LOWER PAHRANAGAT LAKE AS A MODERN ANALOG FOR ANCIENT LACUSTRINE MICROBIALITE FACIES OF THE MIOCENE HORSE SPRING FORMATION IN SOUTHERN NEVADA

Modern microbial mats and Pleistocene microbialites found on the margins of Lower Pahranagat Lake (LPL) in southern Nevada are similar to lacustrine deposits observed in the Oligo-Miocene Horse Spring Formation (HSF) and in other lacustrine microbialites, suggesting that LPL may be a good modern analog for these ancient units. We examined the sedimentary characteristics and mineralogy of a 4 m-long sediment core to further test this hypothesis. We also used five ¹⁴C age dates to explore the climatic and environmental trends in the Pahranagat Valley region spanning approximately the last 2400 years. We gathered x-ray fluorescence (XRF), x-ray diffraction (XRD), and smear slide data to characterize the composition and relative significance of authigenic, clastic and biogenic components for each ten-centimeter increment of the LPL core.

Our results support the hypothesis that this modern shallow carbonate lake setting is a possible analog for a key lithofacies of the HSF, a crinkly-laminated limestone that appears to exhibit a strong microbial influence on it deposition. Specifically, XRD analyses revealed that samples from the LPL are enriched in high-Mg calcite (typically greater than 60%), which compares favorably with the analagous facies in the HSF. Sedimentary characteristics that we observed support this as well. The topmost facies of pink, diffusely-laminated high-Mg carbonate muds are similar to the crinkly-laminated facies of the HSF and represent especially low lake levels at LPL over the last ~800 years. A second facies is a dark brown carbonate mud with evident biogenic materials ranging from gastropod shells to tubular woody fragments, which suggests that from 1,150 to 1,960 years BP conditions were generally wetter. Lastly, a third significant facies is a massive tan carbonate mud with a noticeable increase in clastic materials and decrease in plant materials. This last facies represents a drier time interval, which culminated in environmental conditions that were favorable for significant dolomite formation (reaching 80-90%) at the oldest part of the record, 2,360 to 2,400 years BP. These last two facies are not present in the HSF, suggesting that the LPL only partially serves as an analog for HSF microbialite deposition.