A GLOBAL HIGH-RESOLUTION 87SR/86SR SEAWATER CURVE FOR THE SOUTH TEXAS EAGLE FORD BLACK SHALE DEPOSITION DURING LATE CRETACEOUS VOLCANISM, THE C-T BOUNDARY AND THE OAE2

The mineralogy, major and trace elements of the Eagle Ford (EF) ash beds suggest their source from nearby arc-derived calc-alkaline volcanism. The average ε_Hf(T) = +3.5 of the analyzed ash bed zircons clearly indicate a mantle component in the host magmas of the zircons. Petrographic analyses of shales away from the visible tuff layer show phenocrysts of biotite, alkali feldspar and andesitic rock fragments. The whole rock shale and ash beds with similar concentration of some transition metals indicate contemporaneous arc volcanic activity at the time of shale deposition. XRD analysis of subsurface Eagle Ford bulk shales samples from different depths in 4 cores, show volcanogenic clays, such as montmorillonite, vermiculite, dickite and halloysite 10Å, ranging from 2 to 12% in modal abundance.

Zircons from EF cores yield ages of 93.7±1.9 Ma, 93.2±1.66 Ma and 94.13±1.25 Ma, consistent with the Cenomanian-Turonian (C-T) age of deposition in three contiguous cores, spatially separated by 140 miles. Our zircon ages from volcanic ash study shows that the Eagle Ford was deposited during 10Ma, at a rate of 28ft/Ma covering the Oceanic Anoxic Event 2 (OAE2) at the C-T boundary.We report high-resolution global ⁸⁷Sr/⁸⁶Sr seawater curve derived from separated primary carbonates for a 300ft long subsurface EF core. The ⁸⁷Sr/⁸⁶Sr ratios differ from that found in common literature, showing less radiogenic influx of ⁸⁷Sr/⁸⁶Sr than in the global marine Sr-isotope curve.

The continuous volcanism during the EF deposition of volcanic silicic sediments and carbonates was part of the global continental arc flare-ups in the Cretaceous, responsible for greenhouse conditions and subsequent anoxia during shale deposition. ⁸⁷Sr/⁸⁶Sr sea water curve derived from 28 close spaced carbonates in black shales of EF displays a less radiogenic input of ⁸⁷Sr/⁸⁶Sr during C-T event and leading up to it, in comparison to the most recent global ⁸⁷Sr/⁸⁶Sr sea water curve for the C-T boundary, established by Jenkyns et al. (2016) from platform carbonates in Italy. At the C-T boundary both our EF and Jenkyns et al, Sr-isotopes are identical (0.707454) but at the lowermost Turonian our Sr isotope data are less radiogenic (0.707414) than Jenkyns' (0.707535). In addition, our data extend to the base of Cenomanian increasing gradually to 0.707616. Thus our Sr-isotope data clearly show a decrease of ⁸⁷Sr/⁸⁶Sr at the C-T boundary, probably due to an arc input and remain there throughout the Turonian.