A SUPERBLY EXPOSED LATE MIOCENE LAKE MARGIN IN THE UPPER HORSE SPRING FORMATION OF SOUTHERN NEVADA: PALEOENVIRONMENTAL CONTROLS ON MICROBIAL CARBONATE DEPOSITION IN A HYPERSALINE LAKE SYSTEM

The Miocene Upper Horse Spring Formation near Lake Mead, Nevada, preserves an ancient lake margin in its entirety. Over 4 km of laterally continuous exposure, basinal stratiform stromatolites grade laterally into domal and more complex morphologies that interfinger with alluvial fan conglomerates. Stable isotope geochemistry, lithofacies, and petrological properties of these limestones change dramatically from basin to margin positions and shed considerable light on ancient lacustrine basin processes. A mix of stratiform and small, domal stromatolites that nucleate on conglomerate clasts mark the proximal, marginal facies. Offshore, progressing into a wave-dominated zone, large domal stromatolites are intimately associated with wave rippled beach sandstones. In the basin stratiform stromatolites dominate the lithofacies. The overall trend is one of decreasing morphological diversity from the margin to the basin. Petrographic and SEM analyses support this fundamental conclusion. Marginal stromatolite micromorphologies show well-defined, sub mm scale laminations that exhibit cyclic variations that derive from the inner workings of a complex mat ecology. Basinal samples reflect the lack of variation seen in hand sample and outcrop: discernable, wispy laminations lacking a defined internal structure. SEM reveals increased biodiversity in samples from the margin, possibly in the form of large (100+μm) predatory grazers. δ¹³C values at the margin are significantly enriched compared to the basin. A set of intimately linked environmental variables are responsible for these lateral changes in geochemical signatures, lithofacies, micromorphological structures, and biodiversity. Increased fresh water mixing, light intensity, and diurnal variations in that light at the margin foster a more chemically and biologically stratified microbial mat. This is reflected in the well defined, mm-scale laminations of lake margin stromatolites and explains the lack of these laminae in basinal samples. Increased chemical stratification due to photosynthetic productivity and CO₂ fixation at the margin also explains the enrichment of δ¹³C. This, coupled with increased siliciclastic input and larger grain size, results in increased morphological and lithofacies diversity at the margin.