U-PB ZIRCON DATING OF VOLCANIC ASH BEDS IN THE EAGLE FORD SHALE: IMPLICATIONS FOR REGIONAL CORRELATION AND OAE2 IN THE SOUTHERN CRETACEOUS WESTERN INTERIOR SEAWAY

The Cenomanian-Turonian Eagle Ford shale was deposited within the Cretaceous Western Interior Seaway (KWIS) and records the onset and duration of the Oceanic Anoxic Event 2 (OAE2). Lozier Canyon in Val Verde County, TX provides a unique opportunity to integrate subsurface data and core with a laterally continuous and exposed outcrop section. Six new U/Pb ID-TIMS zircon ages for volcanic ash beds provide age markers for detailed regional correlations as well as calibration of globally recognizable isotopic events.

Stratigraphic positions in the Eagle Ford are measured in feet (meters) above the contact with the underlying Buda Formation. The stratigraphically lowest dated volcanic ash bed is at 1.5 ft. (~45cm) and yielded an age of 97.14 ±0.41 Ma. Other dated Lower Eagle Ford Formation ash beds are located at 64.5 and 80 ft. (19.7 and 24.3 m), which yielded ages of 95.79± 0.26 and 95.23 ± 0.16 Ma, respectively. The 135 ft. (41.2 m) volcanic ash bed in the Upper Eagle Ford Formation is located within OAE2 and only a few centimeters below a major unconformity surface; it yielded a U-Pb zircon age of 94.65± 0.42 Ma. Two additional volcanic ash beds at 155 and 177 ft. (47.3 and 54 m) yielded ages of 91.88±0.18 and 91.23±0.16 Ma, respectively.

The 1.5 ft. (~45 cm) ash bed is among the stratigraphically lowest dated ash beds within the Eagle Ford, and provides a robust benchmark for tracing the timing of transgression throughout the basin, as well as understanding better how the bathymetry of the underlying Sligo/Edwards margin may have affected the onset of Eagle Ford deposition. Rock accumulation rates, determined by cyclostratigraphic analysis between dated ash beds, are 0.7-1.3 cm/ka for the Lower Eagle Ford and 1.6 cm/ka for the Upper Eagle Ford. Average spectral misfit analysis of core images suggests somewhat lower accumulation rates on the order of 0.6-0.9 cm/ka throughout the analyzed segments of the section. Short eccentricity band-pass filtering indicates six cycles within the OAE2 excursion but with possible hiatuses, they may also include some component of obliquity. Without knowing the duration of the hiatus, either a comparably complete record of OAE2 is present in Lozier Canyon or one that is internally truncated by a hiatus. Regardless, a major disconformity is present above the OAE2 interval, inferred to be up to 1.6 Mya.