INFERRING MIOCENE PALEOCLIMATE/PALEOGEOGRAPHY FROM MODERN AND HISTORICAL MICROBIALITE DEPOSITS OF THE LOWER PAHRANAGAT LAKE, ALAMO, NEVADA

The Lower Pahranagat Lake near Alamo, NV is a spring fed shallow lake, no deeper than approximately 1 m. It is an alkaline lake with very high evaporation rates and low precipitation. The average yearly temperature of the Pahranagat area is 15.5°C. Conductivity in January 2014 ranged from 2.52-3.9 mS and salinity from 0.887-1.84 ppt. The lake shoreline is covered in an evaporitic crust, most likely a mix of calcite and halite, that is in turn surrounded by marshy wetlands. Hickson et al. (2013) suggested that the lake is a possible analogue for Miocene lacustrine carbonates of the Horse Spring Formation in the Lake Mead region; our data support this. Microbialite samples were identified from the Paleo-shoreline of lower Pahranagat Lake in addition to modern microbialite samples found on the northern edge of an ephemeral island. Samples were polished and slabbed in order to characterize internal textures and overall microbialite morphology. Morphologies of the Pahranagat lake region include: 1) Domal stromatolites, ranging from 0.5 - 15 cm in diameter; 2) Stratiform stromatolites; 3) Cobble encrusting, layered stromatolites, with varying degrees of post depositional modification by rolling/toppling; and 4) Tufaceous forms with high porosity and surface clots. Three textures and fabrics can be used to describe the internal character of each of these morphologies: A) mm scaled lamination with alternating light and dark layers; B) 0.1-1.0 cm scale domal lamination separated by vertically oriented pores; C) 0.5-2.0 cm layering with internal vertically oriented pores. Pores range in geometry from 0.5-1.0 mm wide and closely spaced, to 1-5 mm wide and more widely spaced. Preliminary XRD analysis shows that the modern stromatolitic crusts are dominated by aragonite, with minor Mg calcite. Older, more lithified stromatolites show Mg calcite with minor quartz as their dominant mineralogy. The Pahranaganat Lake marginal microbialites provide excellent examples of comparable Horse Spring Fm. lacustrine microbialite morphologies and textures, further supporting the hypothesis that this system serves as a strong modern analogue for the Miocene units of the Lake Mead region.