THE USE OF OXYGEN ISOTOPES AND CARBONATE MINERALOGY FOR INTERPRETATION OF SEDIMENTARY FABRICS IN CORES OF QUATERNARY LAKE DEPOSITS IN THE WESTERN UNITED STATES

Detailed analyses of sedimentary fabrics in cores collected from Great Basin lakes provide information on the variability of the depositional environment that is not obtainable from chemical and biological analyses alone. However, there are very few sedimentary fabrics that provide absolute constraints on water depth. Subaerial and wave-formed structures are the best indicators of water depth in cores of Quaternary deposits. The vast majority of sedimentary fabrics can form over a range of water depths even in the same core. We have resolved relative changes in lake water depths through high-resolution oxygen-isotope measurements and carbonate mineralogy combined with detailed sedimentary analyses in cores from Pyramid Lake (Nevada) and Bonneville (Utah) basins. Oxygen isotopes primarily reflect changes in a lake’s hydrologic balance. Smaller d¹⁸O values occur when the volume of isotopically light inflow exceeds the amount of isotopically light water vapor produced during evaporation, and vice versa. Thus, an increase in lake level or rate of spill of water to an adjacent basin is recorded by a decrease in d¹⁸O values in the carbonate precipitated from the lake. Rapid fluctuations in isotopic levels are associated with relatively small lakes, whereas larger lakes have more stable isotopic values. Carbonate mineralogy provides additional information on the water balance with low-Mg calcite forming in dilute water and aragonite and high-Mg calcite forming in lakes that have undergone more evaporative concentration. Sedimentary fabrics, including lamination, thin bedding, and bioturbation, all occur with different isotopic and mineralogic signatures, suggesting that the history of hydrologic change exerts strong control on the type of sedimentary fabrics formed. The presence or absence of lamination is not a measure of lake water depth, but how the hydrologic changes affected bottom oxygen availability, rate of sediment supply, and salinity. In general, layering formed in relatively small and shallow lakes tends to be more irregular, whereas layering formed in large and deep lakes tends to be thinner and more regular to rhythmic.