SEDIMENTOLOGY AND ICHNOLOGY FROM THE EARLY AND MIDDLE TRIASSIC FORMATIONS OF THE CHUGWATER GROUP, CENTRAL WYOMING, USA

The Early Triassic Red Peak and Middle Triassic Crow Mountain (‘unnamed redbeds’) Formations of central Wyoming have long been considered to be nearly devoid of fossil remains. Recent work in these formations has yielded many previously unrecorded invertebrate and vertebrate trace fossils. The diverse vertebrate ichnoassemblage include representatives of most major clades of the ‘modern fauna’ that originated in the Triassic including dinosauromorphs, archosaurs, lepidosaurs, and testudinate track makers (cf. Rotodactlyus, Chirotherium barthii, Rhynchosauroides isp., and cf. Chelonipus, respectively). In addition to vertebrate tracks the Early – Middle Triassic invertebrte ichnoassemblage is consistent with the ichnofauna of the Scoyenia ichnofacies.

In central Wyoming, the upper Red Peak Formation contains strata that are interpreted to be fluvial, lacustine, and crevasse splay deposits as well as associated weakly-developed paleosols; these deposits are conformably overlain by the Alcova Limestone, which gradually grades from limey-siltstone to limestone. There has been some debate as to the depositional nature of the Alcova Limestone, which presents abundant small-scale laterally-linked stromatilites. It has been postulated to either represent a shallow lake of large lateral extent, or fully marine: no diagnostic fossils have been found that help support either claim. The eolian and fluvial Crow Mountain Formation and the fluvial unnamed redbeds of Middle(?) Triassic age overlie the Alcova Limestone. The floodplain deposits of the unnamed redbeds contain vertebrate burrows, abundant adhesive meniscate burrows and an unknown complex burrow reminiscent of those attributed to termites in Triassic and Jurassic strata of the western US. Early to Middle Triassic strata in central Wyoming are yielding new insights suggestive of a much richer biological diversity than previously thought for this region.