TONIAN AND CRYOGENIAN BASIN FORMATION IN DEATH VALLEY, CALIFORNIA AND THE PALEOENVIRONMENTAL CONTEXT OF THE PAHRUMP GROUP (Invited Presentation)

In the Death Valley region of California, the Pahrump Group includes kilometers of Tonian and Cryogenian sediments that provide a well-exposed and accessible record of these dynamic periods in Earth history. Early eukaryotic microfossils, several large carbon isotope excursions, and two glacial diamictites have been reported and studied within these strata. These units have been commonly interpreted – along with the Chuar Group and Uinta Mountain Group basins – as part of a broad, connected seaway established in southwestern Laurentia during the Neoproterozoic as the result of subsidence attributed to the early stages of rifting of the supercontinent Rodinia.

We present stratigraphic, sedimentological, and geochronological data from the Horse Thief Springs Formation, Beck Spring Dolomite, and Kingston Peak Formation that highlight several major unconformities within the Pahrump Group, and subdivide these strata into distinct tectonostratigraphic units. We suggest that these unconformities formed as the result of episodic rifting and successive periods of burial and exhumation that are consistent with deposition in isolated and restricted transtensional basins rather than along an extensive marine rift margin. This revised tectonic model for Tonian and Cryogenian strata in Death Valley and in correlative western Laurentian basins cautions against inferring an interconnected seaway in this region, and rather suggests restriction and geographic isolation, which in turn impacts biostratigraphic and chemostratigraphic interpretations within these basins.