LOWER PAHRANAGAT LAKE: MODERN ANALOGUE FOR LACUSTRINE CARBONATE MICROBIAL FACIES OF THE HORSE SPRING FORMATION?

A common lacustrine carbonate facies found in the Miocene Horse Spring Formation (HSF) of southern Nevada are peloidal grainstones with thin (<1-2 mm) crinkly, filamentous laminae. These laminae may be separated by 1-15 mm of generally peloidal grainstone with admixed micrite. Isopachous calcite cements may form domal structures in discontinuous micro-layers and fenestral porosity is somewhat common. The peloids in these units have been attributed to abiogenic calcite precipitation in the water column, whereas the filamentous, crinkly laminae suggest microbially-mitigated calcite precipitation in the form of a microbial mat. There are few good modern analogues for this facies because most lakes that have been cored lack sufficient carbonate deposition to produce them. In an effort to establish a better actualistic model for HSF lacustrine carbonate deposition, we took a 4 m sediment core and multiple lake margin samples from Lower Pahranagat Lake (LPL), an alkaline lake in east-central Nevada that appears to be dominated by microbially-influenced carbonate precipitation. Surrounding wetlands are dominated by reeds and grasses. Isotopic values of LPL are similar to HSF carbonates of the Rainbow Gardens Member that has been interpreted as having been deposited in a palustrine to lacustrine setting. Fossilized reed beds and paleosols with root traces in palustrine limestones of this member also suggest affinities with LPL. The LPL core intersects a thick (> 1 m) carbonate package that, at first glance, is mainly carbonate mud. On closer inspection, this package is dominated by ~1mm clots of carbonate mud that we interpret as proto-peloids. Dispersed throughout these clots are sub-horizontal, darkened, diffuse layers that show discontinuous ~0.1mm filamentous laminae. We hypothesize that a compacted version of this facies would produce the crinkly-laminated peloidal grainstones of the HSF and, likely, similar facies in lacustrine carbonates globally. O- and C-isotopes from LPL contribute to the global dataset on modern lake sediment chemistry and fill a gap in our knowledge about sedimentation in carbonate-dominated alkaline lakes.