PENNSYLVANIAN-PERMIAN LITHOSTRATIGRAPHY AND ASTROCHRONOLOGY OF THE WESTERN LAURENTIAN MARGIN, CALIFORNIA AND NEVADA, USA

Waxing and waning of continental glaciers on Gondwana during the Late Paleozoic Ice Age induced high-magnitude eustatic sea-level fluctuations from the Late Mississippian to Early Permian and have been attributed to orbitally-driven insolation variation. These changes are recorded globally in cyclic marine strata preserved in paleoequatorial basins. To better understand orbital forcing, glacial-interglacial cycles, and the expression of shelf and slope lithofacies across a deep- to shallow-marine stratigraphic transect, we measured several thick sections of conodont-bearing upper Pennsylvanian through lower Permian strata along the western Laurentian margin. Decimeter-scale stratigraphy and a spectral gamma ray log of the Keeler Canyon Formation were measured at Ubehebe Mine, CA, adding to an existing record at Cerro Gordo, CA. These strata are an ~8 My succession of intercalated gravity flow deposits, turbidites, and marlstones. We also documented the coeval shallow marine Bird Spring Formation at Arrow Canyon, NV, and collected 150 0.5 meter-spaced unaltered carbonate micrite samples across the Pennsylvanian-Permian boundary. Cyclicity at Ubehebe Mine is expressed by prominent oscillations between intervals of debrites and turbidites and thinner marlstone intervals, whereas at Arrow Canyon, shallow water carbonate facies predominate. δ¹³C values vary between 1‰ and 4‰ across the section at the 5 to 40 meter scale and likely correspond to oscillations between interglacial sea-level highstands and glacial sea-level lowstands. The basal Asselian records the largest δ¹³C values and shallowest lithofacies, consistent with a glacial period and sea-level lowstand. Coarse-grained turbidites and debrites from the Keeler basin coincide with higher δ¹³C values, consistent with increased shelf instability and gravity-flow processes during sea-level lowstand. Marlstone intervals typically have lower δ¹³C values and record intervals of shelf inundation. Spectral analysis of gamma ray and δ¹³C time series reveal prominent bundled periodicities that closely match predicted Late Paleozoic orbital cycles. The 405, 123, and 95 Ky eccentricity cycles were identified at all locations, and obliquity and precession are observed in several densely-sampled portions of the Keeler Canyon Formation.