LACUSTRINE MICROBIALITES IN THEIR STRATIGRAPHIC ARCHITECTURAL CONTEXT, MIOCENE HORSE SPRING FORMATION, LAKE MEAD REGION, NEVADA

A characteristic of many ancient outcrop exposures of microbialites is their limited lateral and vertical exposure, both of which degrade considerably over geological time. Many Precambrian microbialites have strong diagenetic overprints that obscure microtextures and geochemistry. In addition, some of these ancient microbialites have been compared to modern marine varieties, but their textures and mesoscale structures have more in common with modern and young lacustrine microbialites. Hence, to improve our ability to interpret these ancient (and possibly off-planet) microbialites, we believe that it is critical to examine them in younger sedimentary basins with the promise of less alteration and better exposure. The Miocene Horse Spring Formation (HSF), exposed north of Lake Mead in south central Nevada, comprises continental siliciclastic and carbonate units formed in rift settings. Incision by the Colorado River leaves sedimentary basin fills well exposed and abundant tuffs provide excellent chronostratigraphic control. Our research group has constrained the litho- and chronostratigraphic framework of these units in some detail. Lacustrine carbonate sediments comprise a significant portion of the HSF, ranging in thickness from less than a meter to well over 200 m. The Lovell Wash Member of the HSF contains lacustrine microbialites with a very diverse array of morphologies and textures. Stromatolitic and thrombolitic microbial build-ups range in synoptic relief from 1 cm to over 1 m and spatial scales of a few tens of centimeters to over 5 m. In this talk we present the lithostratigraphic context of the Lovell Wash microbialites, their lateral variation in morphology and texture, and their association with paleosprings and, possibly, synsedimentary faults. A companion presentation emphasizes geochemical variation within these same units (Frisk and Hickson, 2020).