MAGNETOSTRATIGRAPHY AND GEOCHRONOLOGY OF LACUSTRINE CARBONATES FROM THE SHEEP PASS FORMATION, NEVADA

Ancient lacustrine carbonates offer a means of reconstructing important paleoclimate and paleoenvironmental information, however developing a reliable chronology from them directly can be difficult. The Sheep Pass Formation of eastern Nevada preserves microbial lacustrine carbonates that are being developed as a paleoclimate record for high altitude environments during a greenhouse interval. Previous age constraints for the formation are based primarily on mollusc biostratigraphy, palynology, and limited detrital zircon and carbonate U/Pb dating. Together these support a Late Cretaceous – early Eocene age and suggest that the Sheep Pass Formation may record important climatic and environmental shifts associated with the Cretaceous – Paleogene boundary and/or Paleogene hyperthermal warming events. Incomplete fossil preservation and the absence of volcanic lithologies appropriate for high precision isotopic dating, however, have so far precluded developing a highly resolved chronostratigraphy from these methods. This study seeks to establish a new magnetostratigraphic framework for the Sheep Pass Formation. Over 250 independently oriented paleomagnetic samples were collected from 82 sampling sites across five members of the formation. These members include a variety of lacustrine carbonate facies, including microbial thrombolites, carbonate mudstones, wackestones, and packstones, dolomitic carbonate, and finally carbonate cemented siliciclastics. After applying a combination of thermal and alternating field (AF) demagnetization procedures to remove magnetic overprints, both normal and reverse magnetic polarities appear to be preserved in the formation. However, samples also demonstrate variable degrees of remagnetization and rotation that may complicate the stratigraphic interpretations, but may help elucidate the tectonic history of the formation. Magnetostratigraphic results from this study will be combined with additional new detrital zircon and carbonate U/Pb dates to construct a temporal framework for future paleoclimatic and paleoenvironmental interpretations from the formation.