SEDIMENTOLOGIC AND GEOCHEMICAL ANALYSIS OF A DINOSAUR TRACK SITE, THE DAVENPORT RANCH, BANDERA COUNTY AND COMPARISON WITH OTHER TRACK SITES IN THE LOWER CRETACEOUS GLEN ROSE FORMATION OF CENTRAL TEXAS

Dinosaur tracks are common in the Lower Cretaceous Glen Rose Fm. of central Texas. The Davenport Ranch is a historically important dinosaur tracksite where in 1944 R.T. Bird interpreted herding sauropods. Over the years, these original tracks have become buried. However, 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 original track surface. 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 argillaceous dolomite. Mudrock and argillaceous intervals have elevated concentrations 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.

The facies at the Davenport Ranch are similar to other track sites in both the lower and upper Glen Rose Fm. in which tracks are preserved in marine carbonate and buried by mudrock. Further, the Davenport Ranch site is similar to the Government Canyon site: extensive dolomitization and preservation of the original track surface in the upper Glen Rose Fm. The Davenport Ranch site contrasts with the nearby Mayan Ranch site which occurs in the lower Glen Rose Fm., 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 in central Texas at the Witte Museum.