REDBEDS IN WYOMING: READING LATE PERMIAN-EARLY TRIASSIC ENVIRONMENTS AND CLIMATES FROM THE RED PEAK AND CROW MOUNTAIN FORMATIONS

Thick redbeds with evaporites and thin carbonates were deposited in the midcontinent of North America during the Permian and Triassic. Little is known about the environments or climate of these continental deposits, despite their span over a major critical biological transition. The Goose Egg Formation, Red Peak Formation, Alcova Limestone, and Crow Mountain Formation comprise this section in central Wyoming. Here, we describe trends in paleosols in the Red Peak and the Crow Mountain Formations and attempt a first approximation of environments and climates. Paleosols were recognized in outcrop and thin sections and categorized based on ped morphology, grain size changes, color, horizons, shrink-swell traces, micromorphology, mineralogy, and elemental composition.

The Red Peak Formation, well exposed along most Laramide uplifts in Wyoming, is located stratigraphically below the Alcova Limestone. It is dominated by paleosols and sheet flood deposits. Weathering differences creates couplets of recessive and resistive beds. The sediments fine upward. Few of the paleosols preserve root traces or bioturbation. Within the Red Peak, we notice no trends in ped morphology, though there is a distinct increase in CaCO₃ and trends in trace elements up section.

The paleosols of the Crow Mountain Fm. are well exposed at one location near Lander, Wyoming, and poorly exposed in many locations above the Alcova Limestone. At the well-exposed section, paleosols transition up section from discreet simple paleosols to amalgamated complex paleosols. At the base of the section, thin fining upward beds with well-developed peds and no root traces are separated by non-channelized sediment bodies. The uppermost section is amalgamated paleosols and channelized fluvial deposits. The amalgamated paleosols have abundant root traces, shrink-swell traces, and diverse ped morphology.

Descriptions of these redbeds represent the first steps in attempting to reconstruct climate and environments through the Permian and Triassic sections in Wyoming. We interpret changes in the hydrologic regime over time: specifically, greater availability of water at the surface, and more channelization of that water. This may be indicative of changing climate, from arid to semi-arid, as Pangaea recovered from the Permian-Triassic ecological crisis.