The stratigraphy and biofacies trends of the Lower Mesozoic Gabbs and Sunrise formations, west-central Nevada

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Abstract
The boundary between the Gabbs and Sunrise formations of west-central Nevada is redefined and the two units are combined to form the new Volcano Peak Group. The Nun Mine, Mount Hyatt, and Muller Canyon members are named and assigned to the Gabbs Formation, and six new lithostratigraphic units for the Sunrise Formation are the Ferguson Hill, Five Card Draw, New York Canyon, Joker Peak, and Mina Peak members. The boundary for the Triassic and Jurassic systems is placed above the last Choristoceras and below the first certain Psiloceras. The section at Ferguson Hill is proposed as the system boundary stratotype. The transgressive–regressive curve for the formations points to a strong tectonic influence, but the late Norian and late Hettangian regressions and early Sinemurian, early Pliensbachian, and early late Pliensbachian transgressions are consistent with inferred eustatic events. The facies and faunal data are consistent with an early Mesozoic record in the western Cordillera suggesting widespread anoxia. The high within-habitat diversity in Neogene offshore benthic communities compared with those in the Mesozoic may correspond to the breakdown of widespread marine anoxic conditions as polar regions cooled and worldwide climatic zonation intensified.

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