FACIES ARCHITECTURE AND MINERALOGY OF MIOCENE MICROBIALLY-DOMINATED CARBONATE LAKES OF THE HORSE SPRING FORMATION, SOUTHERN NEVADA

The Oligo-Miocene Horse Spring Formation (HSF) comprises pre- and synextensional sedimentary sequences with significant microbial carbonate accumulations. Its complex tectono-climatic history combined with a predominantly Paleozoic marine carbonate basement/provenance, has yielded a range of facies architectures within lacustrine carbonate deposits. These carbonate sequences nearly all demonstrate a strong microbial influence on deposition, so the HSF also sheds considerable light on the role that microbes play in deposition of thick, carbonate lacustrine sequences. At important reservoir scales, at least three architectural styles are present in the HSF: (1) 50-200 m thick tabular packages with large areal extent; (2) ~100 m thick basin margin packages that exhibit rapid thinning and lateral facies changes into clastic and evaporite sequences; and (3) variable thickness discontinuous packages of carbonate interbedded both laterally and vertically with evaporites and clastics.

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.