Stratigraphy, lithologies, and sedimentary structures of Owens Lake core OL-92

Abstract

Owens Lake, a now-dry lake in southeastern California immediately east of the southern Sierra Nevada, was the site of a coring project designed to obtain a long paleoclimatic record. During the ensuing study, lacustrine deposits were recovered by the 323 m long core designated “OL-92.” The presence of the Bishop ash (ca. 760 ka) and the Matuyama-Brunhes paleomagnetic reversal (ca. 780 ka) near the base of core OL-92 shows that this core represents about 800 k.y. of deposition in Owens Lake. The sediments are dominantly lacustrine clay, silt, and fine sand, although some intervals contain as much as 40 wt % CaCO3. The lowest ~57 m of recovered sediments is mostly silt or clay, but several sand beds are present; the overlying ~60 m of sediment is similar, but its sand content is more dispersed. Together, these two units are composed of ~70 wt % silt and clay and ~30 wt % sand, suggesting deposition in lakes that fluctuated between moderately deep and shallow. Overlying them is ~201 m of sediments that were mostly deposited in deep water; they consist predominantly of silt and clay but include two thin, coarse-sand beds. An oolite bed forms the upper ~4 m of natural deposits, and an anthropogenic salt bed, >2 m thick, forms much of the present surface. In addition to the Bishop ash, several much thinner tephra layers are also present. About 70% of the clastic-sediment units are massive, some clearly because of bioturbation; other units display a thin bedding defined by changes in color or grain size. Rhythmic bedding, observed in numerous segments <1 m thick, seems to represent cyclical events ~100 yr long. Thin color bands caused by the chemical alteration of sediments on each side of hairline fractures create irregular subvertical veins. Clastic dikes, as much as ~2 cm wide and ~75 cm long, characterize some zones. Bioturbation structures, sand pods, ice-rafted(?) granules, small faults, minor discontinuities, and possible turbidity-current structures are also present. Lithologic variations, in combination with other evidence, indicate that from ca. 810–645 ka, Owens was most commonly a moderately deep fresh-water lake; from ca. 645–450 ka, it was more commonly a shallow—but still fresh-water—lake; from ca. 450–5 ka, it was almost continuously a deep, mostly fresh-water lake; and after ca. 5 ka, it was a shallow, moderately saline lake. Other variations in the sediments and their contents, however, indicate additional cycles of average lake-overflow volumes that are not reflected by sediment-size changes.