Abstract
Thick Oligocene-Miocene lacustrine limestones of the Lake Mead region in southern Nevada have few adequate modern analogues that can shed light on their origin. We contend that the Lower Pahranagat Lake (LPAH), a spring-fed, alkaline lake in east-central Nevada, provides such an analogue. Through sediment cores, sampling of the lake margin, and characterization of sediment physical and chemical properties, we show that the LPAH shares a number of lithofacies in common with the Late Oligocene to Early Miocene Rainbow Gardens (RGF) and Miocene Horse Spring Formations (HSF) in the Lake Mead area. These include: stratiform stromatolites, domal stromatolites, reed beds, and intermittently laminated and massive limestones. Oxygen and carbon isotopes from authigenic carbonates support our interpretation in that the LPAH values strongly overlap those of the Oligocene-Miocene units. As a result, we interpret the Oligocene-Miocene RGF and HSF lacustrine carbonates as having been deposited in restricted, alkaline lake basins with high salinity. These basins were not conducive to the ecological success of animals and other, large multicellular organisms and, instead, fostered the growth of significant microbial communities. Lake margins, particularly during RGF deposition, were inhabited by reedy plants and salt tolerant shore grass much as is encountered in the LPAH today. These lakes were likely sourced by springs, but were nonetheless highly evaporative. Some unique aspects of both the LPAH and the Lake Mead region Oligocene-Miocene strata may shed light on the rarity of thick lacustrine carbonates in the rock record. Both of these systems are underlain by thick, Paleozoic carbonate sequences that host the principal aquifers that feed the lakes.