CONTINUOUS VOLCANISM THROUGHOUT THE DEPOSITION OF THE SOUTH TEXAN EAGLE FORD FORMATION AT THE CENOMANIAN-TURONIAN (93-93.5 MA) BOUNDARY: EVIDENCE FROM LA-ICPMS ZIRCON AGE AND XRD ANALYSIS OF EXPANDABLE CLAYS

We analysed by X-ray diffraction powdered black shale samples of a proprietary core from the South Texan Eagleford Formation. Swelling clays were identified throughout the Eagleford Formation in this core from different depths covering the entire Formation. Smectites (Montmorillonite, Dickite, Nontronite and Glauconite), Vermiculites and (d=10Å) Halloysite clays were identified by X-ray diffraction without any prior chemical treatment. The powdered samples were analysed at a relatively slow scan rate of 3⁰ per minute in the 2θ scan range of 3⁰ to 60⁰.

The swelling or expandable clays were distinctly identified from their characteristic 2θ and d-spacing values matching the intensities for the particular mineral involved. The modal analyses were performed manually matching the peak heights and 2θ values from synthetically prepared standards of known modal proportions. The synthetically prepared standards used for reference were prepared in the laboratory using known modal standard compositions of the minerals proven to exist in the samples, namely calcite, quartz and clays (including illite, kaolinite and montmorillonite). These standards were run on XRD and their peaks were matched to calculate the modal analyses for the sub-surface samples, manually.

Automated modal analyses of the diffraction spectra in the Jade 9.0 software had a strong correspondence both in relative intensities and d-spacings with the manually prepared standards of known modal proportions of minerals. We analysed 98 powdered samples of Eagleford formation by this approach, continually checking manually estimated modal amounts from peak heights with the software-generated analyses.

Subsurface Eagleford sample from different depths corresponding to the entire Eagleford Formation show swelling clays ranging from 2 to 12% all throughout, suggesting continuous volcanism during the deposition of Eagleford. U-Pb age of 93.2±1.66 Ma of the zircons in a subsurface ash bed at the biostratigraphically identified Cenomanian-Turonian boundary in the proprietary core agrees well with the GSSP age of 93 to 93.5 Ma for this boundary.