Carbonates and microbialites record a dynamic lake basin evolution in the Late Cretaceous to Eocene Sheep Pass Formation, Nevada, USAJournal Article
Overview
abstract
ABSTRACTAncient terrestrial sediments provide critical information about the responses of continental environments to global scale climate and tectonic perturbations, which are vastly understudied relative to marine archives. The >1 km thick Sheep Pass Formation type section in east‐central Nevada preserves non‐marine carbonates, including microbialites, and lesser siliciclastics, deposited in a tectonically active, high‐elevation basin during the latest Cretaceous through to middle Eocene time, an interval spanning major global greenhouse climate states and warming events. This study combines outcrop and hand‐sample observations, thin section petrography and X‐ray diffraction mineralogical analyses to create a facies framework and interpreted environmental evolution for the Sheep Pass Basin. Together, these observations portray the Sheep Pass Formation type section as a dynamic and highly sensitive basin due to its small size. The dominance of thrombolite boundstones compared to metazoan fossils, which sets the Sheep Pass Formation type section apart from other Palaeogene‐aged lake basins in the western United States, reflects the resilience of microbial mats compared to metazoans in this dynamic setting. The major lacustrine phase of the Sheep Pass Basin records three intervals: a shallow lake with few microbialites, followed by abundant microbialites, before the transition to a marginal setting with evaporative conditions, marking the culmination of this major lacustrine phase. The transition to a microbialite‐dominated interval was likely driven by physicochemical conditions (for example, higher alkalinity), paired with lower competition from metazoan grazers. Although the Sheep Pass Formation type section preserves environmental change in response to both tectonics and climate, similar trends in facies, mineralogy and invertebrate abundance compared to other sedimentary basins from this time suggest that global climate influenced distinct environmental shifts in the Sheep Pass Basin. This work provides a detailed sedimentological framework for a new, high‐elevation palaeoclimate record during a pivotal geological climate interval.
