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Late Paleozoic Tectonism in the Central Great Basin Requires Revision of Stratigraphic and Structural Interpretations

James Trexler, Jr and Patricia Cashman. Geological Sciences and Engineering, University of Nevada, Reno, Mail Stop 172, Reno, NV 89557, phone: 775-784-1504, [email protected]

Recent studies in central Nevada show that stratigraphy of Mississippian through Permian rocks is largely driven by late Paleozoic tectonism. At least eight tectonostratigraphic boundaries have been identified. Most coincide with lithostratigraphic boundaries and many are associated with regional unconformities. Some are dramatic, preserving folding and faulting beneath the unconformity. Others are subtle disconformities. All are regionally important markers recording tectonic activity after the Antler orogeny and before the Sonoma orogeny.

A dramatic example is an upper Carboniferous unconformity at Carlin, NV, where gently deformed Missourian strata overlie highly folded and faulted Desmoinesian rocks. Map-scale folds and thrust faults with 100s of meters of stratigraphic throw record regional NW-SE contraction, with vergence primarily to the NW. Biostratigraphy constrains this deformation to early Missourian. The angular unconformity can be mapped regionally, and correlated with equivalent disconformities and facies changes related to uplift.

Using these tectonostratigraphic boundaries, we are reassessing the upper Paleozoic stratigraphy of the Great Basin. This genetic approach has both simplified correlation, and created some problems. Some units, especially coarse clastic strata related to uplift and reworking, take on new significance as indicators of tectonic activity. Other units need to be redefined because they contain regional unconformities. In addition, the recognition of upper Paleozoic deformation, some of which coincides with Ancestral Rockies deformation to the east, requires revision of tectonic models for western North America.