a middle pleistocene lacustrine authors delta in the kern ... · robert negrini dirk baron janice...

Pacific Section AAPG Publication MP48 95

ABSTRACT

A 200- to 400-ft-thick coarsening-upward sequence of sediments was mapped at depths of 300 to 700 fbgs under the western margin of the Kern River alluvial fan in the southern San Joaquin Valley of California. Sedimentary petrography and x-ray diffractometry of samples from wells containing this unit indicate a predominantly Sierran source for the clastic component of the sediments. The basal part of this unit is typically fine grained (clays and silts) and grades upward into medium sands. The unit thickens toward the Buena Vista Lake terminal basin and is interpreted to have been deposited as a lacustrine delta as part of an alluvial fan-delta system that prograded into its terminal basin. Relatively high amounts of organic carbon and extremely low magnetic susceptibility suggest that the unit was deposited under reducing geochemical conditions. Authigenic euhedral and framboidal pyrite crystals indicate that reducing conditions progressed to the sulfate reduction stage. The coarsening-upward sequence is thickest in a rectangular, prism-shaped trough bounded by the surface projection of normal faults that were previously mapped in deeper sedimentary units. This observation suggests that the deposition of this unit was at least in part structurally controlled. Basinward, the unit laps onto or grades into a thick depocenter clay layer that is tentatively correlated to the Corcoran Clay that was deposited in a valley-wide lake prior to 600,000 years ago. In the Kern Water Bank area, the thickest part of the coarsening-upward unit lies in the same region as a set of wells with anomalously high concentrations of groundwater arsenic. Relatively high concentrations of easily-exchangeable arsenic were found in samples from a well within the coarsening-upward unit. These observations suggest that, in the reducing lacustrine setting of this unit, arsenic was locked up in pyrite reservoirs. Dissolution textures observed in pyrite crystals under a scanning electron microscope suggest a post-depositional transition to more oxidizing conditions and the subsequent release of arsenic into solution in groundwater and in easily-exchangeable sites on mineral surfaces.

AUTHORS

Robert Negrini Dirk Baron Janice GillespieRobert Horton Anne Draucker Neil Durham John Huff Paul Philley Carol Register

Department of Geology California State University Bakersfield, CA 93311

Jonathan Parker

Kern Water Bank Authority 5500 Ming Avenue, Suite 490 Bakersfield, CA 93309

Thomas Haslebacher

Kern County Water Agency P.O. Box 58 Bakersfield, CA 93302

A Middle Pleistocene Lacustrine Delta in the Kern River Depositional System: Structural Control, Regional Stratigraphic Context, and Impact on Groundwater Quality

96 Pacific Section AAPG Publication MP48

INTRODUCTION For the past million years or so the Central Valley of California has received thousands of cubic miles of alluvial sediments transported into the valley by several major rivers, principally from the Sierra Nevada. These sediments serve as important freshwater aquifers that are a source of water for one of the world’s foremost agricultural centers, for wildlife refuges, and for rapidly growing population centers. Therefore, it is becoming increasingly important to understand the details of this aquifer system, including the stratigraphic and geochemical controls on water availability and quality. The uppermost of these sediments were deposited during the anomalously high amplitude climate

swings of the middle to late Pleistocene (e.g., Ruddiman, 2001; Zachos et al., 2001). Thus their study bears on the paleoclimatology of western North America. The current work addresses the above-described goals while focusing on a major stratigraphic feature found within the sediments of the Kern Water Bank (KWB) near the distal edge of the Kern River alluvial fan system (Figure 1). This feature, a combined unit consisting of a coarsening-upward sequence and an underlying uniformly fine-grained unit, appears to have been influenced both by local structures and regional/global climate events. In turn, its lithological features and associated environment of deposition appear to play a significant role in local groundwater quality.

Figure 1. Location map of project area and surrounding features discussed in the text. (a) Generalized geologic map after Page (1986). (b) Well log coverage in project area. Logs with wells shown as black filled circles. Sediment analyses were done on samples from wells 23H and 24K (green filled circles). Magnetic susceptibility was measured on samples from these wells and also measured on samples from wells labeled with filled grey circles. Kern Water Bank boundaries shown as red dashed lines.

Figure 2. Late Tertiary stratigraphic column of southern San Joaquin Valley (after Bartow, 1991). Sediments of this study belong to the younger part of the Tulare Formation.


BACKGROUND The KWB is a large groundwater storage and management facility with a capacity of ~1,000,000 acre-ft located on the western margin of the modern Kern River alluvial fan (Figure 1). The modern Kern River alluvial fan system is located at the southernmost extreme of the Central Valley (Figure 1). The alluvial fan covers an area of approximately 800 mi2 (~2,000 km2). This system has been the subject of many studies, mostly in the form of reports associated with groundwater resources (Mendenhall et al., 1908; 1916; Hoots et al., 1954; Dale et al., 1964; 1966; Manning, 1967; Croft, 1972; Page, 1986; Williamson et al., 1989; Bartow, 1991; PGA and Associates, 1991). The late Pleistocene sediments of the modern Kern River alluvial fan are traditionally assigned to either the uppermost Tulare Formation or the Kern River Formation, both principally units of nonmarine origin. This designation is primarily dependent on proximity of the study area to either the west or east side of the San Joaquin Valley (e.g., Plate 2 of Bartow, 1991). Because our study area is closer to the west side of the valley and because recent work on the Kern River Formation suggests that it is considerably older than previously thought (Miller, 1999; Golob et al., 2005; Baron et al., 2008), we assign the sediments of this study to the Tulare Formation (Figure 2). The lithology of the sediments of this study consists of gravel, sand, silt, and clay with the proportion of fine material increasing away from the mouth of the Kern Canyon (Dale et al., 1966; Croft, 1972). Along the western and southern edges of the fan these easterly-derived sediments interfinger with relatively minor amounts of sediments derived from the Coast Ranges and Transverse Ranges, respectively (Croft, 1972).

METHODSSediment Analyses

Several sediment analyses were conducted mainly on grab samples collected at 10 to 20 foot intervals from wells T30S/R25E-23H and T30S/R25E-24K (Figure 1). These wells were studied because they penetrated the stratigraphic unit receiving primary focus in this study. Also, the wells contained contrasting levels of groundwater arsenic according to laboratory reports on file at the Kern Water

Bank Authority in Bakersfield, CA and Boockoff (2005). Arsenic concentrations in well 23H were measured at 63 ppb in September, 2000, 55 ppb in April, 2003, and 30-33 ppb in February of 2005, whereas well 24K over the same time period has consistently had concentrations that were low enough (1.85 ppb) to be near detection limits.

Grain-size was determined by assigning sand, silt, and clay percentages using the Wentworth scale and standard percentage templates (e.g., Compton, 1985). Mineralogy and petrography of sand- and silt-sized grains were determined using petrographic microscopy augmented by scanning electron microscopy and microprobe analysis. Fifty-four thin sections were point counted (300 points per section) using an automated point-counting stage. Because the grab samples were generally unconsolidated, no textural analysis was possible during thin-section analysis. Mineralogy of fine-grain material in 32 samples was further constrained using x-ray diffractometry.

Total organic carbon (TOC) content was determined at 10-ft depth intervals using the loss-on-ignition method (Dean, 1974). Samples were dried for two days at 45 ˚C in a Fisher Isotemp Oven to expel water and then they were exposed to 600 ˚C in a Lindberg Muffle furnace for 1 hour to combust the organic carbon. The mass of the sample was measured before and after each step.

Magnetic susceptibility (MS) was measured with a Bartington MS2 susceptibility meter after samples were placed in an MS2B bottle sensor. This was converted to mass-specific MS by dividing through by the density of each sample. Because MS can be measured relatively rapidly, these data were acquired for a total of eight wells (including wells 23H and 24K) along a transect parallel to a radius of the Kern River alluvial fan.

Arsenic concentrations of sediment grab samples were measured in a Perkin-Elmer ELAN 6100 ICP-MS after the exchangeable fraction was isolated using the methods from Tessier et al. (1979). This extraction isolates arsenic weakly adsorbed to mineral surfaces that could be released into the groundwater due to changes in pH and ionic strength, or due to increases in the concentrations of competing ions such as phosphate.


Correlation and Mapping of Subsurface Units The primary subsurface mapping data used in this study consists of electric (short normal resistivity) logs gathered from water and oil wells. Most of these logs are from the Kern Water Bank. The database includes electric logs from 162 wells in the KWB and nearby groundwater projects, covering an area of approximately 80 mi2 (~200 km2), most of which penetrate to depths of several hundred feet or more. These electric logs were digitized and then entered into a project in Geographix™, a subsurface Geographic Information System (GIS) software package commonly used in the petroleum industry. Lithologic units were defined based on grain-size distributions inferred from log resistivity values, an assumption that is addressed below in the Results section. These units were then correlated to delineate surfaces corresponding to the bases and tops of stratigraphic units. The resultant surfaces were contoured as structure maps and the thicknesses of the units were contoured as isochore maps.

RESULTS Mineralogy and Petrography Based on thin section analyses of the grab samples, the sediments of the Kern Water Bank in the region of wells 23H and 24K were determined to be arkosic arenites and wackes dominated by quartz, plagioclase, and K-feldspars (orthoclase and microcline) (Figure 3). Plagioclase shows evidence of dissolution at all depths. Coarser samples contain abundant rock fragments; these are mainly microphaneritic grains of granitic composition but these fragments include quartzite, schist, chert, siltstone, and shale. Most fine-grained lithic fragments consist of shale; these are much more abundant in samples from well 23H than those from well 24K (Figure 3). A few samples contain serpentinite grains. Accessory minerals are dominated by biotite and hornblende with lesser muscovite and opaques (including ilmenite and framboidal pyrite composed of cubic and octahedral crystals). Some samples contain epidote, zircon, titanite, rutile, chlorite, calcite (or limestone), tremolite, and /or phosphate. The fine-grained sediments are dominated by smectite (R=0, I/S = 90% smectite) but include illite/mica, kaolinite, and chlorite. Pyrite is common in fine-

Figure 3. Composition of the coarse fraction of sands from Kern Water Bank wells 23H and 24K. Q = monocrystalline, polycrystalline and lithic quartz, F = total feldspars (includes lithic feldspars), L = fine-grain lithics (mainly shale clasts). Plotted following the methodology of Dickinson (1970); lithic quartz and lithic feldspars are crystals in microphanerites and phenocrysts in volcanic grains.

Figure 4. (a) Typical pyrite (white) concentrations of shale images inspected in this study. Pyrite represents ~1-2% of the area shown in this view.


d e

Figure 4. (b-c) Closeup views of spherules of framboidal pyrite and octahedron-shaped authigenic pyrite crystals in shale (polished thin section and grain mount). SEM backscattered-electron image. Well 23h, depth = 690 ft. (d) Microprobe-based back-scattered electron micrograph and wavelength dispersive X-ray element maps of detrital grains from well 23H, depth = 690 ft. Pyrite is indicated by bright spots present in both the Fe and S maps (arrows). Arsenic is present in both pyrites. The framboid on the left show exhibits a low-pyrite core and hig-pyrite rim while the mass at the right shows a spherical low-pyrite core coated with a zone of variable but lower pyrite levels. (e) Microprobe-based wavelength dispersive spectrum of pyrite in framboidal spherule showing La peak for arsenic distinctly above background. Depth = 690 ft. (f-g) Dissolution features in framboidal pyrite (f) and octahdron-shaped authigenic pyrite crystals (g) in shale (grain mount). SEM backscattered-electron image. Well 23h, depth = 690 ft.


grained sediments from both wells, occurring as spherical framboids and irregular masses of euhedral cubic and octahedral crystals (Boockoff, 2005; Negrini et al., 2006). Pyrite in fine-grained sediments from well 23H was found to contain trace quantities of arsenic as high as 0.35 weight percent (Figure 4). Clinoptilolite, gypsum, Fe-oxide, pyroxenes, rare microfossils, garnets, and/or tourmaline are also present in some fine-grained samples. Biotite is most abundant in the shallowest samples, but otherwise no trends with depth were observed in either fine- or coarse-grained samples.

Total Organic Carbon, Magnetic Susceptibility, and Exchangeable As Results for TOC, MS, and exchangeable As (in sediment) are shown in Figure 5a-b for the grab samples

from wells 23H and 24K. TOC ranges from 0 to 8%, MS from 7X10-8 m3/kg to 80X10-8 m3/kg, and exchangeable As from 1 to 15 ppm. TOC is highest and MS is lowest within the middle depth intervals of these wells (~650-250 fbgs). In fact, these anomalously low MS values are within a factor of 2 or so of the instrument sensitivity. Except for a one-sample spike near the bottom of the well, the concentration of exchangeable arsenic in well 23H is at its highest values in the middle interval with maximum values occurring between 400 and 600 ft below ground surface, toward the base of the interval with high TOC and low MS values. Exchangeable arsenic in the 24K well is also elevated slightly in this interval, but not nearly as much as it is in the 23H well. The MS logs for all of the wells surveyed in the Kern Water Bank area are shown in Figure 5c. The MS signature of

Figures 5a and 5b. Total organic carbon content (TOC), magnetic susceptibility (MS), and sediment arsenic concentrations from grab samples of (a) Well 23H and (b) Well 24K. Black lines on the arsenic plots correspond to total arsenic concentrations. Grey lines correspond to arsenic in easily exhangeable sites on mineral surfaces (e.g., clays). The latter is usually associated with higher concentrations of groundwater arsenic.

Figure 5c. Magnetic susceptibility (MS) logs from grab samples of eight wells from the Kern Water Bank. Note that wells 23H and 24K have extremely low MS values in the middle depth ranges.


the 23H and 24K wells was not typical. For the six wells from other regions of the KWB, MS decreased with depth though never attained values as low as those found in the middle depth intervals in wells 23H and 24K. If MS characteristically varies in opposition with TOC as suggested by the data in Figure 5a-b, then the TOC content of the sediments in the region around wells 23H and 24K is anomalously high, particularly in the depth range below a few hundred fbgs.

Electric-Log Resistivity as an Indicator of Grain-Size The formation fluid in the unconsolidated sediments from the upper 1,000 ft in the Kern Water Bank is fresh water with an average TDS value of ~220 ppm (e.g.,

KWBA laboratory reports; Boockoff, 2005). Under these conditions the bulk sediment resistivity should depend principally upon clay content which, in turn, is closely related to bulk grain size. The relationship between clay content in bulk sediment and resistivity is due to the cation exchange capacity of clays and the ease of current flow through the corresponding diffuse layer of exchangeable cations near grain boundaries (Keller and Frischknecht, 1970; Hallenburg, 1984; Ward, 1990). Thus resistivity potentially can be used as a crude estimator of grain-size distribution (e.g., Abu-Hassanein et al., 1996). This presumption has precedent in the study region. For example, resistivity logs were found to give the best grain size distinction between fine- and coarse-grained

Figure 6a. Grain-size of grab samples compared to electric log resistivity for Well T30R25S23H. i. Sand/silt/clay percentage diagram. Percentages were determined from grab samples spaced at 10ft intervals. The symbol “gr*” indicates the presence of gravels and very coarse sands. ii. Short-normal electric log for well T30R25S23H. Resistivity was measured at 0.5 ft intervals. iii. Clay percentage of grab samples. iv. 10 ft averages of electrical resistivity data centered on depths from which sediment grab samples were taken. Dashed lines in iii and iv are smoothed values using a depth window equal to 25% of dataset.

Figure 6b. Grain-size of grab samples compared to electric log resistivity for well T30R25S24K.


alluvial sediments in the Kern River oilfield that lies east of the water bank near the apex of the Kern Fan (Miller, 1986). The Kern River oilfield is a good analog for the Kern Water Bank in that the sediments in question are unconsolidated, are within ~1500 feet of the surface, and have relatively fresh (<650 ppm total dissolved solids) pore waters (Coburn, 1996). We tested this presumption for the sediments of the Kern Water Bank by comparing grain-size with well log resisitivity data (Figure 6). To a first order the proposed inverse relationship between clay content and resistivity holds as shown in Figure 6a.i-ii. The resisitivity is highest (~100 ohm-m) when no clay is present (e.g., sand spikes centered at 275 and 625 fbgs in well 23H) and is relatively low when clay percentage is high (e.g., average resistivity is ~20 ohm-m in the interval from ~600-300 fbgs). In order to account for sampling interval differences between that of the grab samples (10 ft) and logging tool measurements (0.5 ft) in Figures 6a.i-ii and 6b.i-ii, the clay percentage of the grab samples (Figures 6a.iii and 6b.iii) is plotted alongside log resistivity (Figure 6a.iv and 6b.iv) after the latter is averaged over 10ft intervals centered on the depth of each corresponding grab sample. It’s evident from these plots that long-wavelength resistivity changes are associated with similar changes in clay percentage.

Following the approach of Abu-Hassanein et al. (1996), we further investigated the resisitivity/grain-size relationship in well 23H by calculating the average resistivity corresponding to sets of grab samples with identical percentages of fine grains and then plotting this value against “percent fines” (Figure 6c). These data also support a first order, inverse relationship between the percentage of fine sediments and resistivity. In summary, down to a depth of at least 1000 ft, well-log resistivity values in the Kern Water Bank area exhibit a first-order relationship with the percentage of fine-grained sediments. This relationship justifies the inference of relative sediment grain-size from log resistivity values in defining lithologic units from well logs. Based on our results and the previous results of Coburn (1996), we extend this inference to rest of the southern San Joaquin Valley well logs included in this report.

Stratigraphic Correlation and MappingFigure 7 shows a representative resistivity log in which large-scale sedimentary sequences have been identified. One sequence, herein called the “Coarsening-Upward Sequence 2” (CUS2) was chosen for detailed study because its entire depth interval was present in most of the wells of this study, thus allowing its correlation and mapping throughout the

Figure 6c. Electrical resistivity as a function of %fines for well 23H. Resistivity plotted is electric log resistivity from Figure 6.a.iv averaged for all grab samples with corresponding %fines value.


Figure 7. E-log from a well containing the prograding fan delta unit (CUS2) in the Kern Water Bank region. CUS2 was defined to include the interval of uniformly low resistivities usually found below the coarsening upward sequence. To the right of the e-log is a schematic diagram of a vertical sequence typical of prograding fan deltas in Lake Hazar, Turkey (Wescott and Etheridge, 1990). The lake bottom subunit occasionally contains smaller scale fining upward sequences which may be turbidite deposits.

Figure 8. Structure contour maps on the (a) top and (b) bottom of the CUS2 (Coarsening-Upward Sequence 2) unit. (c) Isochore (aka, thickness) contour map of the CUS2 unit. Area mapped is the same as that shown in Figure 1b. Contour labels are elevations in feet relative to mean sea level. Note that, in this figure, the contour fill colors for both structure maps are tied into the same absolute elevation scale. Thus, “deep” elevations for the “Top of LsCus2” map are similar in color to the “shallow” elevations for the “Bottom” map. Interstate Highway 5 (black line), the modern Kern River (dashed blue line) and the outlines of the Kern Water Bank properties (red dashed lines) are shown as landmarks. The base of the CUS2 unit shows a broad, low-elevation feature that roughly follows the modern course of the Kern River and then takes a sharp turn southward toward the location of the modern Buena Vista Lake terminal basin (Figure 1). This observation plus the much lower relief exhibited by the top of the unit and the increasing thickness basinward of the unit support the hypothesis that the CUS2 unit is a sublacustrine delta deposit prograding SSW-ward into a more extensive, ancestral Buena Vista Lake terminal basin.


study area (Figure 8). The uppermost coarsening-upward sequence was not mapped because its top often resides above the dramatically changing water table of the Kern Water Bank through time. Thus the amplitude of the resistivity for this uppermost unit is strongly dependent on the logging date. The CUS2 unit was found in 60 wells that were primarily located in the southwestern quarter of the study area (Figure 8). Based on typical resistivity values and complementary inspections of grab samples from wells, this unit is uniformly fine-grained (clays and silts) at its base and coarsens upward typically into medium-grained sands. The coarsening-upward log signature of this unit was particularly well defined in logs from wells located along the east side and close to the southern margin of township T30S/R25E and adjacent to this region in township T31S/R25E. This location is also characterized by net sand percentages of less than 60% within the Zone 4 depth interval (300 to 150 fbsl) most commonly containing the uniformly fine-grained, bottom half of CUS2 (Figure 9). The structural map on the base of this unit (Figure 8b) depicts a trough-like low that projects into this same region from the east as part of a larger paleochannel. This feature reaches maximum depths at the south end of the area in township T31S/R25E toward the modern site of the Buena

Vista Lake terminal basin (Figure 1). The structural map on the top of this unit (Figure 8a) exhibits markedly less relief. The thickness of the CUS2 unit abruptly increases basinward to >300 ft as it nears the terminal basin as indicated by the isochore map in Figure 8c. A cross section oriented approximately north-south through the CUS2 unit (Figure 10) demonstrates that the southward thickening CUS2 unit shown in Figure 8c continues southward into the Buena Vista terminal basin where its lower part grades laterally into and overlaps a thick unit of uniformly low resistivity (Figure 10).

DISCUSSIONDepositional and Geochemical Environment of CUS2

The CUS2 is a coarsening-upward unit correlated across the Kern Water Bank region. It is best defined and found with more regularity in the south-central portion of the area toward the toe of the Kern River alluvial fan. At this location, the base of the CUS2 unit consists mostly of uniformly fine-grained silts and clays and the unit abruptly thickens basinward (Figures 8 and 10). On the structure map, the bottom of the unit also increases basinward (Figure 8a) and the structural relief of the base of CUS2 is significantly greater than that of its top (Figure 8b). Taken together

Figure 9. Net sand map of Kern Water Bank area for depth interval of 300-150 fbsl (Zone 4).


these observations suggest that CUS2 was deposited in a prograding alluvial fan-delta setting that evolved basinward into a prograding lacustrine delta that includes a distal clay/silt unit (e.g., Wescott and Ethridge, 1990). Out in the Buena Vista Lake depocenter, the CUS2 unit grades laterally into and overlaps a thick unit of uniformly fine-grained resistivity that we interpret to be a coeval lake-bottom clay (Figure 10), the expected basinward facies of the fan-delta system (Wescott and Ethridge, 1990).

The sediment analyses of the CUS2 unit are consistent with the fan-delta hypothesis. First, the CUS2 sediments are relatively high in TOC and low in MS (Figure 5) and this low MS signature appears to be confined to the region where the CUS2 unit is thickest (Figures 1, 5c, and 8). High TOC and low MS is an association that is commonly observed in reducing lacustrine environments due to the dissolution of magnetite during digestion of organic matter by anaerobic bacteria (Cohen, 2003; Evans and Heller, 2003). Second, clusters of pyrite found in fine-grained sediments of well 23H have either euhedral and/or framboidal morphologies that indicate an authigenic origin (Figure 4). All of these observations are suggestive of the reducing conditions expected of a lacustrine depositional environment as opposed to the relatively oxidizing conditions expected for

an alluvial fan setting (Cohen, 2003). During sediment diagenesis, bacterial decay of organic

carbon can quickly deplete pore waters of dissolved oxygen. If this is not replaced by additional oxygenated groundwater infiltrating from the surface, the system quickly turns anoxic. Under these conditions, remaining organic material becomes a food source for anaerobic bacteria. More specifically, as oxygen is depleted as an electron acceptor, a series of bacterially-mediated reactions use first Fe3+ from ferric oxides and oxyhydroxides and then S6+ from dissolved sulfate as electron acceptors for the continued breakdown of organic materials (e.g., Langmuir, 1997). As a consequence, detrital iron oxides and hydroxides dissolve and iron is released into groundwater as Fe2+. For this reason, MS is often used to estimate the extent of iron reduction (Evans and Heider, 2003). In the case of the CUS2 sediments, the disappearance of the MS signal indicates that most, if not all, of the available Fe-oxides and hydroxides were consumed. When all of the iron has been reduced, sulfate acts as the electron acceptor and reduced sulfur from sulfate reduction reacts with dissolved Fe2+ to form authigenic pyrite, thus explaining the framboidal (and euhedral) pyrite crystals found in well 23H (Figure 4; Boockoff, 2005; Negrini et al., 2006).

Figure 10. Cross-section from Kern Water Bank into the Buena Vista Lake terminal basin showing relationship between CUS2 unit and BV lake clay unit. The latter may be correlated to the Corcoran Clay of Croft (1972).


Regional Stratigraphic Context and Hypothesized Age of CUS2As discussed above, the CUS2 unit grades into and overlaps the upper part of a depocenter clay unit in the Buena Vista Lake basin. Clay layers found in the Buena Vista Lake basin at similar depths have been previously correlated to the Corcoron Clay (Croft, 1972; Page, 1986; Kiser et al., 1988). The Corcoran Clay is a widespread lacustrine clay deposited throughout most of the Central Valley of California 800 to 600 kyr ago (Frink and Hues, 1954; Croft, 1972; Sarna-Wojcicki, 1995; Harden, 1999). The lower age of the Corcoran Clay is constrained by the Matuyama/Brunhes magnetic reversal (789 kyr) found just below the Corcoran Clay in a core from near Wasco, CA, and the eruption of the Bishop ash bed (774±2 kyr), which is found just above the base of the clay bed in the same core (Davis, 1978; Sarna-Wojcicki, 1995; Sarna-Wojcicki et al., 2000). The upper age of the Corcoran Clay is constrained by the eruption of

the Lava Creek ash bed (639± 5 kyr), which is found just below the top of the clay in the Wasco core (Davis, 1978; Sarna Wojcicki, 1995; Lanphere et al., 2002). The depth of the Buena Vista Lake clay associated with CUS2 (~700-1000 ft) is consistent with that of the Corcoran clay 60 miles north in the Tulare Lake basin (Croft, 1972) and the depth of coeval units found in the Owens Lake basin 100 miles to the northeast (Sarna-Wojcicki et al., 1997). Thus its presumed correlation with the Corcoran Clay yields a consistent sedimentation rate (~1-2 ft/1000 yr) with other nearby basins in similar depositional settings. Presuming that the depocenter clay in Figure 10 is indeed the Corcoran Clay, then this clay and the stratigraphically-related CUS2 fan-delta deposit were deposited in Lake Clyde (Figure 11), a lake that covered most of the Central Valley of California over a time period of more than 100 kyr during the middle Pleistocene (Sarna-Wojcicki, 1995; Harden, 1999). Notably, the Corcoran

Figure 11. “Lake Clyde,” the Pleistocene lake in which the Corcoran Clay (and CUS2?) were deposited (from Harden, 2004).

Figure 12. Anomalous groundwater arsenic concentrations and surface projection of subsurface faults (Jones and Gillespie, 1997) superimposed upon CUS2 isochore contour map.


Clay has not been found in the region of the Bakersfield Arch, an active structural feature that is responsible for a broad, uplifted, northeast-trending region in the vicinity of Bakersfield (PGA and Associates, 1991; Fig. 1) upon which the Kern River alluvial fan was deposited. It is likely that the Bakersfield Arch region stood above lake-level during most of this time and separated the Tulare and Buena Vista subbasins as it does today. Because CUS2 lies on or near the top of the Buena Vista Lake clay, we hypothesize that the CUS2 unit was deposited toward the end of the lake episode, just after the eruption of the Lava Creek volcanic ash which is found near the top of the Corcoran Clay near Wasco, CA (Sarna-Wojcicki, 1995). If this is the case, then the CUS2 deltaic lobe was deposited into Lake Clyde during Marine Oxygen Isotope Stage 16 (MIS 16). MIS16 marks the highest amplitude glacial event in the Pleistocene Epoch (Reheis, 2002). This extreme glacial episode was likely responsible for the deflection of the northern jet stream storm tracks southward into the latitudes currently occupied by the Central Valley and the Great Basin deserts (Antevs, 1948; Negrini, 2002). Thus it was responsible for the largest extents of pluvial lakes in the Great Basin during the Pleistocene (Reheis, 2002) and, hence, most likely also resulted in extreme pluvial conditions in the Central Valley of California, conditions driven by anomalously high discharge of surface water from Sierran streams (e.g., the Kern River). Sarna-Wojcicki (1995) has proposed that this event resulted in an extreme highstand of Lake Clyde. The lake was then able to overtop a spillover sill into the San Francisco Bay. The highstand, coupled with the anomalously low sea levels associated with the glacial maximum, resulted in an increase in hydraulic head between Lake Clyde and the Pacific Ocean. This, in turn, led to the incision of the modern outlet of the Central Valley hydrologic system into the San Francisco Bay through the Carquinez Straits, an event that drained Lake Clyde soon thereafter. The Sarna-Wojcicki hypothesis is supported by the age of the oldest sediments in San Francisco Bay that exhibit the distinct mineralogy of sediments sourced primarily from the Central Valley (Sarna-Wojcicki, 1995; Harden, 1999). In this scenario, the CUS2 unit was deposited during the high discharge event associated with MIS 16 and the coarsening-upward nature

of this sedimentary package is due to some combination of the prograding nature of the deltaic lobe and possibly the regression of Lake Clyde as it drained into the Pacific Ocean through the newly opened Carquinez Straits.

Association of CUS2 with Elevated Groundwater Arsenic Concentrations The areal distribution of the thickest part of the CUS2 deltaic lobe is remarkably coincident with an isolated, contiguous region in the Kern Water Bank within which groundwater arsenic concentrations are above 50 ppb (Figure 12). The depth interval containing these deltaic sediments also generally coincides with depth intervals characterized by elevated concentrations of groundwater arsenic and of arsenic in the easily exchangeable state in the bulk sediment (Figure 5). The above association may be due to the reducing geochemical environment more likely to be found in lacustrine than alluvial fan settings. In this hypothesis, arsenic is bound in authigenic pyrite formed when reducing conditions are extreme enough and persist long enough for the reduction of groundwater sulfate to occur after the iron reduction phase is completed. Arsenic readily substitutes for iron in pyrite (Savage et al. 2000). A later change to more oxidizing geochemical conditions can potentially dissolve the pyrite and release the arsenic into the groundwater. This hypothesis is supported by preliminary SEM and microprobe analyses reported by Boockoff et al. (2005) and Negrini et al. (2006) wherein As-bearing, authigenic pyrite was found in shale samples from well 23H (Figure 4). Furthermore, dissolution features were observed in some of the pyrite crystals supporting a change to more oxidizing conditions and corresponding release of As to groundwater and/or to easily exchangeable sites on mineral surfaces (Figure 4).

Structural Control of CUS2 The maximum thickness of CUS2 occupies a well-defined 2-3 mile-wide rectangular prism in the part of the mapping area closest to the Buena Vista Lake terminal basin (Fig. 8c). This prism lies within the surface projection of northeast-striking normal faults mapped by Jones and Gillespie (1997) in underlying units. These faults define a series of stair-stepped fault blocks that are down to the


southeast relative to a horst mapped in the southwest corner of township 30S25E (Figure 12). The relief on the base of CUS2 in this region (~100-150 ft of relief at a depth of 700 fbgs) is also approximately consistent with that predicted by the 400-500 ft total throw across the fault system of the older middle Mya unit in the Plio-Pleistocene San Joaquin Formation at a depth of 2500-3000 fbgs (Jones and Gillespie, 1997). From these observations we conclude that the associated fault system was probably active at least up to and throughout the time of deposition of CUS2 and likely controlled site of deposition for this unit.

IMPLICATIONS AND FUTURE WORK Given increasing population stress on water resources over time, there is a growing need to better understand the relationship between groundwater resources, groundwater quality, and underlying geological controls. The work presented here begins to address this need by demonstrating probable links between specific environments of deposition, structural setting and groundwater quality. Much work remains to be done to test and refine these nascent hypotheses. The prograding fan-delta model requires testing with a higher density well log database that should become available as new water wells are drilled and logged. Efforts should be made to core selected intervals from these wells so that age control can be obtained. Two volcanic ash layers (the Lava Creek and Bishop tephra) and a major magnetic polarity boundary (the Matuyama/Brunhes) provide coring targets of the appropriate age (~800 to 600 kyr) with which to test the geologic history proposed herein. Additional SEM and microprobe work is required on samples from both wells 23H and 24K to test the proposed role of the formation and subsequent dissolution of arsenic in the contamination of groundwaters and to help determine why only one of these wells has high concentrations of groundwater arsenic. Such refinements would lead to an extension of this study to elsewhere on the Kern River alluvial fan and the greater San Joaquin Vallley which, in principal, should contain related stratigraphic packages that are controlled by the same underlying geological events and similarly affect groundwater quality and resources in their local region.

ACKNOWLEDGMENTS Funding for this study was provided by the US Department of Agriculture (USDA-CREES Grant #2001-01170) and by the California Department of Water Resources (Assembly Bill 303 “Local Groundwater Assistance” grants awarded to the Kern Water Bank Authority on July of 2003 and 2004). Al Tanabe, Tom Osborn, Elizabeth Powers, Lael Monrian, Cari Meyer, assisted with sediment analyses. Jennifer Shives assisted in mapping the Buena Vista Lake clay unit. Karen Blake was invaluable with respect to setting up the Geographix™ project upon which the well correlations and mapping were based. Conversations with Robert Crewdson and Brian Hirst greatly improved this manuscript. Quantitative XRD analyses were performed by K/T GeoServices, Inc., under the direction of James P. Talbot, P.G. Licenses were made available for the Geographix™ software under a grant to the CSU Bakersfield GeoTechnology Training Center from Landmark Graphics Corporation, Inc. of Houston, Texas. The Hitachi Corporation donated SEM time. Sarah Roeske assisted with the operation of the microprobe at UC Davis Department of Geology facility.

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