MIOCENE EXTENSIONAL TECTONICS AS RECORDED BY THE HORSE SPRING FORMATION, LAKE MEAD AREA, NEVADA: SEDIMENTOLOGY AND ISOTOPE GEOCHEMISTRY OF THE BITTER RIDGE LIMESTONE MEMBER

The Miocene Horse Spring Formation (HSF) appears to record the onset and development of extensional and/or transtensional tectonic activity in the Lake Mead Region. Through detailed sedimentological and geochemical studies of the HSF, we hope to develop a more clear model of basin tectonics in this important sub-region of the Basin and Range extensional province. The Bitter Ridge Limestone (BRL) member of the HSF records the formation of a relatively large, long-lived, closed lake basin. We investigated the lower, thick, ‘granular' limestone unit in two localities in an effort to understand the sedimentology of this lake system, lateral facies relationships, and implications for transtensional tectonic processes. The BRL is predominantly characterized by laminated limestones that suggest microbially mitigated deposition in a relatively shallow marginal lacustrine environment. Teepee structures are relatively common, suggesting periodic subaerial exposure. We conducted mm-scale transects of O- and C-isotopes on microbial laminae at the scale of a single hand specimen. Results suggest a varve-like signature, perhaps recording annual variation in evaporation and precipitation; these fine-resolution transects also lend credence to the interpretation that these rocks have seen very little diagenetic over-printing. In addition, we conducted a detailed O- and C-isotope transect (sampled at 2 m intervals over ~200 m of section) in microbial limestones at one locality. These data show statistically significant variation in isotopic ratios that suggest changes in lake chemistry and biological productivity. Furthermore, there appears to be a general increase in both O- and C-isotopic values. This more systematic trend may be due, in part, to increasing evaporation and aridity and a concomitant decline in primary productivity in the Bitter Ridge lake basin. This interpretation is partially supported by lithofacies that increasingly show evidence of subaerial exposure upward (intense fenestral porosity development, larger-scale teepee structures, more disturbed microbial laminae). ⁴⁰Ar/³⁹Ar geochronology and geochemical fingerprinting of ashes from within the BRL should allow us to correlate sections across a large portion of the BRL basin and shed light on basin tectonics.