Depositional and diagenetic history of the late Triassic Dockum Group, Young Ranch, Nolan County, Texas

The Late Triassic Dockum Group is a series of continental sediments comprised of lacustrine, deltaic, and fluvial deposits. Dockum sediments are found in Eastern New Mexico, Colorado, and West Texas, Oklahoma, and Kansas. The Santa Rosa and Trujillo Formations represent the Dockum Group on the Young Ranch, Nolan County, Texas. Santa Rosa and Trujillo Formations are fluvial deposits consisting of alternating layers of sandstone and conglomerate. These formations are found on the same topographic level on the ranch. Identification of the Santa Rosa and Trujillo formations can be made through petrographic and paleocurrent analysis. The Santa Rosa Formation tends to be more quartzose and arkosic, while the younger Trujillo Formation is more lithic, with an abundance of metamorphic rock fragments and rare volcanic rock fragments. The Santa Rosa Formation has a reported sediment provenance in the present day Wichita-Arbuckle Mountains. Santa Rosa paleocurrent direction on the Young Ranch trend toward the south. The Trujillo Formation has a reported provenance in the lower Ouachita Tectonic Belt. Trujillo paleocurrent directions on the Young Ranch trend toward the north. Therefore, the paleocurrent analysis on the Young Ranch supports the reported Santa Rosa and Trujillo sediment provenances.

Lithofacies types and vertical sequences along with a muitistoried channel stacking architecture indicate a braided fluvial system for the deposition of the Santa Rosa Formation. The presence of the Trujillo Formation at the same topographic level on the ranch leads to the conclusion of deposition in two incised valleys.

Diagenetic history of the Dockum Group is marked by a variety of cementation events. Kaolinite and iron oxide occurred as early, shallow burial cementation. Zoned dolomite cement was emplaced during burial. Quartz overgrowths and microcrystalline quartz cement formed during Early Cretaceous Edwards exposure due to pedogenic silerete formation in the overiying Lower Cretaceous Antlers Sandstone. Dedolomite/Poikilitopic calcite formed because of dissolution of the Kirsehberg Evaporite during Early Cretaceous Edwards exposure. Desiccation of recent freshwater springs precipitated gypsum as the final diagenetic event.