GEOCHEMISTRY AND ENVIRONMENTAL ANALYSIS 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 completed a total station and DGPS survey of the track exposures. We generated gamma ray profiles using a hand held scintillometer. Geochemistry and clay mineralogy were used to evaluate paleoclimate and diagenetic alteration.

Mg is low (180-200 ppm) in the lower half of the stratigraphic section including the track horizon indicating lack of dolomitization. Elevated Mg (>3600 ppm) occurs in dolomitized oolitic grainstone horizons in the upper half of the section. Detrital proxies are elevated in the marl intervals of the lower half of the section: Al (1225-1590 ppm) and Ti (1200-1680 ppm). Proxies for organic productivity (P, Ba, and Cu) are concentrated in the marls of the lower half of the section: P (up to 980 ppm); Cu (up to 0.15 ppm) and Ba (up to 0.11 ppm). Ternary plots of K₂O, Al₂O, and Na₂O indicate high levels of chemical weathering of detrital components supporting extreme global warming during the Cretaceous greenhouse period. δC¹³ fluctuates between 0.8‰ and 2.8‰ except for negative shifts to 0.6‰ and -5.4‰ in the track horizon and in a grainstone 4.2 m above it. δO¹⁸ values range from -1.4‰ to -3.3‰ except for dolomitized intervals in the upper part of the section with values of 1.6‰ and 2.4‰.

The total station survey demonstrated that there is a single track surface (not multiple surfaces) exposed at the HMTHC site. Correlation of geochemical profiles with the section exposed at the Canyon Lake Spillway demonstrates that the track horizon at the HMTHC occurs within the Upper Glen Rose Fm., 44 m above the Corbula zone and occurs at a sequence boundary.