ICHNOLOGY, PALEODEPOSITIONAL ENVIRONMENTS AND PALEOHYDROLOGY OF THE NON-MARINE UPPER CRETACEOUS (CONIACIAN) FRONTIER FORMATION DEPOSITS OF THE WESTERN CENTENNIAL MOUNTAINS, MONTANA

The non-marine Frontier Formation of southwestern Montana has yielded an abundant but still undescribed fossil record mostly composed of dinosaur footprints and invertebrate trace fossils. Recent surveys provide the first descriptions of a fossil assemblage from Frontier Formation deposits exposed in the western Centennial Mountains, Beaverhead County, Montana. In this study, we analyze the sedimentology and trace fossil assemblage of the Price Creek locality, with multiple dinosaur track-bearing horizons reported. A 25 m-thick stratigraphic section was measured and described in detail to define lithofacies and trace fossil assemblages. A sandstone horizon was also excavated, revealing dinosaur tracks preserved at the base of stratum, and was documented with photogrammetry. We used Agisoft™ photogrammetry software to produce 3D models and a false-color depth map of the track horizon. Three main lithofacies were defined: 1) a coarse-grained lithofacies with thin and laterally extensive sandstone channel bodies and characterized by rare vertical, passively filled burrows; 2) a fine-grained lithofacies of overbank deposits, characterized by abundant meniscate, and non-walled horizontal burrows, sinusoidal trails, vertical burrows, and dinosaur tracks; and 3) a lithofacies composed of fining-upward, planar cross-bedded sandstones, associated with abundant carbonaceous silty sandstones and bone fragments. Trace fossils and sedimentological evidences indicate these deposits were part of a distal alluvial floodplain characterized by wetlands with an oscillating ground-water table and formation of low-energy or standing-water bodies. Substrates remained soft to cohesive without desiccation or formation of well-developed soil horizons. Substrate consistencies were likely controlled by variations in seasonal fluvial flow rates and relative ground-water table fluctuations. The presence of herbivorous and carnivorous dinosaur tracks, as well as invertebrate trace fossils assigned to crustaceans, insects, and nematodes, argue for significant faunal diversity in non-marine Frontier Formation paleoenvironments spread across southwestern Montana. This ichnoassemblage represents the richest fossil record reported from non-marine Coniacian deposits of Montana, and represents one of the few non-marine Coniacian fossil records from western North America.