Microfacies and U–Pb ages of Late Cretaceous gas shales in the Sabinas and Parras basins, NE Mexico

Abstract

This work intends to observe the changes in the sedimentation and fauna patterns of the Eagle Ford and Indidura Formations that outcrop in the state of Coahuila. These sedimentary successions were deposited during the Cenomanian-Turonian within the Sabinas basin and are composed of clayey, carbonated, bioclastic, and sandy facies. Nine microfacies (Mf) were described: Mf-1 is formed of mudstone, Mf-2 of shale and wackestone with opportunistic foraminifera, Mf-3 of wackestone from specialist foraminifera; Mf-4 of wackestone and packstone of radiolaria, Mf-5 of wackestone and packstone with filaments, Mf-6 of peloidal grainstone, Mf-7 of bioclastic grainstone, Mf-8 of calcarenites, and Mf-9 of interbedded volcanic tuffs in the sedimentary succession. The occurrence of the Oceanic Anoxic Event (OAE-2) is present within the sedimentary succession, indicated by the deposits of black shale rich in organic matter, the massive death of planktonic foraminifera, abundant filaments, crystals of pyrite, and high concentrations of redox elements such as V, Mo, and Mn. However, the deposit conditions present a variation in the concentration of oxygen, micronutrients, and organic matter throughout the paleogeography. Elements such as Ni and Zn in microfacies 1, 2, 3, and 4 show high productivity, which is associated with the presence of planktonic or radiolarian organisms. Other chemical elements such as Al, K, and Zr show an increase in detrital material within the basin, associated with clayed and sandy microfacies. The organic matter content is higher for microfacies 2 to 5, and it is considered of autochthonous origin. The variation in the oxic-anoxic conditions is notorious due to the planktonic-benthic foraminifera ratio and the geochemical relationship between the Mo, Va, Ni, and S elements. In the Indidura Formation, the ages of U–Pb in zircon grains from tuff layers restrict the deposit between 98.2 ± 0.9 Ma to 91.7 ± 0.8 Ma, during Cenomanian-Turonian, although the presence of Praeglobotruncana at the base restricts the deposit to the late Cenomanian-Turonian. While the age of the Eagle Ford Formation is indicated by the Cenomanian-Turonian genera of Rotalipora cushmani, Hedbergella, and Helveglobotruncana helvética.This work intends to observe the changes in the sedimentation and fauna patterns of the Eagle Ford and Indidura Formations that outcrop in the state of Coahuila. These sedimentary successions were deposited during the Cenomanian-Turonian within the Sabinas basin and are composed of clayey, carbonated, bioclastic, and sandy facies. Nine microfacies (Mf) were described: Mf-1 is formed of mudstone, Mf-2 of shale and wackestone with opportunistic foraminifera, Mf-3 of wackestone from specialist foraminifera; Mf-4 of wackestone and packstone of radiolaria, Mf-5 of wackestone and packstone with filaments, Mf-6 of peloidal grainstone, Mf-7 of bioclastic grainstone, Mf-8 of calcarenites, and Mf-9 of interbedded volcanic tuffs in the sedimentary succession. The occurrence of the Oceanic Anoxic Event (OAE-2) is present within the sedimentary succession, indicated by the deposits of black shale rich in organic matter, the massive death of planktonic foraminifera, abundant filaments, crystals of pyrite, and high concentrations of redox elements such as V, Mo, and Mn. However, the deposit conditions present a variation in the concentration of oxygen, micronutrients, and organic matter throughout the paleogeography. Elements such as Ni and Zn in microfacies 1, 2, 3, and 4 show high productivity, which is associated with the presence of planktonic or radiolarian organisms. Other chemical elements such as Al, K, and Zr show an increase in detrital material within the basin, associated with clayed and sandy microfacies. The organic matter content is higher for microfacies 2 to 5, and it is considered of autochthonous origin. The variation in the oxic-anoxic conditions is notorious due to the planktonic-benthic foraminifera ratio and the geochemical relationship between the Mo, Va, Ni, and S elements. In the Indidura Formation, the ages of U–Pb in zircon grains from tuff layers restrict the deposit between 98.2 ± 0.9 Ma to 91.7 ± 0.8 Ma, during Cenomanian-Turonian, although the presence of Praeglobotruncana at the base restricts the deposit to the late Cenomanian-Turonian. While the age of the Eagle Ford Formation is indicated by the Cenomanian-Turonian genera of Rotalipora cushmani, Hedbergella, and Helveglobotruncana helvética.