INSIGHTS INTO LACUSTRINE THROMBOLITE TEXTURES AND MINERALOGY FROM THE MIOCENE HORSE SPRING FORMATION

Thrombolites, somewhat common in ancient marine systems, also comprise an important lithofacies in some lacustrine carbonate sequences. They can provide important evidence for paleoenvironmental reconstructions of ancient lake systems, as well information useful to petroleum exploration and development in ancient lacustrine basins. However, few studies focus on textural variation within thrombolitic facies and even fewer show lateral variation, hence, we understand relatively little about them. Lake Clifton, a marginal marine lake in southwestern Australia, contains modern lacustrine thrombolites that provide insight into how thrombolites mineralize (Burne et al., 2014), as well as evidence for the controls on lateral variation in thrombolite morphology and texture. Detailed characterization of thrombolitic textures from the same stratigraphic interval from an ancient sequence could provide similar insights into how textural changes occur within these ancient systems. The Lovell Wash member of the Horse Spring Formation in the White Basin near Lake Mead, Nevada contains a record of Miocene microbial communities comprising stromatolites, thrombolites, and dendrolites. Lovell Wash Member thrombolites contain a range of structures and textures. Textural variability occurs in clot size and shape, micrite density, mesoclot packing, presence or absence of detrital grains (ooids in particular), changes in cement fabric (coarse-grained calcite cement vs microspar), micrite content of cement, porosity, and clot fabric/orientation. We report here on this varability and relate it to changes in lake level and proximity to shoreline. We also report preliminary elemental distribution maps associated with thrombolitic textures based on micro-XRF scanning technnology. Our research shows that thrombolitic macrostructures are associated with a range of specific textures. Additionally, preliminary micro XRF results indicate that elemental distributions can be associated with specific textures within Horse Spring Formation thrombolites.