MACROSTRATIGRAPHY OF THE JURASSIC WESTERN UNITED STATES

The Jurassic follows one of the big five extinctions, witnessed the continued break-up and dispersal of Pangaea, and saw a marked diversification of dinosaurs, marine reptiles, and the appearance of mammals. Many aspects of the evolving Earth system are captured by the sedimentary record, but in a spatially heterogeneous way. Here we apply macrostratigraphy to characterize spatiotemporal patterns of Jurassic sedimentation and non-deposition/erosion in the western United States. We synthesized descriptive stratigraphic data and integrated them with current radiometric ages, biostratigraphic constraints, sequence stratigraphic interpretations, and magnetostratigraphic data, to arrive at a regionally comprehensive stratigraphic summary and correlation framework covering 40 geographic regions. This framework was augmented with Paleobiology Database fossil occurrences. Six regional unconformities (J-0 to J-5) have been described from the Jurassic, and each is shown to vary markedly in spatial magnitude and stratigraphic expression. In the region around the Sundance Seaway, there are numerous unconformities of relatively small extent and duration. The Wyoming-Montana region experienced widespread non-deposition in the early Jurassic J-1 sequence, and documents an overall fining upward in grain size, with a shift in deposition from coarse siliciclastic to carbonate and then back to finer siliciclastics. Macrostratigraphic patterns reflect the far-field effects of tectonic events, such as episodic thrusting and forebuldge migration that created the Utah-Idaho trough. Sedimentation rates are generally higher in the western edge of the region, probably as a result of subsidence associated with thrusting from the Nevadan orogen. Fossil occurrences track macrostratigraphic architecture, with turnovers coinciding with shifts in sedimentation from carbonate to siliciclastic and from marine to non-marine. The marine-nonmarine transition, in particular, is responsible for a shift in primary fossil types from invertebrates to vertebrates. Overall, eustatic, tectonic, and paleobiological changes during the Jurassic are relatable to macrostratigraphic quantities, reflecting a combination of sediment preservation-induced sampling heterogeneity and real environmental change.