STRATIGRAPHY AND PALEOENVIRONMENTS AT THE WARM SPRINGS RANCH, FOOT SITE DINOSAUR QUARRY, THERMOPOLIS WYOMING

The Morrison Formation is one of the most extensive and well-known dinosaur bearing rock units in the entire world. It preserves the richest assemblages of dinosaur species from the Jurassic period and covers over 600,000 square miles of the western United States. Distinct stratigraphic members have been identified based on rich data gathered from its southern reaches across Colorado, New Mexico, and Utah. However, the stratigraphy of the formation becomes less well understood in the north-central basins of Wyoming and Montana. During the summer of 2021, 124 rock samples were collected from the Foot Site (FS) Quarry near Thermopolis, Wyoming with the purpose of filling in parts of this “knowledge gap”. Two stratigraphic sections within the quarry were sampled at 5-to-10cm increments measured with a precision Jacobs staff and Brunton. This project is providing high resolution geochemical and stratigraphic data that has not been attempted at the quarry previously. The samples were run through a suite of tests including, X-Ray diffraction (XRD), X-Ray Fluorescence (XRF), Gamma Ray Spectrometry (GRS), and Magnetic Susceptibility (MS).

Preliminary results indicate: 1) alternating calcrete and gray-green siltstone lithologies relating to fluctuations in environmental conditions, with potential(?) volcanic ash layers; 2) predominance of Illite clay mineralogy throughout the section; 3) fluctuations in carbonate content within strata; 4) moderate variation of U/Th values, ranging from 0 ppm to 3.6 ppm, and MS variation from -0.0186 to 0.341.

Future work will include petrographic thin sections and biostratigraphy to further characterize the quarry and may incorporate methods of GIS, geochronology, and a more sophisticated geochemical analysis via ICP-MS. This study is contributing a high-resolution dataset to enrich the stratigraphic knowledge of Morrison strata within the understudied Big Horn Basin. Continued work will help to clarify the relationships between the local quarries and enhance the ability of future researchers to potentially classify these strata into distinct formally named units, as they may persist over a wider geographic area. The data from this work will also add to a concurrent taphonomic study of the FS quarry currently underway by providing a geological description.