Sedimentologic and geochemical analysis of dinosaur track sites of the Davenport Ranch, Bandera County and comparison with other dinosaur track sites in the Lower Cretaceous Glen Rose Formation of central Texas
Josephine E. Tesauro1 , Charles Sheppard1 , Thomas Adams2 , Alexis Godet3 , Dianna Price3 , Justin Sharpe3 , Marina Suarez4 , Daniel J. Lehrmann1
1 Geosciences Department, Trinity University, One Trinity Place, San Antonio, TX 78212
2 The Witte Museum, 3801 Broadway St, San Antonio, TX 78209
3 Department of Earth and Planetary Sciences, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX, 78249
4 Department of Geology, The University of Kansas, Ritchie Hall Earth, Energy & Environment Center, 1414 Naismith Drive, Lawrence, KS 66045-7575
Dinosaur tracks are common in the Lower Cretaceous Glen Rose Formation of central Texas. The Davenport Ranch is a historically important dinosaur tracksite where, in 1944, R.T. Bird interpreted herding behavior in sauropods. Over the years, these historic tracks have become mostly buried, while new tracks and track layers are now exposed. Tracks occur in 3 dolostone beds within the stratigraphic section. The presence of pressure rims, claw and pad impressions indicate the layers represent the true track surfaces and not undertracks. The dolostone contains peloids, ostracodes, microbial laminate, fenestrae and mud cracks indicating tidal flat environments. The upper tracks occur on dolostone with evaporite-solution collapse breccia, indicating subaerial emergence in a restricted hypersaline environment. Dolomitization may have been the product of evaporative concentration of Mg rich brines. The lack of invertebrate fossil diversity further supports restricted hypersaline environments. Each of the track pavements is overlain by a mudrock or an argillaceous dolomite. Mudrock and argillaceous intervals have elevated concentration of Al, Si, Ti, P, Ba and gamma ray values, indicating increased siliciclastic and nutrient flux from land during burial of the tracks. The basal mudrock contains root casts bordered by dense micritic precipitates. Elemental proxies for redox conditions indicate oxidizing conditions, except for elevated V concentration in the lower mudrock interval that indicates local reducing conditions due to decay of organic matter or chlorophyll. Chemical alteration index values decrease upward through the section; this indicates a trend toward more arid paleoclimate over time. 12 Figure 1. Geologic setting and location maps. A) Simplified geological map of the state of Texas (modified from Bureau of Economic Geology, The University of Texas at Austin, 1992). Red star shows the location of the study area. B) Geologic Map of the Bandera Quadrangle, showing the location of stratigraphic sections. Purple dot shows the location of section 1 (Bird’s 1944 herding sauropod locality), and yellow dot shows the location of section 2. Modified from Collins (1997). C) Summary column of the Texas Cretaceous stratigraphy. The red box indicating the stratigraphic level of this study. Modified from Phelps et al., 2014. The facies at the Davenport Ranch are similar to other track sites in both the lower and upper Glen Rose Formation in which tracks are preserved in marine carbonate and buried by mudrock. Further, the Davenport Ranch site is similar to the Government Canyon site as it is extensively dolomitized and preserves original track surfaces in the upper Glen Rose Formation. The Davenport Ranch site contrasts with the nearby Mayan Ranch site which occurs in the lower Glen Rose Formation, and contains undertracks on undolomitized limestone pavements with a diverse marine biota. This project contributes to a larger effort to archive data from multiple central Texas tracksites at the Witte Museum.