Abstract
The Oligo-Miocene Horse Spring Formation-exposed north of Lake Mead, Nevada-formed in restricted basins that pre-dated and accompanied complex transtensional tectonic activity. Such basins are normally buried and poorly exposed, but incision by the Colorado River provides world-class exposure and numerous volcanic tuffs allow for detailed sedimentological and stratigraphic analyses. A mixture of siliciclastic, evaporite, and carbonate facies, this formation contains four significant ( approximately 50-200 m-thick) lacustrine carbonate sequences that are dominated by microbial fabrics and textures. The oldest sequence, part of the Rainbow Gardens Member, is a laterally extensive, pre-extensional deposit comprised by oncolitic and massive carbonates with root traces, suggesting a palustrine depositional setting. Within the Thumb member, a 150 m thick carbonate unit thins rapidly to less than 10 m thick away from an exposed basin margin over less than 2 km, suggesting lake margin groundwater sources. The Bitter Ridge Limestone member is a laterally extensive (>25 km), thick ( approximately 200 m) unit with a tabular architecture. This sequence is characterized by stratiform microbialites, minor domal stromatolites, and extensive teepee structures that illuminate the paleoclimatic evolution of this large lake system. Finally, within the Lovell Wash Member, alluvial fan conglomerates can be traced laterally into a mixed carbonate/siliciclastic, lake margin sequence and, finally, into basinal facies dominated again by stromatolitic sedimentation. Stable isotope analyses of these marginal to basinal facies suggest a systematic delta (super 13) C enrichment toward the basin. Furthermore, we have been successful at using stable isotopic and geochemical signatures to correlate sequences throughout the region and across major transform faults, thereby allowing these lake sequences to serve as important constraints on the timing and nature of deformation in the region. These units can serve as excellent outcrop analogues for other rift-basin lacustrine carbonate.