JURASSIC CARBONATES OF NORTHERN WYOMING: POSSIBLE EVIDENCE OF CLIMATE CHANGE IN THE SUNDANCE SEAWAY

Previous studies have shown that Jurassic strata in the western interior of the United States preserve a long-term climate transition from arid to winter-wet conditions, as recorded by the changeover from evaporites and eolian deposits to coastal plain deposits. Marine deposits of the Gypsum Spring and Sundance Formations of Wyoming similarly record an overall transition from carbonates to siliciclastics, including a middle interval of oscillation between the two. We point-counted 36 carbonate thin-sections from these marine strata, converted them to a percentage scale, and used non-metric multidimensional scaling ordination analysis to identify patterns in grain types and amount of micrite that could reflect changes in oceanographic conditions, such as temperature and salinity. Thirty replicate analyses consistently converged on a three-axis solution in 2–17 runs (median of 4). These solutions consistently returned the same structure, suggesting a strong and relatively simple underlying signal that accounts for 95% of the variation in the data. Axis 1 separates relatively micrite-rich samples from more grain-rich samples (particularly ooids and skeletal grains). Axis 1 primarily reflects depositional environments, with peritidal and open marine facies at one end of the axis, shell beds in the middle, and ooid shoal and eolian facies at the opposite end. Axis 2 distinguishes samples that are dominated by skeletal grains from those dominated by peloids and ooids, and axis 3 separates ooid-rich samples from peloid-rich samples. Samples generally sort by age along axes 2 and 3 (which collectively account for 54% of the variation), with older samples being richer in micrite, ooids, and peloids, and younger samples being more dominated by skeletal grains. This pattern, combined with the occurrence of photozoan organisms (corals, calcareous algae, microbial mats) in older strata, suggest a long-term shift from primarily tropical-type carbonates to temperate-type carbonates. This pattern is consistent with a decrease in water temperature, which may reflect some combination of the poleward drift of as North America and possible global cooling. The parallel increase in siliciclastic mud at this time may also reflect regional climate change, and it may also be a cause of the change seen in carbonates.