Facies stacking and distribution in the Gabbs Formation (Late Triassic, west-Central Nevada, U.S.A.): An environmental baseline to the end-Triassic carbonate crisis

Abstract

The primarily carbonate Gabbs Formation of west-central Nevada, U.S.A., remains an important, rare example of Panthalassan shallow-marine environments from the Late Triassic and through the Triassic–Jurassic boundary. Its relevance as a locality persists, particularly as the end-Triassic mass extinction interval is increasingly recognized as a carbonate crisis, evidenced by the global decline of carbonate facies and calcareous marine organisms. However, important transitions across key stratigraphic boundaries in this region have tended to be evaluated in isolation, producing an incomplete picture of sedimentation through the duration of the Late Triassic leading up to the geochemical crises and mass extinction at the close of the Triassic. In this work, multiple correlative measured sections are used to describe the facies in the mixed carbonate-siliciclastic ramp and characterize controls on carbonate facies. The outer ramp transition to inner ramp facies of the Gabbs represent a transgressive–regressive–transgressive cycle culminating in a transition to terrigenous inputs and a gradual decline of carbonate sediments stratigraphically below the mass extinction interval. This predictable loss of carbonate facies and the near continuous deposition of the Gabbs Formation allow for evidence of facies changes and acidifying conditions in the latest Triassic to be considered independently, in contrast to other global Late Triassic sections where depositional hiatuses or abrupt facies changes often compound these records. Establishing a baseline sedimentation for the Late Triassic demonstrates that the final loss of carbonate facies can be decoupled from the onset of acidifying conditions, resulting in a more precise timeline of latest Triassic environmental upheaval.