M.R. ROSEN studied using a scanning electron microscope. (SEM). .... Trailer Park Well. Feb 22. 1960 ... faces darkly stained with desert varnish. The mid fan ...
Palaeogeography, Palaeoclimatology, Palaeoecology, 84 (1991): 229-257
229
Elsevier Science Publishers B.V., Amsterdam
Sedimentologic and geochemical constraints on the evolution of Bristol Dry Lake Basin, California, U.S,A. M i c h a e l R. R o s e n a' 1
aDepartment of the Geological Sciences, University of Texas-Austin, Austin, TX 78713-7909, U.S.A. (Received A u g u s t 8, 1988; revised a n d accepted June 13, 1989)
ABSTRACT Rosen, M.R., 1991. Sedimentologic and geochemical constraints on the evolution of Bristol Dry Lake Basin, California, U.S.A. Palaeogeogr., Palaeoclimatol., Palaeoecol., 84: 229-257. Two continuous cores over 400 m in length drilled in Bristol Dry Lake, a large playa in the Mojave Desert, California, U.S.A., provide sedimentologic and isotopic information which can be integrated with surface trenches and groundwater analyses. The combination of these data provide three-dimensional data on the facies distribution and geochemical evolution of the basin for the last 4 Ma. At the surface, the basin exhibits a generalized bulls-eye facies distribution. Low-gradient alluvial fans ring the playa. Extensive calcrete and pedogenic calcite associated with halophyte plants cement the mid-to-distal fan gravels and sands. Basinward of the distal fans, in the playa margin facies, an extensive gypsum zone is over 300 m wide. Within the gypsum zone, celestite forms large decimetre-size nodules which may coalesce into metre-size patches. Basinward of the playa margin facies, halite hopper crystals (up to 0.5 m in diameter) form in water-saturated muds of the saline mud-fiat facies. Finally, in the basin-centre a 0.2-m thick chevron halite crust (salt pan) forms from evaporation of ponded ephemeral water. The lateral facies distribution seen at the surface is found throughout the entire length of the cores. The core drilled in the basin-centre exhibits beds of alternating salt pan and saline mud-flat facies, whereas the core drilled in the playa margin exhibits alternating beds of playa margin sediments and saline mud-fiat for most of the length of the core. Towards the base of the playa-margin core, distal alluvial fan sediments are also present. Stable oxygen isotopes of basin-centre calcite concretions from the surface and core indicate that the isotopic composition of the water precipitating the concretions has not changed substantially throughout the evolution of the basin. The sulfur isotopic composition of the surface gypsum, anhydrite and celestite show little variation. However, 634S values of anhydrite from core are isotopicaUy lighter relative to surface gypsum and anhydrite. This isotopic depletion indicates that bacterial sulfate-reduction is involved in the dehydration of gypsum to anhydrite, and is not caused by evolving isotopic composition of sulfate in the brine. At the surface, gypsum ~a4S values show no statistically significant trends with lateral distribution. Chemical analyses from shallow groundwater wells and basin-centre brines from up to 150 m depth indicate that: (1) the brine is dominated by Na, C1, and Ca. (2) Na, Ca, Mg, K, and CI contents increase toward the basin-centre, SO4 increases from the alluvial fan facies to the playa margin and then sharply decreases in the basin-centre, and Si, and HCO3 decrease slightly toward the basin-centre. This pattern is consistent with the observed evaporite mineral distribution in the basin. (3) molar Na:C1 and Ca:CI ratios demonstrate that the simple dissolution of previously deposited halite deposits, as has been suggested for other playa basins, cannot account for the proportions of these ions present in the brine. (4) Chemical budget calculations suggest that CI can be accounted for by atmospheric input, although hydrothermal sources cannot be discounted. The repetitious nature of the alternating shallow brine-pond halite and siliciclastics, the consistency of the carbonate isotopic data from the surface and core, and the agreement of water chemistry data with observed evaporite sequences, indicate a relatively stable brine composition for most of the history of Bristol Dry Lake. All sedimentary structures and primary halite fabrics in the core indicate shallow-water, brine-pond halite alternated with halite-saturated siliciclastic muds in the basincentre. A delicate balance between subsidence, and mechanical and chemical deposition of minerals was necessary to maintain the largely ephemeral environment of deposition during the deposition of over 500 m of basin fill. 1Present address: Div. Water Resources, CSIRO, Private Bag, P.O. Wembley, W.A. 6014, Australia.
230
Introduction Bristol Dry Lake, located in California, USA, is an inland closed-basin (playa) filled with over 500 m of sediment. Several continuous cores drilled over the last 30 years around the basin-centre make Bristol Dry Lake an ideal location for studying relatively long-term geological factors which control the evolution of the basin. In addition to the data from cores, surface exposures, trenches, and hydrologic data provide clues to the overall mineralogic evolution of the basin with time. Recently, emphasis has been placed on geochemical parameters and hydrologic systems in inland evaporite settings in which the variation in evaporite mineralogies in a given basin can be constrained by fluid flow pathways and the availability of ions from the parent solution (Hardie, 1968; Hardie and Eugster, 1970; Hardie et al., 1978; Eugster and Hardie, 1978; Smith et al., 1987; Schmid, 1988). Other studies, such as Wasson et al. (1984), have inferred the geochemistry of the water by studying the coexisting mineral assemblages. The purpose of this study is to determine the hydrologic evolution of Bristol Dry Lake by documenting how depositional environments, mineralogies, and isotopic data relate to the hydrochemical data. The primary reason for this approach is that in ancient evaporite environments, lithologic data are the only information available to the geologist; hydrologic data are generally not available. Abundant data from Bristol Dry Lake are available that address various aspects of the sedimentology (Gale, 1951; Bassett et al. 1959; Durrell, 1953; and Handford, 1982a, b) and hydrology (Thompson, 1929; Shafer, 1964; and Calzia, pers. comm., 1979), yet there is no integrated interpretation of these data. This study represents the first detailed integration of these data. The study also includes new data from trenches and deep (> 500 m) cores that enable the construction of a more detailed three-dimensional model of the basin. The model can then be used to evaluate the sedimentologic and geochemical evolution of the basin.
Geologic setting Bristol Dry Lake is situated in the Mojave Desert region of southeastern San Bernardino
M.R.ROSEN County, California (Figs. 1 and 2). It is the largest (155 km 2) in a system of three N W - S E trending dry lakes (playas) located in a structural trough between mountain ranges in the Basin and Range Province of North America. Bristol Dry Lake is separated from Cadiz and Danby dry lakes by projecting arms of mountain ranges that define the trend of the basin as a whole. At present, each of the three sub-basins have completely separate internal drainage. However, the elevation difference between Bristol Dry Lake and Cadiz Dry Lake is only 2-3 m. It has been suggested that water has flowed from one basin to the other during preHolocene time. This has important implications for the mass balance of ions reaching Bristol Dry Lake. Water chemistry data and observed mineral assemblages support this hypothesis. Unfortunately, at the present time, there are not enough data to fully assess the input from the Cadiz Dry Lake drainage basin into the Bristol Dry Lake over the geologic past. A fourth basin, Alkali Dry Lake (Fig.l), is separated from Bristol Dry Lake by Amboy Crater, a relatively young cinder cone and basalt flow which is thought to be less than 6000 years old (Parker, 1963). Before the basalt flow blocked off the northern end of the drainage area, creating Alkali Dry Lake, this area drained into Bristol Dry Lake. Therefore, for most of the history of the Bristol Dry Lake basin, the total drainage area into the playa was significantly greater (about 4000 km 2) than it is today. The present total drainage area into Bristol Dry Lake, excluding drainage from Cadiz Dry Lake and Alkali Dry Lake, is just over 2000 km 2. Geochemical correlation of a tephra layer at 513 m in the basin-centre core (method of SarnaWojcicki, 1976) yields an age of approximately 3.7 Ma (Rosen, 1989). This core did not penetrate to bedrock when it was drilled. A crude estimate of the depth of the basin was attempted by Rosen (1989) by interpreting the Bouguer gravity map for the Needles, California, quadrangle. Based on this calculation, the total thickness of the basincentre is approximately 680 m. If this calculation is correct, bedrock is approximately 150 m below the bottom of the basin-centre core. Therefore, based on this Calculation and the average sedi-
SEDIMENTOLOGIC AND GEOCHEMICAL CONSTRAINTS ON EVOLUTION OF BRISTOL DRY LAKE, CALIFORNIA
231
Fig.1. Slidelong aperture radar (SAR) image of Bristol Trough showing present surface runoff area (white outline) to Bristol Dry Lake (B) and Alkali Lake (A). Amboy Crater and lava flows (L) are visible separating Bristol from Alkali Dry Lake. Cadiz Dry Lake (C), which is approximately the same elevation as Bristol Dry Lake, is also within the Bristol Trough. Danby Dry Lake is farther to the southeast off the picture. Scale bar is 10 kin.
mentation rate, the basin is at least mid to early Pliocene in age. Previous work
A regional geological study by D a r t o n (1915) and hydrological studies conducted by the
U.S.G.S. on the Mojave Desert region ( T h o m p s o n , 1929) were the first w o r k done in the area. Gale (1951), Durrell (1953), and Bassett et al. (1959) first studied the geology o f the area t h r o u g h trenching and core analyses. However, emphasis was placed on detailed description and not on interpretation o f facies relationships or diagenetic con-
..... .....
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BRISTOL DRY LAKE
\
SACFUS
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Fig.2. Location map of Bristol Dry Lake showing sampling trenches and well locations. Filled circles are water wells. Filled stars are wells with lithic logs and water data (1979, U.S.G.S. wells). The groundwater drainage divide and saline-fresh water interface are from Shafer (1964). The data for these two lines is sparse and open to interpretation. Circled crosses are lithic logs and/or core data. Numbers on wells correspond to names in Table 4 and in the text.
v
~ LAVAFLOWS
.+i +0+ ~+,'?~ • AMBO¥CRATER
AMBOY
bo
SEDIMENTOLOGIC AND GEOCHEMICALCONSTRAINTSON EVOLUTIONOF BRISTOL DRY LAKE,CALIFORNIA
trols. Recently, Handford (1982a, b) conducted a reconnaissance study of the gross sedimentology of the surface and shallow subsurface of Bristol Dry Lake and attempted the first integrated interpretation of the area. This study developed a general facies pattern based on lateral facies distribution, but raised many important questions about how this facies pattern varies with depth. Thompson (1929) proposed that during Pleistocene time a large lake occupied all of the Bristol and Cadiz basins. However, no evidence for the existence of such a large lake has been documented (Bassett et al., 1959; Handford, 1982a). More recently, the Bristol Dry Lake basin has been interpreted to have alternated between subaerially exposed periods and times when shallow ephemeral water bodies covered the playa surface (Handford, 1982a). Overall, even during periods of regionally high rainfall, evaporitic conditions seem to have been dominant. Modern meteorological conditions for the central Mojave Desert and Bristol Dry Lake specifically indicate a mean annual rainfall of less than 100 mm (Table 1), although Thompson (1929) noted that there are periods of 2-3 years when no precipitation has been recorded. TABLE 1 Climatic data for Bristol Lake area Temperature data from Bagdad, Ca. Approx. 30 km NW of Amboy (based on 17 years of data from Thompson, 1929). High: 48°C Low: - 0 . 8 ° C Highest monthly mean (July): 35.1°C Lowest monthly mean (December): l 1.4°C Annual mean: 22.4°C Precipitation data from Bagdad Ca. Approx. 30 km N W of Amboy (based on 17 years of data from Thompson, 1929). Maximum monthly rainfall (February): 81.3 mm. Minimum monthly rainfall (every month): 0.0 mm. (In 17 years it never rained in June). Mean annual rainfall: 57.9 mm (35% of rain falls in Jan.-Feb.). Maximum yearly rainfall: 259 ram. Minimum yearly rainfall: 0.0 ram. Precipitation data from Amboy (based on 4 years of data from 1982-1987 from Dr. G. I. Smith, comm., 1987). Summer average 1982-86:58 mm. Winter average 1982-87:56 mm. Data from Handford (1982a). Humidity: 40-50% (January), 20-30% (July). Mean annual salt pan evaporation: 3200-4000 mm.
233
Methods
Field work was conducted on 6 separate occasions over a 3-year period during the months of October to May. Samples from the basin-centre were collected and stratigraphic sections were measured from extensive pits and trenches excavated by two salt companies. The deepest of these excavations is approximately 20 m. Around the margins of the playa, there were no trenches, so 7 trenches along 2 transects perpendicular to the basin margin were excavated, one transect on the north and the other on the south edge of the playa (Fig.2). Each trench is approximately 2 m deep. The length of each trench is close to 20 m except one continuous trench which is 300 m long. Outcrops of the alluvial-fan sequences which are exposed in wadi channels, and a pipeline trench that has since been closed, provided some control on the source and method of transport of sediment into the basin. Two cores, 3 km apart (Fig.2), taken by Southern California Edison Utility Company in 1985, represent the stratigraphy from 150 to 540 m. These cores, which have virtually 100% core recovery, are open to public viewing and are permanently stored at the core storage facility of the Bureau of Economic Geology, University of Texas, Austin, Texas, U.S.A. One core, CAES #l, was taken on the basinward side of the playa margin sediments which are now covered by the recent basalt flows from Amboy crater. The other core, CAES #2, was taken almost directly in the centre of the basin. The cores were logged and selected samples were used for petrographic and chemical analyses. In addition, the lithic logs of 3 U.S.G.S. cores from the upper 150 m were studied and compared with other data to develop a more complete evolution of the basin. Representative samples from all the facies encountered were cut in oil-base lubricants, impregnated with blue epoxy, and made into 75 x 130 mm thin sections. Thin sections were studied to determine mineralogy, fabric and diagenetic alterations in the sediment. The determination of mineralogies was aided by X-ray diffraction analyses (XRD). Five hundred points were counted on each of 30 halite and 4 gypsum slides to determine quantitative mineralogies. In addition, some samples were
234
M.R. ROSEN
studied using a scanning electron microscope (SEM). Gypsum, anhydrite, and celestite crystals separated from the siliciclastic matrix were washed in distilled water and then a 10% HC1 solution to remove contaminants. Seventy samples were then sent to Coastal Science Laboratories, Austin, Texas, U.S.A., for stable ~34S analysis (Table 2). The error of the analyses is 4-0.5%o CDT. Blind duplicates of 5 samples are within the reported error. Calcite samples were analyzed for the stable isotopes of oxygen and carbon in core CAES #2 (Fig.2, Table 3) following a modification of the procedures outlined in Epstein et al. (1964). The error of the analyses is, +0.3%o PDB. Recently, it has been suggested that fine-grained calcite ( < 5 ~tm) washed in distilled water will dissolve and reprecipitate when dried at temperatures < 60°C (Barrera and Savin, 1987). The calcite reprecipitates from water used in processing. The reprecipitated calcite is enriched in 180 during evaporation and shifts measured oxygen values of the original TABLE 2 Stable sulfur isotope data from the surface and from core. Error is 0.5%0 CDT Surface sulfates Depth (m) 0.00 0.00 0.00 0.08 0.08 0. I 0 0. I 0 0.15 0.18 0.20 0.20 0.30 0.30 0.30 0.30 0.32 0.32 0.40 0.40 0.45
Mineralogy
Sulfur (%0 CDT)
Gypsum Anhydrite Anhydrite Gypsum Gypsum Gypsum Celestite Gypsum Gypsum Celestite Gypsum Gypsum Gypsum Gypsum Celestite Gypsum Gypsum Celestite Gypsum Gypsum
9.4 7.0 7.4 7.7 7.9 7.0 7.6 7.9 7.9 7.5 6.8 8.3 7.9 7.7 8.4 8.2 8.8 7.7 8.4 8.2
TABLE 2 (continued) Surface sulfates Depth (m) 0.55 0.60 0.60 0.61 0.61 0.61 0.75 0.92 0.92 0.95 0.99 0.99 1.00 1.05 1.10 1.22 1.22 1.22 1.22 1.30 1.30 1.30 1.30 1.40 1.40 1.50 1.50 1.50 1.52 1.52 1.55 1.55 1.56 1.65 1.65 1.74 1.83 1.90
Core CAES #1 286.0 319.1 324.1 340.5 424.3 442.4 442.9 443.5 444.1 479.4 480.8 516.3
Mineralogy
Sulfur (%o CDT)
Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Anhydrite Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Gypsum Celestite Gypsum Celestite Celestite Gypsum Gypsum Gypsum
7.4 7.8 6.5 8.1 8.3 7.8 7.5 8.5 8.4 7.4 7.1 7.7 8.9 6.3 8.4 8.0 8.4 8.1 8.5 8.3 8.8 6.8 7.4 7.5 7.1 7.5 7.6 6.1 7.8 7.9 7.2 7.4 7.1 6.4 6.3 7.3 8.6 7.7
Anhydrite Anhydrite Anhydrite Anhydrite Anhydrite Anhydrite Anhydrite Anhydrite Anhydrite Anhydrite Anhydrite Anhydrite
2.9 0.9 1.5 1.5 3.3 3.4 3.9 3.9 3.4 4.0 3.5 3.5
SEDIMENTOLOGIC AND GEOCHEMICAL CONSTRAINTS ON EVOLUTION OF BRISTOL DRY LAKE, CALIFORNIA TABLE 3
Stable oxygen and carbon isotopes from surface and core calcite. Error is + 0 . 3 % o P D B Basin-centre concretions Depth
61s o
613 C
(m)
(%oo P D B )
(%0o P D B )
1.2 -2.3 --0.9 -2.2 -O.l 0.0 - 0.9 -0.5 -2.2 0.6 -0.9 -1.1 -0.5 0.4 -1.0 1.0 0.5 -0.3 -0.6 -0.5 -0.6 -1.4 0.7 0.5 -0.2 0.3 0.0 -0.3 0.4 0.4
-13.1 -10.3 -14.2 -10.7 - 10.2 - 10.1 - 15.2 - 15.4 -8.5 --9.8 -14.3 -12.3 -13.8 -10.3 -14.3 -9.0 -10.2 -8.9 -8.2 -8.8 -11.I -7.6 -10.9 -10.5 -9.5 -11.0 -I1.1 -9.1 -10.5 -8.9
0.20 0.56 0.61 0.61 0.64 0.69 0.74 0.81 0.81 0.86 0.91 0.91 0.97 0.97 0.99 1.02 1.02 1.19 1.22 1.30 1.37 1.37 1.52 1.65 1.75 1.78 1.78 1.83 1.85 2.13
Playama~mpe~genicca&ite 0.15 0.15 0.15
-7.7 -7.8 -8.4
-1.0 -0.9 -1.3
--3.9 -3.7 --3.6 --3.8 --2.8 --0.9 --0.3 --0.6 --2.0 - 2.1 -- 1.2 --2.5
--8.2 --7.5 --7.6 -- 10.3 -4.0 - 12.9 - 15.7 -- 14.8 -- 11.7 -- 15.4 - 16.3 -- 13.7
Core CAES #2 137.92 138.07 138.07 138.30 152.92 182.76 332.23 345.14 347.01 400.96 457.20 518.46
235
sample to heavier values. Samples in this study are generally < 4 lam in size. Therefore, all samples were washed and dried in acetone following the procedures outlined by Barrera and Savin (1987). Water analyses were taken from various sources in the literature (Table 4). Charge-balance calculations were determined for each analysis to determine their accuracy. Although the charge-balance calculations indicate that the analyses are not very accurate (up to 20% imbalance), the analyses are sufficient to draw some important conclusions from these data. Depositional environments In order to understand the hydrochemical evolution of the basin it is fundamental to first understand the lateral and vertical lithologic facies distribution of the basin. Within the saline lake environmental setting, Hardie et al. (1978) recognized 10 genetically related depositional subenvironments. Of these 10 subenvironments, Handford (1982a, b) recognized 4 broad subenvironments in Bristol Dry Lake. They are, from alluvial fan to basin-centre: (1) the alluvial fan, (2) the playamargin sand-fiat and wadi system, (3) the saline mud-fiat, and (4) the salt pan at the basin-centre. These designations differ slightly from the Hardie et al. (1978) subenvironments, but are useful for overall descriptions of the playa geometry. The 4 subenvironments recognized by Handford (1982a) are used in this paper in order to be consistent with his work. However, detailed field work indicates that these broad subenvironments are more diverse than previously described. The following descriptions detail further subdivision of the 4 broad depositional environments outlined by Handford (1982a) into more useful geochemical and hydrologic units.
A llu vial fan The alluvial fan can be divided into three gradational subfacies based on the sediment grain-size. McGowen and Groat (1971) introduced the terms proximal, mid-fan and distal fan as physiographic segments of humid area fans in west Texas. Although the alluvial fans surrounding Bristol Dry
1959 Dec 22 1985 Nov 4 1985 1961
U.S. Geol. Survey Bristol-2 Southern Cal. Edison CAES #2 Southern Cal. Edison CAES #1 California Salt Co. Well No. 2
15 16 17 18
1978 1978 1978 1978
1953 1953 May May May May
Basin centre brine I Durrell (1953) No. 1 2 Durrell (1953) No. 2 13 Br-l-I (28 m) 13 Br-l-2 (152 m) 14 Br-2-1 (66 m) 14 Br-2-2 (151 m) 2 2 9 9
Jan 13 1961
Transitional (distal fan) 3 California Salt Co. Well No. 1
Date of analysis
Apr 26 1962 Jan I1 1961 Nov 20 1963 Feb 3 1 9 6 4 Jan 11 1961 Feb 23 1960 Feb 22 1960 Jan 13 1961 May 25 1956 May 14 1957 Feb 23 1960 Sep 4 1 9 5 8 Jul 30 1959 Aug 15 1910 Feb 23 1960 Sep 27 1910
Well
Groundwater 4 Burris Fresh Water Well 5 Roy Tull Well No. 1 5 5 6 Easley Well 7 McConnel Ranch Well 8 Trailer Park Well 8 9 Riddle Well 9 9 10 Cadiz No. 2 10 11 Cadiz Well No. I 11 12 Archer Siding Well No. 1
No. on Fig.2
Only Only Only Only
0.8578 2.1607 0.7485 0.0649 1.1976 2.5948
0.0164
0.0010 0.0021 0.0033 0.0039 0.0007 0.0013 0.0009 0.0009 0.0006 0.0009 0.0011 0.0012 0.0013 0.0019 0.0010 0.0032
Ca
0.0378 0.0845 0.0972 0.0767 0.0614 0.0588
0.0001
0.0001 0.0001
K
lithic log available core available core available lithic log available
2.0039 2.4955 3.7408 3.3928 3.6538 2.8273
0.1368
0.0037 0.0059 0.0041 0.0043 0.0027 0.0023 0.0027 0.0028 0.0038 0.0040 0.0031 0.0024 0.0020 0.0028 0.0021 0.0065
Na
0.0492 0.0884 0.0987 0.0757 0.2879 0.1892
0.0591
0.0005 0.0016 0.0020 0.0027 0.0006 0.0010 0.0007 0.0010 0.0003 0.0006 0.0008 0.0009 0.0008 0.0002 0.0010 0.0006
Mg
0.0005 0.0002 0.0001 0.0001
0.0002
0.0006
0.0008 0.0003 0.0004 0.0012 0.0004 0.0004 0.0006 0.0004 0.0004 0.0006 0.0006 0.0007 0.0005
SiO2
SO4
0.0004 0.0007 0.0004 0.0005
0.0007
0.0218 0.0044 0.0005 0.0035 0.0004 0.0001
0.0222
0 . 0 0 2 1 0.0010 0.0018 0.0017 0.0025 0.0040 0.0022 0.0059 0 . 0 0 2 1 0.0007 0 . 0 0 2 5 0.0008 0.0022 • 0.0009 0 . 0 0 2 3 0.0009 0.0020 0.0010 0.0022 0.0011 0 . 0 0 2 3 0.0010 0.0022 0.0007 0.0023 0.0006 0.0020 0.0015 0.0023 0.0007 0.0033
HCO3
Na/CI
Source of data
2.9504 4.8769 5.9233 5.0771 6.2054 5.9233
0.1892
0.6792 0.5117 0.6315 0.6683 0.5888 0.4773
0.7229
Bassett et al., 1959 Unpublished Unpublished Shafer, 1964
Durrell, 1953 Durrell, 1953 Calzia, 1979 Calzia, 1979 Calzia, 1979 Calzia, 1979
Sha~r, 1964
0.0021 ,1.7484 Shafer, 1964 0.0061 ~'0.9683 Sharer, 1964 0.0028 1 . 4 6 3 6 Shafer, 1964 0.0026 1.6165 Shafer, 1964 0 . 0 0 1 1 2.4120 Shafer, 1964 0 . 0 0 1 2 1 . 8 5 7 6 Shafer, 1964 0 . 0 0 1 2 2.1730 Shafer, 1964 0 . 0 0 1 5 1 . 8 6 2 2 Shafer, 1964 0.0019 2 . 0 2 5 5 Shafer, 1964 0.0019 2 . 0 3 3 8 Sharer, 1964 0.0017 1 . 8 2 4 8 Shafer, 1964 0.0014 1.6963 Shafer, 1964 0 . 0 0 1 3 1 . 5 0 8 6 Shafer, 1964 0 . 0 0 1 5 1 . 9 2 7 6 Thompson, 1929 0.0012 1 . 8 0 5 4 Shafer, 1964 0.0047 1 . 3 9 3 5 Thompson, 1929
CI
Major element water chemistry data (molar) of groundwater from Cadiz Basin and brines from Bristol Basin. Numbers on left correspond to well locations in Fig.2
TABLE 4
r~
SEDIMENTOLOGICAND GEOCHEMICALCONSTRAINTSON EVOLUTIONOF BRISTOLDRY LAKE, CALIFORNIA
Lake are arid fans, similar to fans described by Bull (1972) and Blissenbach (1954), the physiographic terms used by McGowen and Groat (1971) can be applied to Bristol Dry Lake fans. The proximal fan, includes the part of the fan closest to the mountain source. At present this area is dominated by channelized flow through arroyos incised through the older proximal and mid fan deposits. The older fan deposits consist of coarse-grained gravel, cobble and boulder-size sediment interbedded with gravelly sands. Between the arroyos are cobble- and boulder-covered surfaces darkly stained with desert varnish. The mid fan area is dominated by coarse-grained braided stream deposits. Broad, shallow channels are filled with low-angle trough cross-stratified cobbles, gravels, and sand. Extensive box-work calcrete (Fig.3) is present in poorly-sorted cobble strata of the upper part of the mid fan. The calcrete zone (K-horizon) is best exposed on the north side of the basin in the Bristol Mountain fan arroyos (directly north of Amboy). However, the calcrete zone is widespread throughout the fans. The calcrete zone is approximately 1-3 m below the surface of the fan and it is overlain by the C1horizon of a dark red paleosol. The paleosol is disconformably overlain by a thin 1-3 m unce-
237
mented layer of low-angle trough cross-bedded braided stream deposits. The distal fan is dominated by sheet flood deposits mostly composed of sand-size and finer particles. A thin veneer of the distal fan overlies, and grades into, the playa margin sediments. Pedogenic calcite and gypsum, and small amounts of meniscus halite form in the porous distal fan sediments. The well-developed paleosol and calcrete which extend up into the lower proximal fan are also exposed in the arroyo walls of the distal fan, approximately 1-3 m below the surface fan deposits. At the present land surface, the alluvial fans surrounding Bristol Dry Lake have a low gradient (1.8 m/km), although the northern alluvial fans tend to be steeper than the fans on the south-side of the playa. Yet surface sediment, even in the distal parts of the fan, range up to boulder-size. Some of the larger volcanic boulders may be ejecta from nearby Amboy Crater. Others may represent the lag of large debris flow deposits which have since been deflated by wind erosion. Where cores penetrated the distal portions of the fan, sediments consist of planar laminated, well-sorted, medium- to fine-grained sand with abundant mud partings in the lower half of core
Fig.3. Box-work catcrete in mid-fan area follows horizontal, more permeable, poorly-sorted cobble layers. The horizontal layers are joined by vertical pipes of calcite through better sorted sands. Pen by a vertical pipe for scale.
238 CAES # 1. Planar laminae are defined by concentrations of heavy minerals, but they are neither inversely graded nor do they show other characteristics of wind-ripple laminations (Bagnold, 1941). The planar bedding, fine-scale lamination and mud partings suggest that these sediments were deposited by very shallow sheet-flows in the distal fan subfacies. The California Salt C o m p a n y ' s 400 m well completed in 1961 (No. 18 in Fig 2. and Table4), which was taken along the northern margin of the playa, was logged as containing 30 m of distal fan deposits underlain by 300 m of playa and lacustrine sediments. However, the b o t t o m 70 m of the the core contains boulders, cobbles, gravel, and sand (Shafer, 1964) that probably represent mid-fan to distal-fan deposits. Although small barchan dunes were observed by Handford (1982a) on the playa margin sand fiat, these dunes are no longer present in the Bristol Dry Lake basin. Small coppice dunes are present on the distal alluvial fans and playa margin where plants have trapped sufficient sediment to raise mounds 0.5-1 m high. In general, aeolian deposits are ephemeral in Bristol Dry Lake because the playa surface lacks sufficient moisture to trap sediment. Although the wind regime in the basin is seasonal, the dominant wind direction is to the southeast. Strong winds have plastered sand up onto the sides of the divide separating Bristol Dry Lake from Cadiz Dry Lake. In addition, a large dune field, with some dunes up to 15 m high, is migrating to the southeast directly down-wind of the divide in the Cadiz Basin. This suggests that a great deal of the sand is transported out of the Bristol Dry Lake basin and over the divide into
M.R. ROSEN
the Cadiz basin. No unequivocally aeolian deposits have been seen in the core.
Playa margin Playa margin sediments are deposited in a transitional zone vegetated by sparsely populated halophyte shrubs, between the distal alluvial fan and the saline mud-flat. The sediments vary from silty sands to sandy muds that contain calcite-cemented nodules (some with minor amounts of silica rimming cement) surrounding root holes of former halophyte shrubs. In addition, mud-filled traces of roots and burrows are also present, and blocky prismatic or columnar fractures in the sediment indicate a possible lower Bt or Bn soil horizon (Birkeland, 1984, p. 15). Small root traces and burrows are present in siliciclastic intervals from the CAES #1 well, as deep as 500 m. However, calcrete or silcrete associated with root zones have not been found in the core. Numerous wadi channels and distributary channels (Handford, 1982a) dissect the playa margin and bypass sediment and organic matter from the alluvial fans directly across the playa margin to the playa centre. Handford (1982a) recognized that the distributary channels may be an important mechanism for distributing sediment across the playa. Some of these distributary channels which are 1 m deep and 100 m across are much larger than has been previously reported. The channels extend well out onto the saline mud fiat. Shallow trenches across these wadis and distributary channels reveal flaser bedded sands and muds, also reported by Handford (1982a) from the surface
Fig.4A. Facies map of surface sediments showing location of gypsum-celestite zone and the "bulls-eye" pattern of the evaporite minerals. The playa margin subenvironment extends from the edge of the present playa surface (and under the lava flows) to the edge of the gypsum-celestite zone. Although isolated displacive gypsum occurs in the saline mud fiat, almost all of the gypsum is concentrated in the gypsum-celestite zone. Note that at present there are two salt pans on the surface. B. Cross-section A-A' shown in Fig.4A. The relief on this cross-section is from topographic maps and the position of the different subenvironments at the surface are based on the facies distribution in 4A. The dashed line represents an inferred normal rotational fault. The position of salt beds and facies distributions with depth are schematic based on cores and trenches. Calcite is precipitated as calcrete horizons and nodules in the alluvial fans and playa margin sediments. Gypsum and celestite are confined to the playa margin sediments as well. The saline mud fiat is dominated by molds of small (5-10 mm) displacive halite which are larger and remain filled towards the basin-centre. The modern salt pan consists of vertically aligned chevron halite. Handford's (1982a) cross-section of Bristol Dry Lake (his fig. 12), shows gypsum and anhydrite in the saline mud flat all the way to the basin-centre. Although minor amounts of gypsum and anhydrite are found in the saline mud fiat, the bulk of the gypsum is precipitated in a narrow band around the playa margin. This distinction is important in constructing vertical facies models for the evolution of the basin.
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SEDIMENTOLOGIC AND GEOCHEMICAL CONSTRAINTS ON EVOLUTION OF BRISTOL DRY LAKE, CALIFORNIA
sediments, as well as planar bedded, sheet-flow sands. Where the playa margin passes into the saline mud-flat, a 0.15-0.5 km wide zone of gypsum and celestite is present in a lens-shaped body in a ring around the entire basin (Fig.4a, b). The details of
this zone are presented in Rosen (1989) and Rosen and Warren (1990). Field and chemical data presented below indicate that discharging groundwater in this zone is saturated with respect to gypsum and celestite. Vertically aligned lensshaped gypsum crystals (up to 200 mm along the
A A
