Chemostratigraphic and Tephrochronologic Record of Cenomanian-Turonian Eagle Ford, South Texas, USA

Abstract

The Eagle Ford Formation of South Texas is classified as a carbonate-rich, organic mudrock that was deposited on the South Texas Shelf during the earliest Late Cretaceous. Buda limestone stratigraphically underlies the Eagle Ford Shale at a sharp contact that represents a major transgression during the middle Cenomanian. The contact between the Eagle Ford and overlying Austin Chalk is highly gradational, and the delineation of these two formations often proves to be difficult due to the subtle variation in lithology between the two units. Previous studies use combinations of drill core, thin section and outcrop localities to better understand the rock fabrics, facies architecture and heterogeneities present in this system. Recent studies have also utilized X-ray fluorescence (XRF) to elementally describe chemical units within the Eagle Ford Shale, yielding a direct relationship between rock characteristics and chemistry. Despite heavy interest in this mudrock system, studies often overlook the bentonite ash beds present in the Eagle Ford which serve as regional chronostratigraphic markers across South Texas. Current work suggests that anoxic conditions experienced along the South Texas shelf during this time did not coincide with the well-documented Oceanic Anoxic Event 2, implying that local/regional paleoceanographic conditions played a greater role in the occurrence of anoxia The goal of the present study is to integrate highly-resolved (50.8-mm) chemostratigraphic data, lithologic observations, hierarchical cluster analysis and U-Pb age dates of bentonite beds to accurately determine sedimentation rates and the controls responsible for Eagle Ford deposition. Based on the high resolution geochemical data, fifteen different chemical facies were used to completely define the Eagle Ford Shale and bounding units. This implies a higher variability of distinct facies often not identified or observed in lithologic description. Thus, integration of these fifteen different facies with absolute age dates allows for an improved understanding of the lateral distribution of distinct chemical units across South Texas, highlighting regions of anoxia and potential pay zones.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016