GEOCHEMICAL VARIATION WITHIN LACUSTRINE STROMATOLITES AND THROMBOLITES FROM THE SAME STRATIGRAPHIC INTERVAL: IMPLICATIONS FOR THE INTERPRETATION OF MICROBIALITE FORMATION AND PALEOENVIRONMENTAL INTERPRETATION

Relatively little is known about the variation in mesoscale textures and geochemistry of lacustrine microbialites within a specific ancient lake system. Macroscale morphological variability has been characterized for some systems (e.g. Hot Springs Limestone, Green River Fm.), but detailed textural and geochemical variability is poorly understood. Such data could provide important tools for reconstructing paleoenvironments in ancient lake systems and interpretation of geomicrobiological processes in ancient lakes. The Miocene Lovell Wash Member of the Horse Spring Formation, Southern Nevada, contains limestones with evidence of desiccation and devoid of macrofauna, indicating that shallow alkaline lakes once dominated the area. These contain a variety of microbial morphologies including domal and stratiform stromatolites, thrombolites, and dendrolites. Bernardi et al. (2014) identified a hub of microbial activity within the Lovell Wash Member that provides insight into the formation of microbialites in and around an ancient spring system. We collected extensive data on microbialite morphologies and mesoscale textures within the same 1-5 m thick stratal interval as this inferred spring, over approximately 11 km strike distance. We then used micro XRF mapping techniques to examine elemental distributions within stromatolites, thrombolites, and dendrolites from this interval. Preliminary results indicate that specific elemental distributions can be associated with certain textures and that different elemental distributions vary within similar textures between different morphological types. SEM EDS and XRD analyses confirm our elemental micro XRF mapping results. These data also help link elemental distributions to potential mineralogical sources and/or biochemical/metabolic processes within the microbial mats that we infer were responsible for the formation of these lacustrine microbialites. This study provides insight into specific meso- and microscale textures and their associated elemental distributions through a range of microbialite morphologies. Furthermore, our data shed light on the lateral variation in lacustrine microbialite morphologies, textures, and geochemistry over a narrow stratal interval of an ancient lake system.