SEDIMENTOLOGY AND DETAILED LITHOFACIES ANALYSIS OF THE BITTER RIDGE LIMESTONE FOSSIL LAKE SYSTEM: IMPLICATIONS FOR MID-MIOCENE CLIMATE AND TECTONICS IN THE LAKE MEAD REGION

The middle Miocene-age Horse Spring Formation (HSF) is exposed east of Las Vegas, Nevada along an east-west transect north of Lake Mead and preserves world-class exposures of sedimentary basins that formed in the upper plate of a large detachment fault system. The Bitter Ridge Limestone (BRL) member of the HSF is a fossil lake sequence dominated by carbonate, evaporite, and minor siliciclastic deposits. One exposure of this member is nearly 500 m thick and contains stromatolitic microbialite limestones and other lithofacies that are nearly 100% exposed. This locality provides an unprecedented opportunity to investigate the evolution of a Miocene lake system that experienced both on-going extension and global climate change, including the waning limb of the mid-Miocene climatic optimum, and to explore means by which to extricate tectonic from climatic signals in extensional sedimentary basins. We constrain the age for this part of the BRL between 12 and 15 Ma and have conducted detailed lithofacies and petrographic analyses of this section to begin to draw conclusions about 1) the evolution of the BRL through time and 2) possible tectonic vs climatic signals within this unit. We distinguish 17 principle lithofacies of the BRL, most of which suggest deposition in a fairly shallow, saline to hypersaline, evaporative lake. We interpret these lithofacies in terms of changes in water depth, energy, and salinity, with post-depositional features (tepee structures and fenestral porosity) indicating periods of exposure and subsequent re-wetting. Lithofacies analysis suggests 7-8 episodes of wetting and drying. Some of these wet/dry events correlate to ~20-40 m thick lithostratigraphic packages in the BRL and can be distinguished over much of its outcrop belt; others are more cryptic. However, the scale and estimated time duration of these wet/dry sequences matches predicted solar insolation curves based on orbital forcing models and we tentatively suggest that they represent the ~100 ka eccentricity signal. Relationships between lithofacies and detailed stable isotope curves is considerably more complex and suggests that interpretations of oxygen-isotope records as a proxy for precipitation/evaporation ratios is very challenging in strongly evaporative saline lake systems like the BRL.