Geochemistry And Environmental Analysis Of A Lower Cretaceous Dinosaur Track Site, Heritage Museum Of The Texas Hill Country, Comal County, Central Texas
Asmara A. Lehrmann1, Elliot Blake1, Kexin Sun2, Bailey Welch3, Christopher Ray3, Thomas Adams4, Alexis Godet3, Marina Suarez3, David A. Ferrill5, Ronald N. McGinnis5, Daniel J. Lehrmann1
1Geosciences Department, Trinity University, San Antonio, TX 78212; 2Environmental Science Program, Beijing Normal University-Hong Kong Baptist University United International College, Xiangzhou, Zhuhai, P.R. China; 3Department of Geological Sciences, The University of Texas San Antonio, One UTSA Circle, San Antonio, TX, 78249; 4The Witte Museum, 3801 Broadway St, San Antonio, TX 78209; 5Space Science and Engineering Division, Southwest Research Institute®, San Antonio, TX 78238
Abstract
To investigate the environmental setting that existed as dinosaurs walked along an Early Cretaceous shoreline of the Gulf of Mexico we conducted a detailed sedimentologic and geochemical analysis of a 65 m stratigraphic section of the Glen Rose Formation at the Heritage Museum of the Texas Hill Country (HMTHC).
Facies types include mud-cracked, miliolid-bearing lime mudstone to wackestone, oolitic grainstone, oolitic skeletal intraclastic grainstone, skeletal intraclastic peloid wackestone-grainstone, and calcareous mudrock. Dinosaur tracks occur on the upper surface of a mud-cracked, miliolid-bearing lime mudstone to wackestone bed interpreted to represent a tidal flat environment. The skeletal and oolite grainstone facies are interpreted to represent higher-energy subtidal and intertidal environments, whereas the packstone-wackestone, and calcareous mudrock are interpreted to represent lower-energy subtidal environments. Miliolids within the mud-cracked mudstone-wackestone indicate restricted marine conditions, whereas the biota within the calcareous mudrock and grainstone to wackestone facies indicate stenohaline open-marine conditions. The facies succession 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. Stabilization of the track surface by a microbial mat and early lithification followed by abrupt flooding and rapid burial by mudrock in a low-energy environment all aided in preservation of the tracks.
The geochemical results show that Mg is low in the lower half of the stratigraphic section including the track horizon indicating lack of dolomitization and is elevated in dolomitized oolitic grainstone in the upper half of the section. Siliciclastic proxies are elevated in the calcareous mudrock intervals of the lower half of the section. Proxies for organic productivity (P, Ba, and Cu) are concentrated in the calcareous mudrock of the lower half of the section. Ternary plots of K2O, Al2O3, and Na2O and clay mineralogy indicate high levels of chemical weathering of siliciclastic components supporting humid conditions and extreme global warming during the Cretaceous greenhouse period. d13C fluctuates between 0.8‰ and 2.8‰ and includes a negative shift to 0.6‰ within the track horizon possibly resulting from organic material in the microbial mat or meteoric diagenesis.
Correlation of gamma-ray curves with the section exposed at the Canyon Lake Spillway indicates 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. An oxidized, bored hardground above the track surface in the HMTHC section correlates with a similar surface in the uppermost gorge section.
