ORGANIC MATTER DEPOSITION AND PALEOENVIRONMENTAL CONDITIONS DURING THE TURONIAN-CONIACIAN ON THE COAHUILA BLOCK (NE MEXICO)

Organic-rich limestone and mudstone were accumulated on the Coahuila Block (northeast Mexico) during the Turonian-Coniacian. They are represented by the Indidura Formation, which is equivalent to the Eagle Ford Formation deposited in the Sabinas Basin and South Texas. In this study, the paleoenvironmental conditions and hydrocarbon source rock potential of the Indidura Formation are investigated based on the sedimentological, petrographic, and geochemical analysis of the Las Brujas Section (80 m thick, Cuatro Ciénegas-Coahuila). The lower and middle parts of the section (0–42 m) consist of an alternation of black to dark-gray laminated mudstone and limestone (wackestone/packstone), poorly to moderately bioturbated (Bioturbation Index-BI: 1–3), with oxidized horizons, and pyrite occurring as disseminations and thin horizons. Furthermore, they contain abundant calcispheres and planktonic foraminifera, and exhibit high concentrations of TOC (up to 1.76%), detrital elements (Al, Ti, Zr), redox-sensitive (U, V, Mo), and productivity-sensitive (Ni, Cu, Zn) trace elements. The T_max values from Rock-Eval indicate that the preserved organic matter is immature to the early mature (oil generation window, T_max: 416 to 447 °C). The middle-upper part of the section (42–70 m) is characterized by an intercalation of laminated mudstone with bioturbated limestone (packstone, BI: 3–5) poor in organic matter (TOC < 0.17%) and low trace elements concentrations. A dramatic facies change occurs at the top of the section (70–80 m), represented by thick sandy-limestone with abundant inoceramids, common ammonites, and planktonic foraminifera. The results suggest that the accumulation of organic-rich sediments was favored by an increased detrital input accompanied with nutrients that led to eutrophication and anoxic/dysoxic bottom waters. The δ¹³C_org values range from -27.03 to -23.46 ‰. The resulting curve shows positive excursions that likely reflect the occurrence of Turonian–Coniacian episodes of accelerated global change, mainly the Late Turonian-Coniacian Event (LTCE). This event is characterized by a positive excursion of ~3‰ at the top of the section.