THE ROCKY MOUNTAIN PALEOGEOGRAPHIC MAPS PROJECT: NEW INSIGHTS INTO PERMIAN AND TRIASSIC STRATIGRAPHIC CORRELATIONS IN COLORADO AND WYOMING

We have created a suite of paleogeologic maps of the central Rocky Mountain and high plains region that incorporates newer well log, core, outcrop, isopach, and geochronology data. While some of these maps represent refinements of previous paleogeographic hypotheses, others refine our contextual framework for the region, often revealing stratigraphic correlations that span historic nomenclatural and geographic boundaries. Two examples from this project are presented here, illustrating revised Permian and Triassic depositional, erosional, and tectonic conditions.

Permian deposits in this region are characterized by thick red siltstone units interbedded with thin, laterally extensive evaporites and carbonates. Hallmark units include the Goose Egg Formation in Wyoming, the Lykins and State Bridge Formations of Colorado, and coeval strata from adjacent states. A distinctive element of these successions is the Forelle Member, a ~5 m thick stromatolitic carbonate that is of uniform thickness and extends for nearly 500 km across the Denver-Julesberg Basin. Its distribution implies a flat and occasionally flooded basin east of the Ancestral Rocky Mountains. Such beds extend into the intermontane Eagle Basin, implying loci of clastic deposition are overridden, diverted, or relocated even in close proximity (< 50 km) to the still-eroding Ancestral Rocky Mountains. These data when incorporated into paleogeographic maps, provide a new perspective into the upper Permian system of the western US.

Triassic strata record resumption of primarily fluvial and eolian deposition in Colorado, whereas equivalent strata in Wyoming consist of coastal mudstones and marine carbonates. Exemplifying this pattern is the Triassic Jelm Formation, a unit that has garnered little attention for nearly a half-century. Its lower member, the Red Draw Member, is dominated by eolian sandstones along the northern Front Range, which grade into coastal eolianites and mudstones of the Crow Mountain Formation in central Wyoming. The upper Sips Creek Member consists of fluvial sediments flowing from northern Colorado to western Wyoming. New detrital zircon signatures and stratigraphic analyses suggest that the Sips Creek Member of the Jelm could be partially equivalent to lower members of the Chinle Formation to the south and southwest.