FACIES ARCHITECTURE AND MINERALOGY OF MIOCENE MICROBIALLY-DOMINATED CARBONATE LAKES OF THE HORSE SPRING FORMATION, SOUTHERN NEVADA
We found that the tabular palustrine to lacustrine carbonates of the Rainbow Gardens Member showed a mixed mineralogy, with 44% of samples showing calcite, 25% high-mg calcite and 6% containing dolomite; the remaining 25% of samples were either mixtures of high-Mg calcite or dolomite with calcite. The much thicker Bitter Ridge Limestone tabular deposit showed no high-mg calcite or dolomite in samples that were from either the type locality or well-confirmed correlative sequences. This package is also lithologically more homogeneous than the others. Samples from the basin margin package represented by the Thumb Member carbonate sequence were comprised entirely of calcite as well, and may reflect the proximity of this sequence to a lake margin that is comprised of a sequence of Paleozoic limestones. Perhaps the most heterogeneous lacustrine sequences were the discontinuous packages of the Lovell Wash member. Samples from these packages (N=50) showed 38% with calcite, 16% with high-mg calcite, and 2% with dolomite; however, the remaining 44% of samples showed various mixtures of calcite, high-Mg calcite and dolomite. These results suggest that the HSF lakes were not only very different in terms of areal extent and geometry, but were also chemically quite distinct.