Stratigraphy and Depositional Environments of the Middle Jurassic (Callovian) Ralston Creek Formation, Beulah-Wetmore Area, South-Central Colorado

Eleven stratigraphic sections of the previously unassigned post-Foundation (Pennsylvanian), pre-Morrison (Upper Jurassic) strata along the eastern flank of the Wet Mountains, Colorado, have undergone a detailed sedimentological and stratigraphic analysis. These rocks are now assigned to the Middle Jurassic (Callovian) Ralston Creek Formation. Seven major lithofacies are defined within the Ralston Creek sedimentary package and include an arkosic pebbly conglomerate (Lithofacies A), a laminated siltite (Lithofacies B), a cryptalgal laminate (Lithofacies C) , a laminated subarkosic sandstone (Lithofacies D), an arkose (Lithofacies E), interbedded sandstones, siltstones, and arenaceous micritic limestone (Lithofacies F), and a very thickly bedded galauconitic sandstone (Lithofacies G). Depositional environments, as well as paleogeographic and paleotectonic implications of the Ralston Creek Formation have been determined based on lithofacies associations in the Beulah-Wetmore area on lithofacies associations in the Beulah-Wetmore area of south-central Colorado. Sediments of the Ralston Creek Formation were deposited along the southeastern margin of the Jurassic Western Interior embayment adjacent to a positive area formed by the remnants of the Ancestral Rocky Mountains during the Middle Callovian transgression. An initial episode of depositional quiescence can be inferred form the restricted, hypersaline marine, tidal flat and intertidal deposits of Lithofacies A, B, C, and D. The appearance of Lithofacies D marks an increase in sedimentation rates within the Ralston Creek basin that was possibly triggered by localized tectonic uplift. Nearby tectonic uplift in the area of the present Wet Mountains ended the period of quiescence in the Ralston Creek basin and supplied the coarse arkosic clastics that comprise the upper lithofacies (Lithofacies E and F) of the Ralston Creek Formation. Lithofacies E, F, and G are sediments deposited on a northeasterly prograding fan-delta complex and represent both subaerial (alluvial fan) and subaqueous (marginal marine) environments and processes. The Tectonic activity and associated fan-delta deposition of the Upper Ralston Creek Formation may have initiated the final Jurassic marine regression form the southern Western Interior embayment region.