Chronostratigraphic and Geochemical Characterization of the Cenomanian – Turonian Eagle Ford Group in West and South Texas, USA

Organic-rich mudstone depositional environments commonly were thought to require low-energy and persistent benthic anoxia for accumulation and preservation of large amounts of organic matter. However, more recent studies indicate that organic-rich, mudstone-dominated successions commonly were deposited in environments at least episodically influenced by more energetic bottom currents (e.g. storms, turbidites, debris flows, contourites). Integrated redox-sensitive trace element (RSTE) geochemistry, mineralogy, organic geochemistry, and thin section petrography from outcrops and cores characterizing the Upper Cretaceous (Cenomanian –Turonian) Eagle Ford Group in west and south Texas (USA) indicate its organic-rich, mudstone-dominated depositional environments were heterogeneous, having different depositional processes simultaneously affecting coeval deposits in different parts of the same basin. The outcrop-to-subsurface correlation is constrained by high-precision isotope dilution thermal ionization mass spectrometry (ID-TIMS) uranium – lead (U-Pb) zircon geochronology. ID-TIMS analyses of individual zircon grains characterizing sixteen volcanic ash beds from three outcrops and two petroleum industry cores were integrated with regional biostratigraphy to provide a comprehensive outcrop-to-subsurface geochronology for the Eagle Ford Group. The 206Pb/238U ages range from 96.45 ± 0.18 Ma near the base of the Eagle Ford Group in west Texas to 88.9 ± 0.12 Ma near the base of the Austin Chalk in Karnes County, Texas. This interval encompasses uppermost Lower Cenomanian, Turonian and upper Lower Coniacian, spanning ~7.5 m.y. of deposition from the base of the Eagle Ford Group to the base of the Austin Chalk, and provides important age constraints that tie the Eagle Ford Group outcrops in west Texas to subsurface oil-and-gas producing wells in south Texas. The Ford Group outcrops at Lozier Canyon also contain several volcanic ash deposits with abundant apatite crystals. Over 230 apatite samples characterizing nine volcanic ash beds in the Eagle Ford Group in west Texas were analyzed for trace element concentrations using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). When the apatite trace element concentrations are integrated with U-Pb zircon geochronology, the Cenomanian and Turonian ash beds can be distinguished on the basis of the Eu/Eu* on chondrite-normalized REE plots and on cross-plots using a combination of (Th/U)CN, ∑(La, Ce, Pr, Nd)CN and (Ce/Yb)CN