Abstract:
Ordovician to Middle Devonian rocks at the Nopah Range provide a record of depositional environments on the eastern margin of the Cordilleran miogeocline. This interval includes: Lower to Middle Ordovician Pogonip Group (517 m); Middle Ordovician Eureka Quartzite (63 m); Upper Ordovician to Lower Silurian Ely Springs Dolomite (266 m); upper Lower to Upper Silurian (Lower Devonian?) Hidden Valley Dolomite (91 m); and the basal 76 m of the Devonian Lost Burro Formation. Ages are based on conodonts and macrofossils.The lower 133 m of the Pogonip contains abundant cryptalgal structures that formed on restricted carbonate intertidal flats. The upper 384 m formed in shelf and shoal environments. The Eureka records shallowing from subtidal to intertidal environments. Subsequent transgression resulted in deposition of carbonate mud and bioclastic storm layers of the Ely Springs, which formed in restricted lagoons. Evidence of higher energy normal marine conditions in the upper Ely Springs and lower Hidden Valley reflect the development of an open carbonate platform. The upper 40 m of the Hidden Valley represents restricted platform conditions. The lower Lost Burro records an initial transgressive event and subsequent shallowing. Channelized intertidal dolomitic sands are overlain by quiet subtidal carbonates. Shallowing is indicated by high energy barrier bar sands overlain by quiet peritidal carbonates. Examination of strata provides a good record of depositional environments and sea-level fluctuations at a location transitional between craton margin and miogeoclinal facies of the Ordovician to Lower Devonian in the southern Great Basin.
