SEDIMENTARY EVIDENCE FOR AN ICE-FREE OCEAN DURING SNOWBALL EARTH GLACIATION AND NEOPROTEROZOIC-EARLY PALEOZOIC OROGENIC ACTIVITY ALONG WESTERN LAURENTIA

The Neoproterozoic fragmentation of Rodinia along western Laurentia was synchronous with two episodes of Snowball Earth glaciation from 717-662 Ma (early Cryogenian “Sturtian”) and 635 Ma (late Cryogenian “Marinoan”), and followed by a ~400 million year period of passive margin sedimentation. Evidence includes widespread miogeoclinal strata, Fe-rich Neoproterozoic glaciogenic deposits, and overlying cap-carbonates located throughout Cordilleran mountain belts. However, two problems regarding Snowball Earth glaciation and the evolution of the Laurentian passive margin include (1) Cryogenian deposits suggesting sediment transport to ice-free oceans, and (2) the unlikelihood of ~400 million years of uninterrupted sedimentation given the regular interval of tectonic activity following the Devonian-Mississippian Antler orogeny. Based on U-Pb geochronology and detailed stratigraphic studies, we provide a challenge to the Snowball Earth hypothesis, given the correlation of the Neoproterozoic Kingston Peak Formation and overlying Noonday Dolomite of Death Valley with the nonglacial Wyman Formation and overlying Reed Dolomite of the White-Inyo Mountains to the northwest. The Kingston Peak Fm. and Noonday Dolomite are characteristic Snowball Earth deposits consisting of shallow-marine passive-margin sediments, volcanic rocks, and glacial diamictite capped by dolomitic carbonate. The absence of glaciogenic deposits in the Wyman Fm. along the same horizon, outboard of the Death Valley region, supports the existence of partially ice-free oceans, contrary to the idea of global ice-cover in the Neoproterozoic. In addition, our field analysis of paleocurrent indicators within Precambrian-Cambrian strata of the White-Inyo Mountains reveals east to southeast-directed flow during >50 million years of sedimentation, suggesting a topographic high to the west. East to southeast-directed paleocurrents coupled with structural analysis of asymmetric, east-verging folds observable within the basal Wyman Fm. but not overlying Reed Dolomite suggest western Laurentia experienced a previously unrecognized Neoproterozoic-Early Paleozoic orogenic event following rifting and birth of the proto-Pacific Ocean.