Core OL-92 from Owens Lake, southeast California

Sediment Size Analyses of the Owens Lake Core

Kirsten M. Menking
Hannah M. Musler
Earth Sciences Board, University of California, Santa Cruz
Jeffrey P. Fitts
James L. Bischoff
U.S. Geological Survey, Menlo Park, California
Robert S. Anderson
Earth Sciences Board, University of California, Santa Cruz

Contents:

Abstract

 Sedimentary analyses of a 323-m core from Owens Lake, southeastern California, show variations on many different time scales. Grain size analysis was determined on point samples taken at 2 to 3 m intervals down the core. In addition, sand plus gravel, silt, and clay percentages were measured on 3.5-m-long channel samples to supplement point sample grain size data. The upper 195 m of sediment consist of interbedded fine silts and clays. The lower 128 m, however, comprise interbedded silts and fine sands. Fluctuations in mean grain size of sediments in the top 200 m of the core correlate well with variations in carbonate content of channel samples.

Introduction

 Owens Lake is the first in a chain of lakes lying to the east of the Sierra Nevada. The lake, which lies at the southern end of the topographically closed Owens Valley, receives runoff primarily from the Sierra Nevada via the Owens River and its tributaries. During "pluvial" (high ratio of precipitation to evaporation) periods, Owens Lake exceeded the confines of the southern Owens Valley and spilled over a sill into the China and Searles Lake basins (Smith, 1984). During drier periods, the lake shrank and remained nearer to or at levels lower than the spillover sill. In this paper, we report results of grain size analyses conducted on sediments from a 323-m-long core of Owens Lake which reflect the variations in pluvial-interpluvial climate in the Sierra Nevada.

Sample Types

 Two types of samples were taken from the Owens Lake core for sedimentary analyses. "Point samples," which represent about 2 to 3 cm of core length, were collected at the drill site at 1 to 2 m spacings down the core. These samples comprise about 60 g of bulk sediment and have been used to determine water content, pore water chemistry, organic carbon and carbonate content, and grain size; grain size is reported here. "Channel samples" were produced in the laboratory and represent integrated ribbons of sediment, each spanning 3.5 m of core and comprising about 50 g of sediment. Organic carbon/carbonate, grain size, clay mineralogy and bulk chemical analyses have been performed on these samples; grain size is reported here.

Methods

Grain Size Analysis--Point Samples

About 10 g of each point sample were placed in a beaker with 100 ml of deionized water. The samples were lightly disaggregated with a glass stirring rod. To remove carbonate and organic material, 150 ml of Morgan's solution (1 L solution of deionized water, acetic acid, and sodium acetate, containing 27 ml of glacial acetic acid buffered to pH 5 with 82 g of sodium acetate) and 20 ml of 30 wt.% H2O2 were added (Note: we have assumed that fine grained carbonates were chemically precipitated in the lake rather than detrital in nature. In order to assure complete disaggregation of detrital clastics, then, it was necessary to remove any binding cements). Morgan's solution was used in place of dilute HCl because it is considered less damaging to clay minerals (Mark Johnsson, personal communication), and the clay-sized fractions of some grain size samples were collected for X-ray diffraction analysis. Samples sat in this solution for two days and were stirred once every 8 to 12 hrs. Heating to 150°C for 2 to 4 hours removed excess H2O2. Samples were centrifuged for 30 minutes at 5000 rpm and the supernate discarded. The remaining sediment was wet sieved to separate gravel, sand, and silt plus clay fractions. U.S. standard sieves of 20 and 230 mesh (equivalent to -1 and 4 ø grain sizes) were used in the sieving process. The clay and silt fraction of each sample was collected in a 1000 ml graduated cylinder. Sands and gravels were poured into evaporating dishes and dried in an oven at 60°C.

Dried gravels (-2 to -1 ø) were weighed and then sieved at 0.5ø intervals to determine their grain size distributions. Sands (-1 to 4 ø) were weighed and then split to arrive at 0.1 to 0.5 g representative subsamples. These subsamples were introduced into specially-constructed settling tubes owned by the U.S. Geological Survey Pacific Marine Geology Branch. Each settling tube consists of a 2 m long section of PVC pipe closed at one end and mounted vertically into a metal scaffold. The pipe is filled with water to a level resulting in a grain fall height of 205 cm. Near the base of each tube, a small plastic disk is suspended from a metal rod connected to a strain gauge. Sand grains are spread onto a wetted plate which is lowered into the PVC pipe, initiating settling. As settling proceeds, grains are deposited onto the plastic disk displacing the metal rod downward. The resulting strain gauge voltage signal is recorded on a chart recorder which plots voltage versus time. A suite of calibration standards allows the conversion of these voltages to grain sizes. Further descriptions of the settling tube can be found in Galehouse (1971), Cook (1969), and in Felix (1969). Sand grain sizes were determined at 0.5 ø intervals by this method.

To prevent flocculation, 5 ml of 5% sodium hexa-metaphosphate ("calgon") dispersant solution were added to each clay and silt solution (<4 ø) and the graduated cylinders filled to 1000 ml with deionized water. Each solution was agitated for two minutes, then allowed to settle for 20 seconds, after which 20 ml were removed with a pipette submerged to 20 cm depth (as described by Folk, 1968). To determine the weight of silt and clay in each sample, the 20 ml aliquot was placed in a previously weighed aluminum dish and allowed to dry for two days in an oven heated to 40°C. After drying, the resulting mass value was multiplied by 50 to derive the total mass of silt and clay held in the graduated cylinder. Each solution was re-agitated for two minutes and another 20 ml sample drawn off after 20 seconds at 20 cm depth. The resulting aliquots were introduced into a Cimax Inc. Model TSS8005-H Hydrophotometer at the U.S. Geological Survey Pacific Marine Geology Branch.

The hydrophotometer uses both Stokes' Law of settling and the Beer-Lambert Law to calculate grain sizes of sediments in suspension. The instrument consists of a series of 12-cm-long tubes bored into a transparent slab of plexiglass. The tubes have windows near their bases on each side of the slab. Suspended sediment is introduced into each cell until all are filled to a known height (we used a grain-fall height of 8 cm). A light beam is shined through the windows of each tube into a detector on the other side of the plexiglass slab, and the light transmittance through the suspended sediment is calculated by a microprocessor. Light transmission is measured at specified time intervals determined by Stokes' Law which states that:

where d is the particle diameter, g is the acceleration due to gravity, Delta-p is the difference in density between a sedimentary grain and the liquid in which it is suspended, n is the viscosity of the liquid, and V is the velocity at which the grain is falling (Jordan, Fryer and Hemmen, 1971). Given that the fall height of the grains is known, the settling time can be calculated by dividing the fall height by the velocity at which the grain is settling. In this way, the hydrophotometer can measure light transmittance at those times when each successive grain size class has fully settled out of suspension. Conversion of light transmittance values to weight percents of individual grain size classes results from the Beer-Lambert Law:

where I with subscript zero is the intensity of the beam emanating from the light source, I with subscript n is the intensity of the beam transmitted through the sample cell and hitting the detector, alpha is a constant related to grain shape, c is the concentration of particles of size dx in grams per cubic centimeter, L is the length that the light beam traverses through the sample cell, Kx is an empirically derived constant dependent on grain size, and Nx is the number of particles, per gram of sample, of size dx (Jordan, Fryer and Hemmen, 1971; and Simmons, 1959). Using this technique, grain sizes were determined at 0.5 ø intervals. For a further explanation of hydrophotometers and the theory behind them, see Jordan, Fryer and Hemmen (1971), and Simmons (1959). Because of the rather lengthy nature of grain size analyses, only a few replicate measurements have been produced as of yet. These analyses indicate an average precision of about ±0.25 ø. Torresan (1987) has determined a precision of about ± 0.5 ø for the hydrophotometer used in these analyses.

Grain Size Analysis--SDSZ program

The U.S. Geological Survey computer program SDSZ, written by Graig McHendrie, welds the sieve, hydrophotometer and settling tube data sets into one cumulative grain size curve on which the program calculates statistical parameters such as mean and median grain size, sorting, skewness, and kurtosis. The information necessary to construct the cumulative curve consists of the weight fraction of each grain size class, i.e. the weights of gravel, sand, and silt plus clay in each sample, and the size distribution of each grain size class (as determined by sieving, settling tube or hydrophotometer). The program employs the graphical statistic equations developed by Folk and Ward (1957), Inman (1962), and Trask (1930) and was used to analyze the Owens Lake core point-sample grain size data. The statistical equations used by the program follow:

Folk and Ward (1957) statistics:

where ø with subscript 16 refers to the ø grain size at the sixteenth percentile on the cumulative grain size curve, other subscripted numbers referring to corresponding percentiles.

Inman (1962) statistics

Trask (1930) statistics

In plotting our data (see results section), we have chosen to use the Folk and Ward (1957) statistics because they take the "tails" of the grain size distribution into account (however, we have converted the ø units they use into mm). According to these workers, sorting values from 1.00 to 2.00 ø units are classified as poorly sorted. Skewness values between -0.10 and +0.10 are defined as symmetrical while skewness values between +0.10 and +0.30 are positively skewed. A positive skewness implies that samples are weighted toward fine grains while negative skewness implies a weighting toward coarse grains. Kurtosis is a ratio of the degree of sorting of the central part of a grain size distribution to the sorting of the extreme ends of the distribution and can also be thought of as the "peakedness" of a grain size distribution. Values of 1.11-1.50 are classified as "leptokurtic" meaning the central part of the distribution is better sorted than the ends. Values between 1.50 and 3.00 are considered "very leptokurtic" implying that the central part of the distribution is extremely well sorted compared to the ends.

Sand, Silt and Clay Contents--Channel Samples

Sand plus gravel, silt, and clay contents were determined on ninety-one 3.5 m long channel samples. Ten-gram splits were subjected to the same chemical treatments used on the point samples. Sand and gravel were separated from silt and clay by wet sieving with a sieve of U.S. standard mesh size 20 (-1 ø). The sands and gravels were collected together in evaporating dishes and weighed after drying. Silts and clays were collected in 1000 ml graduated cylinders. A scaled-down pipette analysis was used to determine concentrations of coarse silt, fine silt and clay in each sample (Galehouse, 1971): twenty milliliter aliquots were removed at various time intervals based on the grain size of interest and the temperature of the solution. These aliquots were dried and weighed and their weights multiplied by 50 to estimate the weight of sample in each size fraction.

Results

 Grain-size analysis of point samples defines two distinct depositional regimes (figure 1). With the exception of a coarse grained oolite layer at the top of the core, mean grain size fluctuates between 5 and 15 mm (clay- to silt-sized material) between 7 and 195 m in depth. In contrast, between 195 m and the base of the core at 323 m, mean grain size fluctuates between 10 mm (medium fine silt) and 50-100 mm (coarse silt to fine sand). Sand plus gravel, silt, and clay contents (measured as weight percents) of 3.5 m long channel samples broadly mimic the point sample trends, with fine silts and clays predominating from the top of the core to 190 m depth and then changing to coarser grained material between 190 and 323 m depth (figure 2). Clay content (figure 3) of the channel samples varies widely from less than 10 weight percent to nearly 80 weight percent. A closer look at the top 200 m of the core reveals periodicity in mean grain size with depth in the point sample record (figure 4). Comparison of the mean grain size versus depth curve to the channel sample carbonate content versus depth curve reveals great similarities in trends (figure 5). For example, lows in carbonate content are matched with very fine mean grain sizes, while highs in carbonate content correspond to coarser grain sizes.

Figure 1: Mean grain size of point samples (in micrometers) versus depth (in meters). Note the change in depositional style at 195m from interbedded silts and clays at the top of the core to interbedded sands and silts at the base. Mean grain size was calculated using Folk and Ward's (1957) statistic and then converted to micrometers.

Figure 2: Sand plus gravel content of channel samples (in weight percent) versus depth (in meters). Note the change in depositional style at 190m depth from samples with little sand and gravel at the top of the core to samples with high quantities of sand and gravel at the base of the core.

Figure 3: Clay content (in weight percent) versus depth (in meters). Note the great variability.

Table 1 lists mean grain size, sorting, skewness, and kurtosis for the top 195 m of the core (excluding the very coarse oolite layer at the top) and the bottom 128 m of the core. The difference in depositional style between these two core sections is evident in the mean grain size and skewness parameters, while the sorting and kurtosis are much the same in the two sections.

Table 1: Grain size parameters for the top 195 m of the core and the bottom 128 m (Folk and Ward, 1957, statistical parameters). 

Statistical Parameter  top 195m       bottom 128m
mean grain size (ø)    7.40 ± 0.79    5.73 ± 1.56
mean sorting (ø)       1.80 ± 0.43    1.90 ± 0.52
mean skewness          0.16 ± 0.14    0.04 ± 0.25
mean kurtosis          1.53 ± 0.39    1.41 ± 0.60
In the Owens Lake core, all sediments fall into the poorly sorted category whether they are the fine grained sediments in the top of the core or the coarser grained sediments of the lower core. Those sediments in the top 195 m of the core are positively skewed implying a weighting toward fine grains. Sediments in the bottom 128 m show a basically normal grain size distribution. Both sections of the core are leptokurtic.

Discussion

 A particularly striking feature in the Owens Lake core is the change from sediments composed largely of interbedded sands and silts in the lower third of the core to sediments consisting primarily of silt and clay in the upper two-thirds. It is possible that the lower, sandy section of the core represents a shallow lake present in a drier climate. On the other hand, it is also possible that the change in depositional style at 195 m occurred as the result of tectonic factors, namely valley deepening or uplift of the spillover sill with respect to the surface of the lake, rather than from climate change. At this point, we do not have enough information to determine the nature of the depositional change. Grain size statistics collected to date show little difference between the sediments from 0 to 195 m and those from 195 to 323 m.

Like the channel- and point-sample carbonate records, the mean grain size record in the top 200 m of the core exhibits oscillation in deposition. Mean grain size may reflect variations in lake level, with coarse-grained materials deposited during times of lake lowstands, when the shore of Owens Lake more closely approached our core site, and fine-grained sediments deposited during highstands, when the shoreline was at a greater distance from the core site.

References

Appendix: Tables of observations and statistics

 The tables are given as tab-delimited ASCII. Since the tab character (ASCII 9) is used to delimit columns on each line, the values may not line up as expected when displayed outside of a spreadsheet program. It is expected that these tables will be imported into spreadsheet programs.
  1. Raw point sample grain size data
    Raw point sample grain size data in the format accepted by the program SDSZ. The first four columns, -2.0 phi through -0.5 phi, are gravel weights in grams. The next twelve columns, -1.0 phi through 4.5 phi, are cumulative percentages of sands measured with a settling tube. The remaining 11 columns, 4.5 phi through 14 phi, are light transmission values determined by hydrophotometer on silt and clay samples. Some overlaps in grain size between different methods exists resulting in the two values each for -1.0 phi, -0.5 phi, and 4.5 phi.
  2. Mass of gravel, sand, and silt plus clay used in each point sample grain size analysis
  3. Statistical measures of grain size
    Statistical measures of grain size produced by the program SDSZ. FW refers to statistics developed by Folk and Ward, 1957; I refers to statistics developed by Inman, 1962; T refers to statistics developed by Trask, 1930.
  4. Channel sample grain size data
HTML encoding by Peter Schweitzer