8.50). Potassium feldspar shows an average abundance of 8.99 percent (s.d. = 4.49). Abundance is again related directly to the shoreline rock type where boundaries can be sharp. Potassium feldspar is present in two varieties, twinned and un twinned, with the untwinned being predominant. Twinned varieties show the characteristic "gridiron" twin pattern of microcline. Trace amounts of perthite also occurs. Greater than two-thirds of the plagioclase feldspar shows twinning. Pittman (1969) noted that a lack of twinned feldspar is common in fluvially- transported sediment, probably due to breakage. In this case, a predominance of twinned plagioclase feldspar may indicate a minimum of transport, fluvial or otherwise. Twinning is predominantly albite, although some feldspar grains do show Carlsbad twinning. The plagioclase composition of the plutonic rocks surrounding the lake was determined by Loomis (Burnett, 1968). The plagioclase of the diorites and 26 gabbros has been identified as andesine (An 4 q_5 o)> whereas the plagioclase of the granodiorite has been identified as oligoclase (An 20-30^* Feldspar alteration is extremely variable. Alteration can vary from sample to sample, or within one particular sample. In one case the feldspar grain may look fresh, whereas in another instance, and possibly in the same thin section, sericitization has completely obliterated the grain. It is very likely that this observation is due to the mixing of fresh bedrock material and more weathered Quaternary shoreline mixture. Iron staining is common along the east shore and is very notable along the south shore where the sand is characteristically orange in color. Sample color changes, in general, are shown in Figure 5, where, again, these changes may be attributed directly to shoreline rock types. Opaque Minerals (Magnetite-ilmenite) This iron mineral was found to have an over-all abundance of 1.57 percent (s.d. = 5.31) and is a relatively minor constituent of the total "variable" population. Locally, however, it can be important; and in some samples magnetite is the major constituent (Fig, I 26-B, 55-F, 68-0, 15*). The average abundance of pyroxenes is 2.13 percent (s.d. = 5.6) and has a range from 0.0 percent to 37.5% (Table 5). Mineral types include augite and hypersthene, with the former being most abundant. Hypersthene occurs as pale green to pale red euhedral crystals. Augite occurs as euhedral crystals with light green color and pyroxene cleavage. In most cases, higher concentrations of pyroxene are associated with higher concentrations of volcanic rock fragments. 27 5 0 ^ 4 9-& ^ 47-B A VARIOUS VALUES OF GRAY O VARIOUS VALUES OF ORANGE □ VARIOUS VALUES OF BROWN 43-A 9 7,9 8 37-BQ BACKSHORE FORESHORE OFFSHORE NAUTICAL MILES Amphiboles Components included in this category are hornblende, basaltic hornblende, and crystals showing definite amphibole cleavage (Kerr, 1959), but of an undetermined mineral species (listed under "Amphibole", Table 5). Mean abundances are 2.2 percent, 0.2 percent, and 0.3 percent respectively. Hornblende shows very strong pleochroism from green to brown, and occurs as both euhedral and subhedral crystals. Basaltic hornblende is very easily distinguished due to its unique reddish brown color and strong pleochroism. Basaltic hornblende is commonly found as as euhedral phenocrysts in volcanic rock fragments, whereas common hornblende is ubiquitous. Mica Biotite was the only mica positively identified, although phlogopite may be an important mica component at Glenbrook Bay. The optical distinction between the two micas is difficult, at best, (Kerr, 1971) and was not noted. Biotite shows little weathering or alteration and has an average abundance of 2.0 percent (s.d. = 3.7). Sphene Sphene is easily identified because of very high relief and birefringence. The mineral commonly occurs as euhedral crystals, but some anhedral crystals are also evident. There are two notable occurrences of sphene along the shoreline; one near Zephyr Point (Fig. I, 26-B), and one at State line Point (Fig. I, 68-0). Hyne (1972) noted that the sphene at State line Point is clearly derived from the granodiorite that forms the point. Rock Fragments Rock fragments are, for the most part, common to all samples and were 30 designated into four components; volcanic rock fragments, plutonic rock fragments, metamorphic rock fragments, and clastic rock fragments (sandstone, siltstone, and shale). Table 6 shows the range, mean, and standard deviations (s.d.), in percent, of all rock fragments found in the littoral sand. Volcanic rock fragments show the greatest range and highest mean percent (Table 6), without nearly the subaerial extent of the other major rock types. These high percents of abundance may be due to sorting differences (size, shape, or density), lack of mixing, and/or erosional factors. The volcanic lahars and granodiorites are both highly susceptible to physical weathering (Burnett, 1968). Plutonic rock fragments also show a wide range and a relatively high mean percent abundance. This is not surprising since most of the basin is composed of this type of rock (Fig. 4). Plutonic rock type is predominantly granodiorite with small bodies of quartz diorite, diorite, and gabbro located near metamorphic terranes (Fig. 4) (Burnett, 1968). In thin section, any grain that was an amalgamation of feldspar and quartz was included in the plutonic rock fragment cateogory. Metamorphic rock fragments may actually be more abundant than shown (Table 6), but when working with the medium sand size (0.35 mm to 0.50 mm) it is difficult to discern between plutonic rock fragments and metamorphic rock fragments (Folk, 1974). The metamorphic rocks are, however, limited in subaerial extent to near Glenbrook Bay and east of Fallen Leaf Lake (Fig. 4). With the limited drainage and no shoreline exposure, the amount of metamorphic rock fragments noted in thin section seems reasonable. 31 m lo c v j ro lO in c v i ro o > (O (/) > C V I (/) O The same is not true for detrital sediment. There is considerable exposure of Pleistocene morainal sediment and Quaternary lake deposits along the western and southern shoreline. Inasmuch as the sediments are derived largely from the surrounding plutonic and volcanic terranes, differentiation of source terranes is difficult in thin section. Once broken down, the sediments are classified into constituent grains such as feldspar and quartz. Figure 6 shows the distribution of rock fragments in the littoral sand as found through pétrographie examination. Minor Accessory Minerals Two minerals were categorized as minor accessory minerals, epidote and tourmaline. Percent mean abundance for epidote and tourmaline is 0.4 percent. The lack of accessory minerals is most likely due to the size fraction studied. Most accessory minerals tend to concentrate in sand finer than the medium-grain size that was studied here (Folk, 1974). Epidote is easily distinguished by its high relief, crystal habit, and anomalous blue birefringence under crossed nicols. Tourmaline is also easily discerned by its high relief and characteristic purple pleochroism. Epidote seems to be rather ubiquitous, whereas tourmaline is found in only two sample locations, 15-B and 41-0, 27’ (Fig. I). Both areas are adjacent to metamorphic rock bodies, so it is possible that fluvial transport is responsible for the presence of tourmaline at these locations. Shape Roundness The ”rho” (p) scale of Folk (1955), as it applies to the verbal roundness scale of Powers (1953), was used to visually estimate the roundness of quartz 33 Figure 6. - Plufonic-voleanic-mefamorphic ternary diagram. o CL UJ ro iô r o uf ro r o r o 00 Zoou) C J 00' 00 % If) C V J lO C V J if> tn lOO O ) O grains. Accordingly, there are six visual classes to which grains are assigned and are as follows: very angular, 0-1; angular, 1-2; subangular, 2-3; subrounded, 3-4; rounded, 4-5; well-rounded, 5-6. In order to facilitate computations, only mid-point values were used. The average roundness for littoral quartz grains was found to be 1.8 (angular) and ranges from 1.4 (angular) to 2.2 (subangular). A range of 0.8 is considered to be very small. Sphericity An estimation of sphericity was carried out by using the visual comparison charts of Rittenhouse (1943). Sphericity values range from 0.71 to 0.76 and average 0.74. As with roundness, the sphericity range is again very small. The small range in both cases probably reflects a minimal amount of transport. Discussion Using the procedure set forth by McBride (1963), the derived compositional data were plotted on a Quartz-Rock Fragment Feldspar (QRF) ternary diagram (Fig. 7). The five compositional groups are readily differentiated. Groups I, II, and III all fall within McBride's (1963) lithic arkose classification for sandstone. Group IV is entirely within the feldspathic litharenite class, and group V falls mostly within the litharenite class. Although McBride's (1963) classification is for sandstone, it is also useful in defining the composition of sand. Approximately 81.2 percent of all of the pétrographie samples fall into one of these five groups. QRF group I is petrographically characterized by a mean abundance of 18.8 percent monocrystalline undulose quartz, 40.1 percent plagioclase, 11.5 36 Figure 7. - Quartz-rock fragments - feldspar ternary diagram. 37 O) ï g g i < c D ti)X :^ =)= )= )!zo : o c o c n _ j< t i l l — curOTf iD § î t t = y g < < < T ^ , CVJ — \ CO # C LU Li _ 38 percent potassium feldspar, and 14.2 plutonic rock fragments. A total of 25 samples were assigned into this group. Figure 8 shows the littoral distribution of group I. A major, distinct cell, composed of this group, extends from Stateline Point (sample locality No. 68) to Deadman Point (sample locality No. 40). Engstrom (1978) noted that this shoreline segment receives approximately 33 m etric tons per km (58 tons per mile) of shoreline each year from fluvial sources. Petrographically, no direction of transport can be inferred, but two facts are clear: I) sand is not being transported south of Deadman Point; and 2) the steep underwater c liff (Fig. 2) at the northern end of the segment is an effective barrier to littoral transport around Stateline Point, as shown by the sharp cut-off of group I at that point. As previously mentioned, a northerly direction of sediment transpx>rt is suggested by Engstrom (1978) on the basis of predominant wind direction, but southward trending sandspits were observed during the sampling period indicating that there is also a south trending component of littoral drift. Smith (1959) also notes a southward trend of littoral drift that dissipates near Deadman Point. QRF group II has a higher proportion of quartz than group I with near subequal values of monocrystalline undulose quartz and plagioclase (33.9 percent and 31.9 percent, respectively). As in QRF group I, the mean abundance of potassium feldspar and plutonic rock fragments are again close, 12.4 percent and 11.9 percent, respectively. The littoral distribution of this group is shown in Figure 9. Group II is confined mostly to the segment between Zephyr Point (sample locality No. 27) and Eagle Point (sample 39 Figure 8. - Quartz-rock fragments - feldspar group distribution. 40 NAUTICAL MILES I I I _________ i______ 0 GROU GROUP GROUP GROUP GROUP /-BACKSHORE /^FORESHORE [/% > -O F FSHORE Figure 9. - Shoreline sediment textures. 184 3 8 4 384 2 8 3 3,4,2 I - SAND 2 - GRAVEL 3- COBBLE 4 - BOULDER 82 28 I— : 281 GRANITE CLIFF NAUTICAL MILES 43 locality No. 8). The shoreline, predominantly represented by group II (South Lake Tahoe), receives some 2,000 m etric tons (2175 tons) of fluvial sand per year for every mile of existing shoreline within this segment (Engstrom, 1978). Field estimates (California Division of Soil Conservation, 1971) show that the total sediment yield available for transport by the Upper Truckee River and Trout Creek is approximately 27,000 m etric tons (30,000 tons) per year, and that both are capable of transporting an even higher amount of sediment, should it be available. Petrographically, the shoreline beach sand (group II) and the Upper Truckee River sand are compositionally very similar which lends further support to fluvial sediment contribution. The direction of littoral sand transport along this segment is not readily apparent using only pétrographie information from the QRF groups. Engstrom (1978) and Smith (1959) both note that there is strong evidence for on easterly direction of littoral transport, with occasional episodes of westerly sediment transport (Budlong, 1971). Based on the south shore distribution of group II and evidence presented by other authors, three facts are relatively clear: I) the easterly drift of littoral sand seems to dissipate between Elk Point (sample locality No. 24) and Zephyr Point (sample locality No. 27), presumably due to the tunneling of sand into deeper water by the canyons present at the eastern end of the segment; 2) any westward movement of sand is effectively trapped in spaces between boulders (Fig. 9) at the far western extent of the segment; and 3) the beach sand is derived directly from the glacial-outwash deposits (Fig. 4) by fluvial processes. QRF group III (Fig. 7) has a composition intermediate to groups I and II but with greater proportion of rock fragments. Monocrystalline undulose quartz 44 has a mean abundance of 28.6 percent, while plagioclase abundance Is 24.7 percent. Potassium feldspar and plutonic rock fragments each have mean abundances of 12.2 percent and 21.6 percent, respectively. The littoral distribution of group III (Fig. 8) is somewhat scattered. However, there is a significant concentration of group III samples at Sugar Pine Point (sample localities 98 to 105). Engstrom (1978) estimates that this shoreline receives approximately 220 m etric tons (389 tons/mi) of fluvial sand per year for every km of shoreline; and that, at this location, c liff erosion also may be an important contributor of beach sediment. The unconsolidated glacial deposits that form Sugar Pine Point (Fig. 4) are easily erodable by fluvial and wave action, should the wave energy levels be high enough. Only a southerly direction of littoral transport is indicated, by I) the mixing of groups just south of Sugar Pine Point, and 2) the sharp cutoff between groups III and V north of Sugar Pine Point (Fig. 8). Because of this sharp boundary between groups, it is probable that littoral transport is not active north of Sugar Pine Point. Engstrom (1978) also notes that a southward drift of littoral sand is probable, based on the predominant northeast wind direction that affects this shoreline. QRF group IV is significantly different from the first three groups due to a high mean abundance of volcanic rock fragments (51 percent). Characteristic mean abundances of plagioclase, monocrystalline undulose quartz, potassium feldspar, and plutonic rock fragments are 20.9 percent, 4.9 percent, 2.3 percent, and 9.2 percent, respectively. The presence of chert, with a mean abundance of 0.8 percent and a high mean abundance of 45 pyroxene at 3.0 percent, is significant. Group IV obviously represents sediment derived from the volcanic rocks located predominantly in the northwest portion of the basin (Fig. 4). Littoral sand transport cannot be inferred by using group IV location points. Engstrom (1978) has noted that the littoral drift directions ore highly variable in this portion of the lake. One fact, however, is evident concerning fluvial transport. Stream samples No. 3, 20, and 28 are all assigned to group IV yet samples taken from near the mouths of these streams belong to other groups, or ore unassigned. This seems to suggest that, in some coses, fluvial contribution of sediment to the littoral sand budget is negligible or strongly diluted. Work by Engstrom (1978), however, suggests 373 m etric tons of fluvial sediment per year is deposited on the beach for every km of shoreline. The volcanic labors in this area, especially around Dollar Point, ore easily weathered and eroded, and, therefore, available for transport. QRF group V exhibits a high mean abundance of volcanic rock fragments, 75.9 percent, and a high mean abundance of pyroxene, 11.7 percent. Also evident are low mean abundances of; monocrystalline undulose quartz, 1.0 percent; plagioclase, 5.4 percent; potassium feldspar, 0.5 percent; and plutonic rock fragments, 0.4 percent. Chert has a mean abundance of 0.2 percent, which is slightly lower than group IV. Group V shows a very limited littoral distribution and is concentrated near Tahoe Pines (Fig. 8, sample localities 92 to 96). Immediately evident from Figure 8 is, again, the sharp boundary between group V and group III. As mentioned previously, this sharp boundary seems to suggest that littoral drift is negligible in this portion of the lake. The northern boundary of group V 46 seems to represent a direct compositional change in the shoreline vo I conics, grading from a volcanic flow with high concentrations of pyroxene to a flow with high concentrations of plagioclase. A stepwise discriminant analysis of the QRF groups was performed to determine if the groups are statistically valid. This Fortran IV analysis is used in defining a subset of variables that maximizes group differences. One at a tim e, the variables are entered into a classificatory function until the separation of groups does not improve noticeably. In this case, the QRF groups were known; and so the author was only interested in this analysis as a m ultivariate test for group differences, using the F approximation to lambda test for equality of variances. Table 7 shows the F-m atrix and classification m atrix for the QRF groups (l-IV). At the 0.01 level of significance, all QRF groups were found to have separate and distinct sample populations. From the classification m atrix (Table 7), it is noted that 98.4 percent of the cases were correctly classified into the proper groups. A Fortran IV cluster analysis was executed, using all of the pétrographie variables, to determine whether or not a more statistically distinct set of groups could be formed. This m ultivariate analysis is used when one suspects that the data is heterogeneous and could possible be reduced or classified into groups. In this instance, Q-mode clustering was performed on cases. This type of clustering permits a m ultivariate summary of the data in display form. From this analysis, six clusters formed (Fig. 10), with each cluster having its own partieluar set of pétrographie characteristics. Cluster I is characterized by almost equal mean amounts of monocr y st a 1 1 i ne undulose quartz, 31.6 percent, and plagioclase, 32.9 percent. Approximately 47 C M C M (0 lO C D 00 < D (d C D r^ C M C M C M C D L U C M C M C M 48 Figure 10. - Q-mode cluster analysis. 49 CASE NO. LABEL I 3 -B 2 1 II8-F 27 4 1 - 0 . 2 7 II If ORDER OF AMALGAMATION 6 4 ST-1 54 los e s ST-9 42 1-0 47 16-F 52 2 2 - 23 2 3 - I6 -B # l 31 3 0 4 5 -0 31 4 7 -0 .2 1 38 6 0 -B I t 4 0 6 4 -F 33 5 1 -8 2 % 7 68A-B t i 40^8 2 9 4 2 -0 .2 7 u r n , . a fA ÎI-B 17 l- F - I 18 71 1 8 ^ 2 0 10 82 -B IJ:|' 7 9 1 0 6 -F ?l II:? 3 1 8. 20 I0-8 if it p 5 6 8 -0 .1 5 6 7 7 9 -F il n I Ë - 4 2 6 -8 5 3 ./ 4 9 . / 50 equal mean abundances of potassium feldspar, 12.3 percent, and plutonic rock fragments, 14.0 percent, are also evident. Figure II shows that the littoral distribution of Cluster I is confined to areas composed of glacial sediment (Fig. II). A net eastward littoral drift pattern is clearly evident in the southern portion of the lake, by noting that no samples of Cluster I are found west of the mouth of the Upper Truckee River (Fig. II). Petrographically, and distributionally. Cluster I is very similar to QRF group II being made up of largely the same samples. Cluster 2 has slightly different pétrographie characteristics from Cluster I. Very unequal mean abundances of monocrystalline undulose quartz and plagioclase exist, comprising 17.5 percent and 47.1 percent respectively. The mean abundances of potassium feldspar and plutonic rock fragments are 11.5 percent and 14.5 percent respectively, much like Cluster I. The composition and littoral distribution of Cluster 2 (Fig. II) is almost exactly the same as QRF group I (Fig. 8). Cluster 3 is distinctly different from the first two groups. This group is characterized by having a mean abundance of 23.2 percent monocrystalline undulose quartz, 34.8 percent plagioclase, 19.9 percent plutonic rock fragments, and 7.8 percent potassium feldspar. The mean abundances of hornblende and biotite are significant in this group, having percentages of 5.9 percent and 3.5 percent respectively. The littoral distribution of Cluster 3 (Fig. II) does not coincide with any one QRF group (Fig. 8). Interestingly enough, the boundary of Cluster 3 and 2 is at exactly the point where Smith (1959) predicted the convergence of northerly and southerly littoral drift directions. 51 Figure II. - Map showing distribution of clusters I through 6. 52 75X7372 70 69 68 GROUP 1-0 GROUP 2-0 GROUP 3 -A GROUP 4-A GROUP 5-0 GROUP 6- 3 6 ^ 12 13 14 BACKSHORE FORESHORE ^ O F F S HORE NAUTICAL MILES 111 J I I Cluster 4 is very similar to Cluster I. The mean abundance of monocrystalline undulose quartz, 30.7 percent, is nearly equal to the mean abundance of plagioclase, 26,8 percent. The mean abundances of potassium feldspar and plutonic rock fragments are nearly equal, having 15.2 percent and 13,2 percent respectively. If the littoral distribution of Cluster 4 is compared with Figure 8, it is evident that QRF groups II and III are best related to Cluster 4, By comparing the distribution of Cluster 4 (Fig, II) with the shoreline geology (Fig, 4), it is evident that this cluster is derived from lake bed deposits. Curiously, this relationship suggests a westerly direction of littoral drift in the southern portion of the lake. Cluster 5 is similar to Cluster 4, Monocrystalline undulose quartz has a mean abundance of 26,3 percent, and plagioclase displays a mean abundance of 29,6 percent. Potassium feldspar and plutonic rock fragments are nealy equal in mean abundance, comprising 12,0 percent and 14,8 percent respectively. The most significant characteristic may be the higher amount of magnetite at 2,1 percent mean abundance, Compositionally, Cluster 5 seems to be related to QRF group II, and is probably derived from the glacial deposits. Cluster 6 is distinctly related to the volcanic source rocks of the Tahoe basin. In this cluster, volcanic rock fragments have a mean abundance of 40,2 percent, whereas monocrystalline undulose quatz, plagioclase, plutonic rock fragments, and potassium feldspar comprise, in mean abundance, 11,0 percent, 18,8 percent, 6,3 percent, and 3,1 percent respectively. Furthermore, highly characteristic of Cluster 6 is the presence of 0,6 percent chert. Only in cluster 6 is chert present. Higher mean abundances 54 of pyroxene and magnetîte-ilmenîte comprising 4.8 percent and 3.1 percent respectively, are inherent. Figure II shows that Cluster 6 is relatively widespread along the littoral zone. Some shoreline occurrences of this cluster are many miles from the nearest volcanic source rock. This leads to one of two conclusions: I) the computer grouping is incorrect; or 2) littoral drift is active. Compilation of evidence supports the second conclusion. A fter clustering, another stepwise discriminant analysis was carried out using the six clusters. This was done to determine if, in fact, these clusters are significantly different from one another. Table 8 summarizes the classification and F matrices of the clusters. A t the 0.05 significance level, Clusters I, 3, and 4 do not significantly differ. At a significance level of 0.01 , Clusters I, 2, 3, and 4 do not significantly differ. Some interesting possibilities exist: I) since all pétrographie variables are weighed equally in this cluster analysis (i.e., pyroxene is just as important as plagioclase), these clusters may in fact be a better representation of the natural system; or 2) the clusters are not significant, in this case, the lack of discrimination seems to indicate the reworking of an older unit into a younger one. The classification m atrix presented in Table 8 shows that 79.5 percent of the cases were correctly classified, whereas 98.4 percent of the cases of QRF groups were correctly classified (Table 7). The results of the pétrographie analysis suggest that the littoral sand is compositionally grouped into shoreline segments. In many cases, each shoreline segment has its own particular littoral d rift patterns associated with the predominant wind direction affecting that segment. Since the basin can be divided into five lithologie types (granitic, volcanic 55 CJ —I O C —I O ^ Z _ l C J < 0 ” < O C Ü H (0 o — C J O O r O 00 —I O CM 1 1 1 l u —I C M G O I- K - X Z o = UUJ </> to CJ lO CM —I —I —I —I —I 56 and metcmorphîc rocks, glacial and lake bed sediments) with limited drainage (Fig. 4) (Burnett, 1971), it is probable that the principal minéralogie components of each rock type, along with rock fragments, are potentially important in defining littoral sand distributions. The stepwise discriminant analysis which was performed using the QRF groupings, shows that the quartz-rock fragment-feIdspor groups are statistically valid at the 0.01 significance level. In the cluster analysis of cases, all variables are considered equally, and therefore the importance of minerals such as pyroxene (mainly in the form of augite) and magnetite-ilmenite are amplified. Subtle compositional changes that are not evident from a QRF diagram suddenly become evident. Even though a stepwise discriminant analysis of the six computer- derived clusters shows that at the 0.01 significance level, three and four groups respectively are derived from the same sample population, important information concerning littoral drift patterns and compositional changes is conveyed. The discriminant analysis of the computer derived cluster groups shows that groups I through 4, at the 0.01 level of significance, are statistically homogenous and therefore were derived from the same or very similar parent population. Since the glacial and lacustrine deposits were largely derived from the surrounding plutonic rocks (Burnett, 1971), the discriminant analysis correctly shows this relationship. Based on the preceeding discussion, it is suggested that both the QRF and cluster groupings are correct, and useful in defining littoral petrofacies trends, which, in turn, are useful in defining littoral drift directions and limitaitons. 57 Textural Results and Discussion Table 9 is a summary of the overall grain size parameters of the littoral zone and streams of Lake Tahoe. Backshore sand averages 0.96 mm in diameter and is moderatley sorted (Folk, 1974). The backshore samples are generally positively skewed and extremely leptokurtic. One can also see that the foreshore and backshore grain characteristics are almost a mirror image of one another. Sand sampled from the offshore portion of the littoral zone is distinctly different. There the average grain size is 0.65 mm and the sorting is moderate. Skewness is nearly symmetrical, but the kurtosis is again extremely leptokurtic. Fluvial sand averages 0.88 mm in diameter and is poorly sorted. Skewness is strongly positive whereas the kurtosis is very leptokurtic. The littoral sand in general has mean grain diameter of 0.84 mm and shows moderate sorting. The sand is also finely skewed, but extremely leptokurtic. An R-mode factor analysis was performed using the mean grain size (mm), standard deviation, skewness, and kurtosis as the variables. An R-mode factor analysis makes a variable-by-variable comparison of data (Harbaugh and Merriam, 1 968). The analysis isolates theoretical factors that account for the greater portion of the observed variance, and indicates the importance of each factor to each variable. The interpretation is somewhat simplified by rotation of the factor axes into alignments that show the most distinct factor loadings. A high positive value for factor loadings (0.500 to 1.000) shows that two or more variables are highly correlated with the positive end of the factor axis. A highly negative value for factor loadings (00.500 to -1.000) indicates that this group is highly correlated with the m G O 00 Cs J lU (/) Û O S 0 ) 0 C M G O G O Q . S I > O M " CO CM CO C M GO O O JN: 3 0) O O ) JQ negative end o f the fa c to r axis. Factor loadings near zero denote variables th a t are essentially independent o f the computed fa c to r axis. If a fa cto r grouping shoud have tw o highly positive or negative fa cto r loadings, the inte rpretation is th a t the variables vary in d ire ct proportion to one another. If a fa cto r grouping has a highly negative loading and a highly positive fa cto r loading, it can be said th a t these two variables are in d ire ctly proportional or m u ta lly exclusive. When interpreting each fa cto r axis, the investigator must decide what natural process m ight account fo r the inclusion or exclusion of pa rticular variables. this questions can be answered, a theoretical model fo r geological processes can be tenuously presented. A fte r com putation, using a Fortran IV program (Biomed), it was found that tw o factors are needed to explain 80 percent o f the to ta l variance of the moment data for the entire lake sample population. Table 10 shows the rotated fa cto r loadings fo r the set of moment measures. By convention, only fa cto r loadings w ith values greater than or equal to 0.500, and less than or equal to -0.500 are considered fo r interpre tation . Factor I accounts fo r 42 percent o f the to ta l variance and shows high positive rotated fa c to r loadings fo r skewness (a measure o f graphic asym m etry) and mean grain diam eter (mm). Factor No. 2, which accounts fo r 38 percent o f the variance, shows a highly negative rotated fa cto r loading for standard deviation (sorting) and a highly positive rotated fa ctor loading fo r kurtosis. B ivariate scatter plots of skewness versus mean grain diam eter (Fig. 12) and kurtosis versus standard deviation (Fig. 13) show some crude graphic trends. It is evident th a t as the mean grain diam eter increases so does the skewness, which becomes more postive. As the value for 60 Figure 12. - B ivariate scatter p lo t- mean grain diam eter vs. phi skewness. 61 lO CM ro m CM CM iO CM CM CM ro CM CM CM CM CM CM — ^ ro — — fO ro — lO CM CM — ■ — c m X , CM — — — — C M ,C M CO — — — — CM — C M CM CM C M CM CM CM CM CM O lO ro o o C O o ID CM O o CM O to o o o o in < > z < LU O O M " CM O O CM O CO o o o CD CM I O O CM I C O L U ^ ( X i r ro ) 62 Figure 13. - Bivariate scatter plot- phi kurtosis vs. phi standard deviation. 63 cviro to CM I p lO -/C M CM C M ro CM CM CM fO CM CM CM ro CM to CM CM. CM CM CM CM O lA S tO — CM CO O O If) O I f ) CM I f ) CM O O O O I f ) 0 0 O O I f ) N - O I f ) C M CO O 8 I f ) O I f ) h - ro O O I f ) CM O I f ) CM O O O CM O C < > > U 1 a o H c o CM 00 CO CM = > 0: H > < 0: 64 C M O O CD >1 O o CM Û Z CCO c/> < Û C < > 3 l - l l l k: c /)û standard deviation increases (becoming more poorly sorted) the kurtosis decreases (becoming less peaked). A similiar R-mode factor analysis was performed on each portion of the littoral zone (backshore, foreshore and offshore) and the stream sand samples. Factor loadings, similar to those found for the entire sample population, are found in each portion of the littoral zone, namely the backshore, foreshore, and offshore (Table 11). The stream rotated factor loadings are different, in that the variables correlative in rotated factor No. 1 are skewness, kurtosis, and mean grain diameter (Table 11). In rotated factor No. 2, standard deviation and mean grain diameter are directly proportional. This trend differs from the rest of the sample population significantly. These facts are clarified in the "Discussion" section of the following pages. Discussion Results of the textural analysis (Fig. 14) suggest that the sand packages and littoral drift patterns are difficult to explain by standard textural analysis. This is particularly true of the Nevada shore zone. Textural trends indicate that the net littoral drift patterns are more complex than suggested by Engstrom (1978). The only clear trend evident from the textural data is that there is a definite offshore fining. From State line Point to Sand Harbor (Fig. 14, sample localities 67 to 54) the textural packages are too complex to define the transport direction of littoral sand. The presumed direction of transport is eastward (Engstrom, 1978). From Sand Point to Dead man Point (Fig. 14, sample localities 54 to 41) the 66 Table II. - Rotated Factor Loadings OFFSHORE MEAN (mm) 1 FACTOR 1 .875 FACTOR 2 .240 STANDARD DEVIATION 2 .179 .869 SKEWNESS 3 .902 -.028 KURTOSIS 4 -.011 -.861 FORESHORE MEAN (mm) 1 .890 .089 STANDARD DEVIATION 2 .121 .870 SKEWNESS 3 .882 -.147 KURTOSIS 4 .182 -.855 BACKSHORE MEAN (mm) 1 .899 -.056 STANDARD DEVIATION 2 .024 -.848 SKEWNESS 3 .892 .149 KURTOSIS 4 .108 .855 STREAMS MEAN (mm) 1 .552 .778 STANDARD DEVIATION 2 -.129 .924 SKEWNESS 3 .872 .319 KURTOSIS 4 .863 -.171 67 Figure 14. - Grain size distribution of the Lake Tahoe littoral and stream sediments. 68 86 87 88 89 90 O FINE (0.125mm-0 .2 50mm) O MEDIUM (0.250mm-0.500mm) COARSE (0.500mm-1.000mm) A VERY COARSE ( I.OOOmm- 2.000mm) I0|| ,12 1 3 1 4 BACKSHORE 'l i yy-FORESHORE y%^OFFSHORE NAUTICAL MILES I I textural analysis is again inconclusive, except for the general trend of offshore fining. The shoreline sediment is predominantly boulders (Fig. 9) and the offshore sand packages are all very coarse to coarse sand (Fig. 14). The presumed direction of sand transport is north (Engstrom, 1978), but aerial reconnaissance during sampling showed sandspits forming southward. Since sandspits form in the direction of longshore currents (Komar, 1976) it must be assumed that at least during sampling the d rift direction was south. From Deadman Point to South Point (Fig. 14, sample localities 41 to 37) there is no justifiable cone I use ion regarding the direction of sand transport. Texturally there is some fining southward, but the available data is contradictory and inconclusive (Fig. 14). Textural data derived from the patchy sand beaches (Fig. 9) between South Point and Zephyr Point (Fig. 14, sample localities 37 to 27) suggest elements of sand transport in both northerly and southerly directions. Smith (1959) also noted that there might be two directions of transport operating near this location. Engstrom (1978) reports no presumed direction of transport between Zephyr Point and Elk Point (Fig. 14, sample localities 27 to 24), but a general fining of the offshore sand southward was noted. This southward trend in fining may indicate that a southward transport trend may be operational. A southern trend in littoral sand movement also is evident in that sand is accumulating in the southern portion of the Zephyr Point - Elk Point shoreline segment (U.S. Dept, of Commerce, 1971). Along the shoreline segment between Elk Point and Eagle Point (Fig. 14, sample, localities 24 to 8) an eastward sand transport trend suggested by 70 Engstrom (1978). The sand fines somewhat to the east, where there is a tremendous accumulation of sand. Since there is good textural correlation between the littoral sand and the river sand, it is likely that a major portion of the river sand is transported to the littoral system by the Upper Truckee River, even though the river actually empties into a large swampy area just south of the beach itself. It is postulated that during the spring runoff, most of the sediment being carried in suspension or as bed load is deposited in the lake rather than the swamp, due to the large discharge of the water. Records kept by the California Division of Soil Conservation (1969) reveal that more than 27,216 m etric tons (30,000 tons) of sediment per year, may be carried in suspension and as bed load, by the Upper Truckee, which would ultim ately be deposited as sediment in Lake Tahoe. Texturally, the littoral zone and the river sediment are very coarse (Fig. 14). As seen in figure 9, this segment has for the most part a sandy shoreline. North of Baldwin Beach (sample locality No. II), data is inconclusive due to the lack of samples, boulder shoreline, and steeply sloping lake bottom. The cell represented from Eagle Point to Rubicon Point (Fig. 14, sample localities 8 to 4) is essentially without data. The shoreline is boulder (Fig. 9) and very steeply sloping (Fig. 2). Seismic evidence (Hyne, 1968) shows that a great thickness of sediment covers the bedrock at this locality. From Rubicon Point to Sugar Pine Pont (Fig. 14, sample localities 4 to 102) there is southward direction of littoral sand transport (Engstrom, 1978). The textural data from this area is inconclusive as far as determining a direction of net sand transport, but it should be noted that there is a considerable accumulation of sand within this shoreline segment. 71 Textural data collected between Sugar Pine Point and Dollar Point (Fig. 14, sample localities 102 to 82) suggests that transport may actually occur in two directions, south from Madden Creek (Stream No. 5) and north from Ward Creek (Stream No. 3). However, since this hypothesis is based on only a few widely spaced samples, and since the apparent direction of transport is south (Engstrom, 1 978), the author reserves judgement in this case. Seismic profiling done by Hyne (1968) shows that a large deltaic outwash complex exists just south of Dollar Point. From Dollar Point to Flick Point (Fig. 14, sample localities 82 to 75) the direction of sand transport is northward (Engstrom, 1978). The textural data, which shows a slight northerly fining of sand (Fig. 14), appears to also support a northward direction of net sand transport. The shoreline segment represented from Flick Point to State line Point (Fig. 14, sample localities 75 to 68) is Well defined texturally (Fig. 14), in that the cell is very homogenous. Engstrom (1978) suggests an eastward direction of net sand transport, however there Is little textural evidence to substantiate this concept. Texturally there is a very slight fining to the east, which may or may not indicate an eastward trend of transport. Seismic profiling (Hyne, 1968) shows that at State line Point there is a tremendous fault scarp, so it would seem impossible for sand to be transported around Stateline Point. Results of the R-mode factor analyses on all textural data indicated that two factors explain 80 percent of the variance for the total littoral sample population (Table 10). Rotated factor No. I associates mean grain diameter (mm) with phi skewness (Table 10). A bi varia te scatter plot of the two variables (Fig. 12) shows that 72 the mean grain diameter is directly proportional to skewness. More positive values for skewness indicate that the sand population has an excess amount of fines. As previously noted there is a definite fining offshore. As wave action sorts the foreshore littoral sands, the finer fractions are carried offshore leaving the coarser material behind (Reineck and Singh, 1975). This offshore fining seems to be represented by rotated factor No. I. Rotated factor axis No. 2 displays an inverse relationship between phi kurtosis and phi standard deviation (Table 10), whereas the remainder of the variables are essentially independent of this factor. Graphical evidence (Fig. 13) shows that as the value for phi standard deviation increases, thus becoming more poorly sorted, the phi kurtosis decreases (graphically less peaked). This inverse relationship between phi standard deviation and phi kurtosis is what one would expect to see. Table 9 shows that the fluvial samples show a significant increase in phi standard deviation and decrease in phi kurtosis when compared with the remainder of the littoral samples. This may imply that rotated factor No. 2 represents the fluvial sediment contribution of the littoral sand population. One problem with this reasoning is that quite possibly the effect of the relatively few fluvial samples would be largely masked by all of the lake samples (offshore, foreshore and backshore) in the R-mode analysis. One also must consider time and intensity of mechanical energy at various points along the shoreline to determine how much the fluvial input has been modified. Figure 15 shows a summary of the sediment transport directions derived from the textural and petrographical analyses of this study. Since this lake is probably not strongly affected by the moon's gravity, it is likely that the 73 direction of net transport is dependent solely on the prevalent direction of the wave-generating winds (Fig. 15). When waves break on shore at an oblique cngle, a longshore current flowing parallel to the shoreline is generated (Komar, 1976). In a study of Lake Michigan, Fox and Davis (1976) found that the longshore current there was dependent on the first derivitive of barometric pressure. Therefore, the waves and currents that modify beach and nearshore topography are strongly influenced by major storm tracks and local weather features. In some instances two directions of net sediment movement were noted in Lake Tahoe (Fig. 15). Komar (1976) found that the net littoral movement of sediment is due to the summation of all of the individual wave trains striking the beach from numerous wave-generation areas, and that the direction of transport can be seasonal. By noting the differences in prevalent wind direction in Figure 15 and the frequency of wind direction in Table I, it is easy to see that various directions of longshore movement due to wind generated waves is possible. 74 Figure 15. - Map summarizing most likely net sand transport directions. 75 ST-28, ST-29 ST-27 CRYSTAL BAY S T -3 TAHOE PINES ST-25 ST-24 AGATE BAY S T -23 ST-30 S T -22 TAHOE CITY TRUÇKE R ST-2 ST-2 1 DEADMAN FT. GLENBROOK BAY PREDOMINANT W IN D DIRECTION R E L A T IV E % S T-20 PINE P T 4-I02 ST-8 SUGA S T -9 ZEPHYR COVE ST-10 / RUBICON PT ST-18 EMERALD BAY ST-12 ST-13 S T -17 FALLEN 1 l e a f l a k e n a u t ic a l m il e s CONCLUSIONS A pilot investigation was conducted along the littoral zone of Lake Tahoe to better define net littoral sand transport and associated compositional trends. Standard textural and pétrographie analyses, along with computer analysis, were used on sand samples collected from the offshore, foreshore, and backshore sub-environments of the littoral zone, along with samples collected from various fluvial sources. The results of this investigation are summarized as follows; 1 . There is a definite offshore fining of littoral sand which suggests a net offshore movement of sand. 2. The longshore d rift of littoral sand is confined to shoreline segments between headlands, much like the marine "cell" environment. The direction of net transport is dependent solely on the prevalent direction of the wave- generating winds, and can be quite variable. 3. The littoral sand is compositionally clustered into at least five groups, using McBride's (1963) triangular graph of sandstone compositions; or six groups, using multivariate clustering techniques. Although there was somewhat of a lack of discrimination among the computer cluster groups, it was found that both groups contribute useful information concerning net littoral sediment transport. The lack of discrimination probably indicates reworking of older units into younger ones. 4. Fluvial transport and c liff erosion are important in supplying sand to the littoral environment of Lake Tahoe. The relative importance of each depends on the location in question. The beach sand of south Lake Tahoe is 77 derived from fluvial erosion and transport of the southernmost glacial deposits. The littoral sand of Rubicon Bay is directly related to the erosion of the adjacent backshore cliffs, because no fluvial sources are evident, nor is there much evidence for longshore transport. Because of the renewed interest in the environment of Lake Tahoe, further studies, more specific than this one, will be required to obtain complete information on all aspects of the littoral zone. Specifically, the author suggests heavy mineral analysis, Fourier grain shape analysis, and surface micro texture analysis for a more complete understanding of sediment provenance and transport. Also suggested is an artificial tracer grain study used in conjunction with current meters for further verification of offshore and longshore movement of sand. ACKN O W LEDG EM ENTS The author would like to thank Dr. R.H. Osborne, Dr. D.S. Gorsiine, and Dr. J.L. Anderson for their guidance and consturctive criticism on this project. The author is particularly indebted to Dr. R.H. Osborne and the personnel in the Sedimentary Petrography laboratory who helped collect and sort the basic data for this thesis. Funding for this thesis came in part from the "Geological Sciences Graduate Research Fund." I would also like to thank Tracey Thomas for typing the final manuscript. My deepest thanks go to my wife Wendy for her understanding, patience, and typing skills. 79 References Basu, A., S.W. Young, L.J. Suttner, W.C. James, and G.H. Mack, 1975, Re- evaluation of the use of undulatory extinction and polycrystallinity in detrital quartz for provenance interpretation: Jour. Sed. Petrology, v. 45, p. 873-882. Bateman, P.E., and Wahraftig, C., 1966, Geology of the Sierra Nevada, in Bailey, E.H., ed.. Geology of Northern California, C alif. Div. Mines and Geology Bull. 190, p. 107-172. Birkeland, P.W., 1963, Pleistocene voIconics and deformation of the Truckee area, north of Lake Tahoe, California: Geol. Soc. America Bull., v. 74, p. 1453-1464. ______ 1964, Pleistocene glaciation of the Northern Sierra Nevada, north of Lake Tahoe, California: Jour. Geology, v. 72, p. 810-823. 1968, Mean velocities and boulder transport during Tahoe-age floods of the Truckee River, California-Nevada: Geol. Soc. America Bull., v. 79, p. 137-142. Bud long, Gerald M., 1971, Processes of beach change at Tahoe Keys, California: an example of man and nature as geomorphological agents: unpublished M.A. Thesis, Chic State College, Chico, California. Burnett, John L., 1967, Geologic map of Lake Tahoe, southern half: California Div. Mines and Geology open-file report. 1968, Geology of the Lake Tahoe basin, in Evans, J.R., and Matthews, R .A ., eds.. Geological studies in the Lake Tahoe area, California and Nevada: Sacramento, Annual Field Trip Guidebook, Geol. Soc. Sacramento, 99 p. 1971, Geology of the Lake Tahoe basin: California Geology, v. 24, p. 119-127. Court, J.E., Goldman, C .R ., and Hyne, N,, 1972, Surface sediments in Lake Tahoe, California-Nevada: Jour. Sed. Petrology, v. 42, p. 359-377. Durrell, Cordell, 1965, LaPorte to the summit of the Grizzly Mountains, Plumas County, California: Geological Society of Sacramento, field trip guide book. Engstrom, W., 1978, The dynamics of physical shoreline change, in The Cumulative Impacts of Shorezone Development at Lake Tahoe: Phillips, Brandt, Reddick, McDonald, and Grefe, p. 4-2 - 4-15. 80 Folk, R .L., 1955, Student operator error in determination of roundness, sphericity, and grain size: Jour. Sed. Petrology, v. 25, p. 297-301. 1974, Petrology of Sedimentary Rocks, Hemphill Publishing Co., Austin, Tx., 182 p. Fox, W.T., and Davis, R.A. Jr., 1971, Weather patterns and coastal processes, in Davis, R.A. Jr. and Ethington, R .L. (eds.). Beach and Nearshore Sedimentation, Soc. Econ. Paleontologists and Mineralogists Spec. Publ. No. 24, p. 1-23. Galehouse, J.S., 1971, Point counting: in Carver, R.E. (ed.). Procedures in Sedimentary Petrology, John Wiley and Sons, Inc., New York, p. 385- 407. Har bough, J.W. and D .F. Merriam, 1968, Computer Applications in Stratigrphic Analysis, John Wiley and Sons, Inc., New York, 282 p. Hudson, F.S., 1948, Donner Pass zone of deformation, Sierra Nevada, California: Geol. Soc. America Bull., v. 59, p. 796-800. Hutchinson, G.E., 1957, A Tretise of Limnology, v. I, Geography, Phsyics and Chemistry: John Wiley & Sons, Inc., New York, 1015 p. Hyne, N.J., Chelminski, P., Gorsiine, D.S., and Goldman, C.R., 1972, Wuaternary history of Lake Tahoe, Co I if omi a-Nevada: Geol. Soc. America Bull., v. 83, p. 1435-1448. Jenkins, Olaf P., 1938, Geomorphic map of California: California Division of Mines and Geology Map Sheet. K err, P.F., 1959, Optical Mineralogy, McGraw-Hill Book Co., New York, 442 P- Komar, P.D., 1971, Evaluât in of longshore current velocities and sand transport rates produced by oblique wave approach, In Davis, R .A . Jr. and Ethington, R.L. (eds.), Beach and Nearshore Sedimentation, Soc. Econ. Paleontologists and Mineralogists Spec. Publ. No. 24, p. 1-23. 1976, Beach Processes and Sedimentation, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 429 p. Krumbein, W.C., and F.A. Grayblll, 1965, An Introduction to statistical models In geology: McGraw-Hill Book Company, San Francisco, 660 p. Le Conte, J., 1875, On some ancient glaciers of the Sierra: Am. Jour. Sci., 3rd Ser., v. 10, p. 126-139. Lîndgren, Waldemar, 1896, Description of the Pyramid Peak quadrangel, California: U.S. Geological Survey Geol. Atlas, Folio 31. 1897, Description of the Truckee quadrangle, California: U.S. Geological Survey Geol. Atlas, Folio 39. 1 91 1, The Tertiary gravels of the Sierra Nevada of California: U.S. Geological Survey Prof. Paper 73, 226 p. Loomis, A.A., I960, Petrology of the Fallen Leaf Lake area, California: unpublished Stanford Univ. Ph.D. dissertation. Louder back, G.D., 1911, Lake Tahoe, California-Nevada: Jour. Geography, v. 9, p. 277-299. 1923, Basin range structure in the Great Basin: California Univ. Pubs. Geol. Sci., V. 15, p. 1-44. 1924, Period of scarp production in the Great Basin: Univ. of C alif. Publications in geological Sciences, v. 15, p. 1-44. Matthews, R.S., 1968, Geological hazards of Lake Tahoe basin area: in Evans, J.R. and Matthews, R.A ., Geological studies in the Lake Tahoe area, California-Nevada, Annual field trip guidebook of the Geological Society of Sacramento, 1968, p. 14-26. McBride, E.F., 1963, A classification of common sandstones: Jour. Sed. Petrology, v. 33, p. 654-669. Pittm an, E.D., 1969, Destruction of plagioclase twins by stream transport: Jour. Sed. Petrology, v. 39, p. 1432-1437. Powers, M .C., 1953, A new roundness scale for sedimentary particles: Jour. Sed. Petrology, v. 23, p. 117-119. Rittenhouse, G., 1943, A visual method of estimating two dimensional sphericity; Jour. Sed. Petrology, v. 13, p. 79-81. Smith, R.M ., 1959, Littoral d rift - a study: Lake Tahoe Area Council, No. I, 8 p. Tahoe Regional Planning Agency, and U.S. Dept of Agriculture, Forest Service, 1971, A guide for Planning, 21 p. Van der Plas, L. and Tobi, A .C ., 1965, A chart for judging the reliability of point counting results: Am. Jour, sci., v. 263, p. 87-90. 82 Wahrhaftig, C., 1965, Tahoe C ity to Hope Valley: in Wahrhaftig, C., Morrison, R.b. and Birkeland, P.W., Guidebook for field conference I, Northern Great Basin and California, International Assoc, for Quaternary Research, p. 59-71 . Wolfe, J., 1968, Earthquake History near Lake Tahoe, in Evans, J.R., and Matthews, R .A ., ed.. Geological studies in the Lake Tahoe area, California and Nevada: Sacramento Annual Field Trip Guidebook, Geol. Soc. Sacramento, 1968, p. 27-36.
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Sand transport and petrofacies of the Lake Tahoe littoral zone
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