ABSTRACT On this field trip, we will examine a modern lake in central Nevada, the Lower Pahranagat Lake, and lacustrine carbonate outcrops of the late Miocene, upper Horse Spring Formation. Both of the modern and ancient systems hold significant microbialite populations and we interpret that the Lower Pahranagat Lake is a possible analog for the ancient unit. Both systems are or were spring-fed from a similar Paleozoic carbonate aquifer. Both have evidence of microbially influenced sedimentation, probably related to spring activity. Both are dominated by the deposition of carbonate to the exclusion of nearly all siliciclastic material. In the Lower Pahranagat Lake, we will focus on the Holocene depositional record of the lake and the microbialites that are found therein. Molecular genetic data from three sites near the Lower Pahranagat Lake suggest that carbonate deposition could be strongly mediated by varying and complex microbial communities, and that simple interpretations of carbonate geochemistry probably neglect this influence. In the Lake Mead area, we will examine both the vertical (stratigraphic) and lateral relationships between a wide diversity of microbial macro- and mesostructures, to critically evaluate the relative effects of climate change, variable lake chemistry, and the role of microbial mat metabolisms on microbialite geochemistry.

Abstract Little is known about the effect of thrusting on lithological and petrophysical properties of reservoir sandstone. Here we use field observations, probe permeability measurements and thin-section analysis along ten transects from the Muddy Mountain thrust contact downwards into the underlying Jurassic Aztec Sandstone to evaluate the nature and extent of petrophysical and microstructural changes caused by the thrusting. The results reveal a decimetre- to metre-thick low-permeable (≤50 mD) and indurated (0–3% porosity) zone immediately beneath the thrust contact in which dominant microscale processes, in decreasing order of importance, are (1) cataclasis with local fault gouge formation; (2) pressure solution; and (3) very limited cementation. From this narrow zone the petrophysical and microstructural effect of the thrusting decreases gradually downwards into a friable, highly porous (c. 25%) and permeable (≤2 D) sandstone some 50–150 m below the thrust, in which strain is localized into deformation band populations. In general, the petrophysical properties of the sandstone as a result of overthrusting reveal little impact in overall primary reservoir quality below some tens of metres into the footwall, except for the relatively minor baffling effect of deformation bands.