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Core OL-92 from Owens Lake, southeast California

Field log of Core OL-92

George I. Smith
U.S. Geological Survey, Menlo Park, California
The 323m core designated OL-92 was recovered between April 22 and June 9, 1992, from the south-central part of (now-dry) Owens Lake, Inyo County, California (fig. 1A). The drill site is approximately 140 m west and 420 m north of the southeast corner of Sec. 9, T. 18 S., R. 37 E. [M.D.B.M.] (fig. 1B). Permission to drill the core hole was obtained by the U.S. Geological Survey from the California State Lands Commission which has jurisdiction of State lands that include the surface of Owens Lake. The drill site and a large amount of land around it is leased by the State of California to the Lake Minerals Corp.; they permitted our group to use their roads and were helpful in numerous other ways.

Five other cores have been obtained from Owens Lake. The 278.5-m-long core obtained by the U. S. Geological Survey in 1953 (Smith and Pratt, 1957, p. 5-25) was from a site about 2 km NE of the OL-92 site (fig. 1B). Although that core represents approximately the same depth of lake fill as was anticipated from the 1992 coring effort, a new core was desired because only 66 percent of the section at the 1953 core site was recovered, and many techniques have been developed during the intervening years that improve our ability to date sediments, determine sediment and interstitial water chemistry, interpret the environmental significance of fossils, and reconstruct the environments of deposition. Four shorter cores that were recovered more recently from Owens Lake (Newton, 1991; Lund and others, 1991) were studied with the goal of reconstructing climatic events and magnetic excursions during the latest Pleistocene and Holocene.

Core recovery for Core OL-92 was better than 80 percent. Because of drilling technicalities, Core OL-92 is actually three cores from essentially-adjacent sites; Cores OL-92-1 and -2 were recovered from the surface of a man-made drill-pad (which was used as the "core-depth datum" throughout the project, even though actually 0.94 m above the dry-lake bed), and Core OL-92-3 was recovered a few meters east of the drill pad. The depths of their cored intervals overlap slightly. Core OL-92-1 extends from 5.49 m to 61.37 m below the pad surface (85 percent core recovery), and OL-92-2 extends from 61.26 m to 322.86 m below that surface (79 percent recovery). Core OL-92-3 extends from 0.94 m (depth of the lake surface below the drill pad) to 7.16 m (100 percent recovery).

The rotary drill rig used for Cores OL-92-1 and OL-92-2 recovered 3 in. (7.6 cm)-diameter cores with a core barrel equipped with a split-spoon liner. The coring-bit size was 4.88 in. (12.4 cm) outside diameter, and the PVC (polyvinyl chloride) casing (set in OL-92-2 to a depth of about 60 m) was 6-in. (15.2 cm) diameter. Maximum core length was 15 ft (4.6 m) but most runs were two-thirds or less that length. Drilling mud was a bentonite type. Core OL-92-3 was obtained using two techniques. The upper 2.58 m of sediments (below the drill pad elevation) consists of salts and well cemented oolites; they were sampled using a rotary, sawtooth-edged drill used by Lake Minerals Corp. to test the thickness or composition of near-surface layers. The remaining part of the oolite bed, which was nearly uncemented, and the soft, dark, clay-sized sediment beneath it, were sampled using a thick-walled, 3-in. diameter, 20-ft-long PVC pipe that was pushed down, using a large backhoe, through the bottom of the hole created by the rotary drill. While the PVC pipe was still a short distance above ground level, a cap was cemented on the top of the pipe and a chain was attached to it using a clove hitch. The tube was then pushed down to its maximum depth, a short distance into an excavated depression below lake surface, the attached chain enabling the backhoe to pull it back up.

On recovery, the cores were carried a few meters, in one half of the split- spoon liner, to a 25-foot long, partially refrigerated, truck-type trailer where a 16-ft-long work table was set up along one side. On this table, the core was first transferred from the split-spoon liner into 3-in diameter PVC tubes that had been pre-sawn into 1.5 m lengths and longitudinally into halves. Core-top directions were indicated by arrows marked on the outside surfaces of each segment of PVC tubing. Using an improvised "cheese cutter" or a coping saw, the core was then split longitudinally into a "working" half and an "archive" half, and finally cut into 1.5 m long (or less) segments that filled (or partially filled) the pre-cut PVC tubes.

The logs are based primarily on field observations of both core halves; the samples for chemical, isotopic, and sedimentary analyses, fossil identification, paleomagnetic study, and other investigations were mostly taken at the same time from the working half. Those samples were placed in rigid-plastic cube-shaped containers (for paleomagnetic study) or numbered glass bottles with air-tight "Polyseal" caps. After their positions were entered into the log, the samples were stored in the refrigerated section of the trailer. Both halves of the core were then wrapped in "Saran Wrap" to retard evaporation, and the working and archive halves were re-united. Plastic caps were placed over, and taped to, the ends of the re-united PVC tubes, holding the two halves together; additional tape was wrapped around this new unit in two or three intermediate places to make it more rigid. Labels were placed on the caps and also into each tube. The cores were then also put into the refrigerated end of the trailer which was separated from the working area by thick, floor-to-ceiling sheets of insulating material.

The field log was made primarily by visual inspection supplemented by hand- lens and binocular-microscope study. Wentworth's (1922) terminology and size limits for clastic rocks were followed. A petrographic microscope and index oils were available and used to confirm the isotropic character of suspected volcanic glass, but with no electricity, the instrument was found difficult to use without the built-in light from below. The numerical designations of the colors of the unoxidized and still-wet cores, included in the field log, are based on the numerical system used by the "Rock-color chart" that is distributed by the Geological Society of America (1991). Within a few days after logging, colors changed as the cores oxidized, and in the months since then, some have dried noticeably (even though they have been kept in closed tubes and refrigerated), changing both their "lightness" and "chroma" but rarely their "hue". The descriptive terms for the colors in the Rock-color chart are not included in the log because they do not lend themselves to numerical extrapolation of the present core colors back to the logged core colors or vice versa, and some of the color names suggested by the chart are so general that they would be equally appropriate for several nearby color chips in the Rock-color chart. The logs of these three cores are tabulated below (tables 1, 2, and 3). An abbreviated written log, and a graphic log based on it, are presented in figure 2.

Each core segment was logged in the field in terms of its "drive" number (which represented the times the empty core barrel had re-entered the core hole) and "slug" designation (the letter representing each 1.5m-long core segment from that drive, with "A" at the top); starting and ending depths of each drive (in feet and inches) were based on the driller's log; they were later converted to meters to the nearest 0.01 m. Unrecovered core, logged as "No core", was arbitrarily placed at the base of each drive.

References

Field logs of Owens Lake Core OL-92 in tabular form

The logs are presented here as tab-delimited text files. Each log consists of four columns: the top of the described interval in meters, bottom of the described interval in meters, thickness of the interval in meters, and a textual description of the sediment.
  1. Table 2-1: Log of Core OL-92-1
    Between 5.49 m and 61.37 m (total = 55.88 m), "no core" entries in log account for 8.50 m; thus, 85 percent of this interval was recovered as core (although some of it was suspected of being "cuttings, not core").
  2. Table 2-2: Log of Core OL-92-2
    Between 61.26 m and 322.86 m (total cored thickness = 261.60 m), "no core" entries in log account for 54.71 m; thus, 79 percent of this interval was recovered as core (although some of it was suspected of being "cuttings, not core").
  3. Table 2-3: Log of Core OL-92-3
    1. All listed depths are converted to "drill-pad datum depths" which are 0.94 m above lake surface.
    2. From 0.94 m to 3.52 m, core was recovered as cuttings or short segments that were logged but not saved; from 3.52 m to 7.16 m, core recovery was 100 percent, all of which was logged and archived.
    3. X-ray diffraction and thin section study of oolite samples reveal a significant percentage of the saline mineral gaylussite (Na2CO3.CaCO3.5H2O). In thin section, the outer edges of many oolites are coated with relatively large, high-birefringence crystals that we think are probably gaylussite; similar crystals are also found attached to cavities inside the oolites. We interpret this phase to be one that formed early in the 20th century, when the Owens River was diverted into the Los Angeles Aqueduct causing Owens Lake to desiccate, producing a brine that had a sufficiently high density to displace the original interstitial fluids and replace it with brines having the composition and salinity that would crystallize gaylussite.
    4. At room temperature, these salt crystals dehydrated to white powder that XRD tests showed to be mostly thenardite (Na2SO4), but tests with acid show that they also contained some carbonate mineral; the "white powder" was probably a mixture derived from the dehydration of mirabilite (Na2SO4.10H2O) and natron (Na2CO3.10H2O) that crystallized from the interstitial brines after the cores were first brought to the surface and refrigerated.

U.S. Department of Interior, U.S. Geological Survey
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Last modified: 03.01.01 (krw)