ABSTRACT: Limestone Microfacies and Sequence Stratigraphy: Eagle Ford Group, North-Central Texas Outcrops

DAWSON, WILLIAM C.

The Eagle Ford Group is a mixed siliciclastic/carbonate unit that records a Late Cretaceous (Cenomanian-Turonian) second order transgression. Eagle Ford deposition occurred within a major period of condensation during cycle 2.5 of the UZA-2 supercycle. Eagle Ford strata are overlain disconformably by the Austin Chalk, and the Eagle Ford-Austin contact represents the Turonian-Coniacian boundary. Regional lithostratigraphic variations allow the recognition of two major Eagle Ford depositional packages: lower Eagle Ford (transgressive) unit dominated by dark well-laminated shales exhibiting minor evidence of bioturbation; and upper Eagle Ford (highstand/regressive) unit consisting of thinly interstratified limestones, shales, carbonaceous quartz siltstones, and numerous bentonites. In outcrops near Austin and Dallas, thin limestones are conspicuous within regressive Eagle Ford units. Petrographically, basal regressive Eagle Ford limestones (Austin outcrops) are fine-grained, dense (calcite-cemented), argillaceous, inoceramid grainstones. Foraminifers, echinodems, and fish debris are accessory bioclasts. Concentrations of glauconite and collophane occur locally. Microstylolitization and pyritization are common diagenetic features. Stratigraphically superjacent regressive limestones (Dallas outcrops) consist of arenaceous, foraminiferal, inoceramid grainstones which contain abundant megafossils (bivalves and ammonites) and trace fossils (e.g., Rhizocorallium). Lenses of peloidal grainstones occur in underlying transgressive shales. Eagle Ford limestones are microporous and could yield oil shows. However, because of inherently low permeabilities and low volumetrics, Eagle Ford limestones appear to have limited reservoir potential.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana