Carbonate Sedimentology and Conodont Biostratigraphy of Late Pennsylvanian–Early Permian Stratigraphic Sequences, Carlin Canyon, Nevada: New Insights into the Tectonic and Oceanographic Significance of an Iconic Succession of the Basin and Range

The Gzhelian (Upper Pennsylvanian) to Kungurian (Lower Permian) succession around Carlin Canyon, northern Nevada, in the Basin and Range province of the western USA is a relatively undeformed wedge of fossiliferous marine carbonate and fine-grained calcareous and cherty clastic rocks that rests with profound angular unconformity on Mississippian to mid-Pennsylvanian sedimentary rocks that had been uplifted, faulted, folded, and eroded prior to the Late Pennsylvanian transgression. This wedge of sediments, which tapers over less than 2 km from 1341 m in the west to 588 m in the east, comprises the Strathearn, Buckskin Mountain, and lower part of the Beacon Flat formations. These units form a second-order sequence within which five third-order unconformity-bounded transgressive–regressive sequences are nested. These sequences are Gzhelian, early to late Asselian, latest Asselian to late Sakmarian, latest Sakmarian to late Artinskian, and latest Artinskian to late Kungurian in age based on the determination and biostratigraphic interpretation of 26 conodont taxa, including two new species (Adetognathus carlinensis n. sp. and Sweetognathus trexleri n. sp.). Each sequence records sedimentation on a westward-dipping ramp along which significant facies change occurs with inner-ramp coarse-grained algal and bioclastic photozoan grainstone to the east passing westward into mid- to outer-ramp heterozoan carbonate, and ultimately into deep-water fine-grained mixed clastic–carbonate facies with no fossils except sponge spicules, representing deep-water sedimentation in a basinal area that underwent repeated episodes of rapid subsidence associated with each sequence. Accommodation during sedimentation of Gzhelian–Kungurian sequences around Carlin Canyon was repeatedly created in response to flexural subsidence caused by tectonic loading west of the study area. Each sequence recorded the simultaneous foundering of the basinal area in the west and uplift of the basin margin in the east. Individual sequences overlap the underlying sequence to the east, while flexural subsidence from the Gzhelian to the earliest Artinskian led to a basin in the west that became deeper over time. A lull in tectonic activity associated with each sequence allowed carbonates to prograde from east to west, partially filling the basinal area until the early Artinskian, and completely filling it to sea level during the late Artinskian and then again in the late Kungurian. The Gzhelian–Kungurian carbonate succession of the Carlin Canyon area bears much resemblance with the coeval succession that occurs all along the northwest margin of Pangea, from Nevada in the south to the Canadian Arctic islands in the north, and down from the Barents Sea to the central Urals to the east. That broad area was affected by the same oceanographic events, the most significant of which was the earliest Sakmarian closure of the Uralian seaway, which prevented warm water from the Tethys Ocean from reaching the northwestern Pangea margin as it did before; this led to much cooler oceanic conditions all along western North America, even in the low tropical paleolatitudes where northern Nevada was located, in spite of a globally warming climate following the end of the late Paleozoic ice age.

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