STRATIGRAPHY, SEDIMENTARY PETROLOGY, AND DEPOSITIONAL ENVIRONMENT OF THE CHUGWATER GROUP, NEAR DUBOIS, WYOMING

Exposures of the Triassic Chugwater Group near Dubois, Wyoming, yield exceptionally well preserved fossils of temnospondyl and stem-caecilian amphibians in two recently discovered bonebeds. Study of the strata at the Dubois locality was conducted to determine local paleoenvironment, as well as to improve understanding of the stratigraphy of the two fossil sites. The depositional environments of the formations in the Chugwater Group are somewhat controversial due to ambiguous lithologies. Deposition of the Red Peak Formation has been hypothesized to be marine, either nearshore or offshore. Laterally expansive beds with constant thickness are consistent with an open marine paleoenvironment, but clay drapes (suggesting variable depositional energy) may indicate tidal influence. The depositional history of the Crow Mountain Formation is similarly controversial, but indicators such as stromatolitic limestone at the base of the unit are consistent with shallow marine deposition. Up section, the Crow Mountain Formation contains high angle trough cross stratification and lateral accretion deposits consistent with fluvial deposition. The “Unnamed Red Beds” and the Popo Agie Formation are decidedly fluvial in origin at the Dubois locality. In thin section, the framework mineralogy of the Chugwater Group is characterized by high concentrations of quartz, with rare feldspars and lithic fragments. When compared with compositional data reported in previous studies, the low concentration of feldspars in these rocks may reflect a greater distance from potential source areas.

Paleosols have not been described from the Chugwater Group previously. At the Dubois locality, paleosols are present near the top of the purple unit of the Popo Agie Formation. These ancient soils potentially provide important insights that relate to the hypothesized origin of the bonebeds. In outcrop, paleosol indicators include slickensides and meter scale faults. In thin section, the paleosols display wedge-shaped/prismatic peds, clay aggregates, and Fe/Mn oxide nodules. These features are consistent with vertisols and a seasonal paleoclimate. A sub-humid to sub-arid environment with strong seasonality is consistent with previous reconstructions which link the amphibian bonebeds to drought and mass mortality.