Calc alkaline volcanism-aided phytoplankton growth and organic matter production of the Cenomanian-Turonian Eagle Ford Formation of South Texas

The kerogen-rich Eagle Ford (EF) black shales of south Texas formed in the Late Cretaceous, overlapping the Cenomanian-Turonian (C-T) boundary and the Oceanic Anoxic Event 2 (OAE2). This study proposes a continuous calc-alkaline volcanogenic model of oil-generation for the EF by petrographic, geochemical, and geochronological studies of sub-surface and surface EF shale cores. Biotite, alkali feldspar phenocrysts, andesitic rock fragments and swelling clays observed, and X-ray diffraction (XRD) analyses of bulk samples indicate continuous volcanism during EF deposition. Zircons yield U-Pb ages of 93.2 ± 1.66 Ma, 94.13 ± 1.25 Ma and 93.7 ± 1.9 Ma, consistent with the C-T age of deposition. The εHf(i) of zircons averaged at +1.23, indicating a crustal component in the arc volcanism. In the 187Os/188Osinitial at 93 Ma, the time of EF deposition is 2.47–5.11 indicating a continental-crustal origin of the osmium (Os). Continuous arc volcanism during the EF deposition of volcanic-silicic sediments and carbonates was simultaneously a part of the Cretaceous global continental arc flare-ups, responsible for greenhouse conditions and anoxia during black shales deposition. We conclude that the black shale of the EF is dominated by lamalginites (unicellular solitary or colonial phytoplankton), produced during algal blooms falling as ‘marine snow’ augmented by calc-alkaline volcanism at the C-T boundary.