SAUK III PLATFORM MARGIN EVOLUTION IN EASTERN CALIFORNIA

Reconstruction of the Cambro-Ordovician history of a proposed rimmed-platform margin is based on detailed sequence-stratigraphic analysis in a transect from the mid-platform Nopah Range section westward to the outer platform and platform-margin settings in the Cottonwood Mtns (CM), Talc City Hills (TCH), and southern Inyo Mtns (SIM).

The occurrence of microbial and associated carbonate buildups and paleokarst in the carbonate successions in the distal platform setting contrasts with previous models of a vast, gently westward-sloping to distally steepened ramp. Moreover, a major basinward shift of this platform rim is recorded by westward progradation of the shallow outer platform facies, from latest Cambrian through early Ordovician. Several sequence boundaries established by Cooper and Keller (2001), in more inboard sections from the inner miogeocline towards the craton, can be traced westward into these outboard sections, thus bridging previously independently studied parts of the platform.

Near the end of Sauk II, there was a platform-margin high expressed by a prominent dissolution surface at the top of the Bonanza King Fm in the CM. After deposition of the subtidal Dunderberg Shale and Halfpint Members of the Nopah Fm, there was rapid shallowing and exposure as indicated by pronounced local karst within and at the top of the upper Nopah Fm (Smoky Member) in this area. During deposition of the overlying Early Ordovician Goodwin Fm on the mid platform, the thrombolitic facies typical of the upper Nopah Fm prograded westward to the TCH area where it persisted into the Early Ordovician. The shallowest Goodwin facies were deposited further west in the SIM. The rimmed margin of the platform remained in this region until the exposure surface at the top of the Tamarack Canyon Dolomite (=top of Goodwin Fm and top of Sauk III) was flooded and covered by silty shales of the Al Rose Fm (Ninemile Shale equivalent).