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Determining Lithologic Variations, Provenance, and Depositional Environments of the Del Rio Formation in West Texas

Abstract

The Cenomanian Del Rio Formation is an interbedded calcareous shale and skeletal limestone that outcrops in west Texas and extends laterally throughout the subsurface dipping gently towards the Gulf Coast. This project integrates outcrop studies with detailed wireline log cross-sections is to further understand the regional stratigraphy, thickness variations, provenance and depositional environments of the Del Rio Formation across the Maverick Basin and west Texas. Eleven outcrops of the Del Rio Formation were measured, and their thicknesses vary from 0′ on top of the Terrell Arch, to 20′-80′ flanking the arch. Hand-held gamma ray profiles were generated for each measured section in order to correlate the formation exposed at the surface with subsurface wireline log signatures. Hand samples were collected of the representative facies within the Del Rio Formation for petrographic, geochemical, and stable isotope analysis. The dominant Del Rio Formation lithofacies is a calcareous shale that is brittle and easily weathered. The Del Rio Formation also contains thin interbeds of sandstone, nodular limestone, and silty laminated skeletal packstone with abundant hummocky cross-stratification. These facies record deposition along a low relief storm-dominated ramp immediately above the Georgetown or Edwards Group. The Del Rio Formation is unique because it records a relatively thick siliciclastic unit sandwiched between the underlying carbonate-rich Edwards/Georgetown Group and the overlying carbonate-rich Buda, Eagle Ford and Austin Chalk. In addition, detrital zircons will be analyzed to determine the provenance of the coarser siliciclastic sediments to test the hypothesis of whether sediment source was local (Llano Uplift; Terrell Arch) or more distal (Trans Pecos; Ouachitas). The prominent siliciclastic unit likely records a long-term sea level lowstand following deposition of the Edwards/Georgetown sediments and prior to the late Cenomanian transgression that deposited the Buda, Eagle Ford, and Austin Chalk units. Variability in the thickness of the Del Rio Formation is likely to irregular paleotopography of the underlying Edwards/Georgetown Group carbonate platform.