Integrated stratigraphy and paleoenvironmental reconstruction of a dinosaur track-bearing interval of the Glen Rose Formation at Government Canyon State Natural Area, Bexar County, Texas
During the Cretaceous, unique tectonic and climatic conditions favored the creation of the shallow Western Interior Seaway that divided the North American continent into two distinct landmasses. This vast shallow-marine environment promoted the deposition of thick series of platform carbonates, which constitute an excellent record of ecological crises resulting from the high sensitivity of carbonate producers to environmental changes. The Glen Rose Formation is a thick shallow-marine carbonate succession deposited during the Albian (Early Cretaceous) period over South Central and North Central Texas as a part of a large, extensive Central Texas carbonate platform represented by the Trinity, Fredericksburg, and Washita sequences. The Glen Rose Formation is particularly well known for containing abundant dinosaur tracks and trackways that can be observed in outcrop at numerous sites throughout South Central and North Central Texas, such as Canyon Lake Gorge (Comal County). Recently, well-preserved tracks and trackways (theropod and sauropod) have been described at Government Canyon State Natural Area, northwestern Bexar County near San Antonio, TX. This is a significant discovery as it is the first Glen Rose track site to be identified within Bexar County. It is the aim of this study to determine the position of this dinosaur track-bearing section within the overall stratigraphy of the Glen Rose Formation and to assess the paleoenvironmental and paleoclimatic conditions that prevailed in this location during the time of deposition. Based on the evolution of carbon isotopes within the section and the outcrop gamma-ray profile, the section is correlated to the lowermost portion of the upper Glen Rose Member, constraining the age of the section to 109.8-109.5 Ma. Bulk-rock geochemical data suggest that these dinosaurs were living under very warm and humid conditions characteristic of intertropical regions. The integration of data permits the development of a sequence stratigraphic framework for the section: a third-order sequence boundary separates the section into a late highstand systems tract and an early transgressive systems tract. Dinosaur tracks and trackways are preserved at the tops of parasequences belonging to this late highstand systems tract.