The Cenomanian–Turonian Eagle Ford Formation of northeastern Mexico: insights on depositional and environmental conditions

Global environmental change during the Cenomanian–Turonian interval caused the widespread deposition of organic-rich mudstones in marine basins. In the southern part of the Western Interior Seaway and the northeastern portion of the Mexican Interior Basin, these rocks are represented by the Eagle Ford unit. A multi-proxy approach was applied to two drill cores of this formation in northeastern Mexico to provide insights into its depositional and environmental history and hydrocarbon source-rock potential. The uppermost part of the underlying Buda Formation was deposited in a carbonate-dominated outer shelf to slope environment characterized by the settling of planktonic organisms from mesotrophic to eutrophic surface waters onto a well-oxygenated seafloor. The organic-rich mudstone of the lower Eagle Ford member began to be deposited between the final part of early Cenomanian and the middle Cenomanian in a slope to open-marine setting with high terrigenous input, and mostly under eutrophic surface and anoxic bottom water conditions. This unit was formed by the settling of fine-grained particles and subsequent reworking by bottom currents. Possible turbidites, as well as soft-sediment deformation caused by density inversion, and synsedimentary faulting associated with down-slope gravity movements, affected the sedimentary record. The bioturbated limestone of the upper Eagle Ford member was deposited in a shallower environment under oxic conditions. Bottom currents were stronger and more active during deposition of the upper part of the Eagle Ford Formation and its transition to the overlying Austin Formation. Bentonite in the Eagle Ford Formation resulted from the alteration of volcanic ash of intermediate composition derived from the Cretaceous Cordilleran arc of western North America. Vertical changes in the gamma-ray trends throughout the studied units correlate with those documented in Texas. The organic matter of the Eagle Ford Formation in the northern part of the Sabinas Basin is typically in the post-mature state.