--> Defining the Age of the Eagle Ford Formation With U-Pb Geochronology: Guidelines for Improved Correlations and Definition of Facies Architecture

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Defining the Age of the Eagle Ford Formation With U-Pb Geochronology: Guidelines for Improved Correlations and Definition of Facies Architecture

Abstract

The Upper Cretaceous Eagle Ford Shale and equivalent Boquillas Formation contain abundant bentonitic, volcanic ash beds of varying thickness. The abundance of melt-derived, non-detrital zircon crystals in these deposits provides an opportunity to develop a high-resolution geochronology of a mudrock system with U-Pb dating that can be used as a basis for testing the accuracy and validity of other methods of stratal and temporal correlation. Here we present the first extensive collection of U-Pb dates yet obtained for this system and discuss their implications. We collected, processed, and analyzed 14 ash beds from outcrops and cores from three areas in a 350 mi transect along the South Texas shelf from around Austin, Texas on the east to outcrops near Comstock, Texas on the west. Zircon abundance ranged from 27 to 164 crystals per sample (average: 60). Zircons were analyzed using laser ablation ICP-MS procedures at the University of Texas at Austin. Ash bed ages range from about 87 to 97 Ma. Comparisons of our new age dates with biostratigraphic determinations of Cenomanian/Turonian boundary show that our data are largely consistent with those reported elsewhere at about 93–94 Ma. However, U-Pb dates suggest that the Eagle Ford/Boquillas section varies greatly in temporal duration locally, ranging from as much as 10 Ma in some areas to possibly as little as 5–6 Ma in others. Although ash bed LA-ICP-MS U-Pb dates do not provide sufficient temporal resolution needed to develop bed-to-bed correlations, they do place important constraints on regional correlation frameworks based on biostratigraphy, chemostratigraphy facies, and wireline logs. Not surprisingly, these data illustrate that the upper and lower contacts of the Eagle Ford are time transgressive - findings previously discounted by many - casting doubt on wireline log-based correlations. But U-Pb data are also locally in disagreement with more robust methods of correlation, including carbon isotope and redox trace element profiles. Collective comparison of different data sets suggests that although more extensive biostratigraphic data are needed, additional U-Pb zircon geochronology using LA-ICP-MS and higher resolution thermal ionization mass spectrometry (TIMS) combined with redox sensitive trace element chemostratigraphy may form the best basis for defining temporal correlations and mudrock facies architecture in this complex depositional system.