SEDIMENTOLOGY AND STRATIGRAPHY OF A LOWER CRETACEOUS DINOSAUR TRACK SITE, HERITAGE MUSEUM OF THE TEXAS HILL COUNTRY, COMAL COUNTY, CENTRAL TEXAS

Dinosaur tracks are abundant in the Lower Cretaceous, Glen Rose Fm. of Texas. Over 300 tracks and 28 trackways are preserved at the Heritage Museum of the Texas Hill Country (HMTHC). These tracks are interpreted to have been made by theropod and ornithopod dinosaurs. They are preserved in a marginal marine carbonate facies deposited along the shoreline of an epicontinental sea.

We described a 65m stratigraphic section and made detailed observations of thin sections and polished slabs. We generated spectral gamma ray profiles using a hand held scintillometer.

Facies types within the succession include mud cracked, miliolid bearing lime mudstone to wackstone, dolomitized oolitic grainstone, skeletal intraclastic peloid grainstone, skeletal intraclastic peloid, packstone and wackstone, and calcareous mudrock (marl). Dinosaur tracks occur on the upper surface of the mud cracked, miliolid bearing lime mudstone to wackstone bed interpreted to represent a tidal flat environment. The skeletal grainstone and dolomitized oolite grainstone facies are interpreted to represent higher energy subtidal and intertidal environments, whereas the packstone-wackstone, and calcareous mudrock are interpreted to represent lower energy, deeper subtidal environments. Miliolids within the mudcracked lime mudstone indicate restricted marine conditions, whereas the biota within the grainstone to wackstone facies and marls indicate stenohaline open-marine conditions. Facies stacking patterns indicates shallowing upward and exposure on the track horizon. Exposure was followed by abrupt flooding and deposition of an open marine calcareous mudrock over the top of the track horizon. Following mudrock deposition, the succession shallowed again to high energy subtidal and intertidal environments. Early lithification of the trackway surface, abrupt flooding and rapid burial by mudrock in a low energy environment all aided in preservation of the tracks.

Gamma ray curves indicate that the trackway horizon at HMTHC is correlated with the trackway at the spillway in the Canyon Lake Gorge and occurs at a sequence boundary. An oxidized, bored hardground surface and oolitic horizons above the track surface in the HMTHC correlate with similar facies in the uppermost gorge section.