Abundance and Character of Organic Carbon in Cretaceous Rocks of Central Texas

S. M. Clark and S. I. Dworkin
Baylor University

Cretaceous rocks in central Texas exhibit a wide range of organic carbon content. Because one of the basic tenants of organic geochemistry states that significant accumulation of organic matter will not occur in well-oxygenated aqueous environments, the changing abundance of sedimentary organic material in Cretaceous rocks probably reflects changes in marine and terrestrial conditions on Earth. Such climatic changes could be driven by variations in temperature, atmospheric composition, sea level, nutrient abundance, seawater circulation patterns, seawater chemistry, or orbital variations. This study will document the abundance and character of sedimentary organic matter in Cretaceous rocks of central Texas with the goal of reconstructing temporal changes in the environmental, chemical, and depositional conditions responsible for organic carbon accumulation.

Organic carbon content ranges from .2 to over 1.5 weight percent. Rocks from the Eagle Ford group have the highest amount of organic carbon and are probably correlative with oceanic anoxic event II. C/N ratios range from 5 to over 25. The C/N ratios are low in the Grayson and Pearsall formations whereas high C/N ratios are characteristic of the Walnut Clay and Lake Waco formations. High C/N ratios maybe indicative of terrestrial organic matter input or unusual organic matter production or preservation during oceanic anoxic events. Carbon isotope ratios of organic matter range from –28.8 to –22.7 ‰ (PDB) with lower values occurring in the middle-Cretaceous. Lastly, δ¹⁵N of organic matter ranges between –0.5 and 5 ‰ (AIR) with most organic matter hovering between 1 to 3‰.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas