Abstract

The Cenomanian–Turonian Eagle Ford Shale of south Texas occupies an important gateway between the Western Interior Seaway (WIS) of North America and the Gulf of Mexico. While the Eagle Ford north of the San Marcos Arch and its stratigraphic equivalents to the east of the Sabine Arch are shallow-water sediments dominated by terrigenous clastics, the more distal localities in south Texas are dominated by hemi-pelagic carbonates draped over an Early Cretaceous carbonate platform, called the Comanche Platform, and adjacent submarine plateaus and basins. This region was strongly affected by major oceanographic changes during the Cenomanian-Turonian, particularly a significant transgression that drove localized upwelling and organic matter burial in the Lower Eagle Ford prior to the global Oceanic Anoxic Event 2 (OAE2). These pre-OAE2 organic-rich shales are the basis of Eagle Ford shale gas play, which has spurred commercial and academic research into many aspects of the geology of the Eagle Ford Group. Much of this research has been fairly locally focused, and little effort has been made to understand the timing of events across the platform. We compared new data from three study sites across south Texas—Lozier Canyon in Terrell Co.; Bouldin Creek in Travis Co., near the San Marcos Arch in the center of the Comanche Platform; and Swift Energy's Fasken Core in Webb Co., off the platform on the Rio Grande Submarine Plateau—as well as published data from near Big Bend National Park on the western margin, and from Atacosta and Karnes counties on the eastern margin. Using these data we document the occurrence of key foraminiferal species across the platform and present a regional biostratigraphic scheme incorporating five global planktic foraminiferal zones (and contemporaneous occurrences that may serve as proxies for the zonal markers, which tend to be rare in Texas) and four local origination or acme events that serve as useful secondary markers. The succession of events is: 1) highest occurrence (HO) Favusella washitensis, 2) lowest occurrence (LO) Rotalipora cushmani, 3) “Benthonic Zone”, 4) HO R. cushmani and/or Thalmaninella greenhornensis, 5) “Heterohelix shift”, 6) LO “Anomalina W”, 7) LO Helvetoglobotruncana helvetica, 8) HO H. sp., and 9) LO Dicarinella concavata. Overall, we show that lithologic and geochemical trends through most of the Eagle Ford, particularly the oxygenation at the onset of OAE2 and the concurrent shift to more carbonate-rich lithologies, are synchronous across the Comanche Platform. However, the transition from the Eagle Ford Group to the Austin Chalk varies in age. While Austin Chalk deposition began in the middle Turonian Marginotruncana schneegansi Zone on the Rio Grande Submarine Plateau, a transgressive surface on the Comanche Platform (known as the “Rubble Zone” in central Texas) represents a condensed interval at the top of the Eagle Ford that ends in the upper Turonian D. concavata Zone. This is part of a transgressive disconformity that extends north through the WIS, where it is associated with the Juana Lopez Calcarenite.

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