ABSTRACT

The Eagle Ford Group is a mixed siliciclastic/carbonate unit that records a Late Cretaceous (Cenomanian to Turonian) second order transgression. A major (third-order) condensed interval (cycle 2.5 of the UZA-2 supercycle) is present within the Eagle Ford Group. Eagle Ford strata disconformably overlie the Woodbine Group and are overlain disconformably by the Austin Chalk. The Austin/Eagle Ford contact represents the Turonian/Coniacian boundary (89 ma). The Cenomanian/Turonian (92 ma) boundary occurs within the Eagle Ford Group. Regionally, the Eagle Ford Group consists of two major depositional units: a lower (transgressive) unit dominated by dark well-laminated shales (exhibiting only minor evidence of bioturbation); and an upper (regressive) unit consisting of thinly interstratified (high-frequency cycles) shales, limestones, carbonaceous quartzose siltstones, and numerous bentonites. This study is based on petrographic characterization of limestone strata from Eagle Ford outcrops near Austin and Dallas, Texas.

At Austin, thin limestones are interstratified with uppermost transgressive Eagle Ford shales. These limestones consist of fine-grained, dense (calcite-cemented), foraminiferal (globerigerinid) grainstones. Inoceramids, echinoderms, and fish debris (teeth and bones) are accessory bioclasts. Concentrations of glauconite and collophane nodules occur locally. Microstylolitization and pyritization are common diagenetic features. Concretions (micritic limestones) occur mainly in underlying, organic-rich, transgressive, Eagle Ford shales. Stratigraphically superjacent regressive limestones, known locally as the Kamp Ranch Limestone (Dallas outcrops), consist of arenaceous inoceramid grainstones which contain abundant megafossils (bivalves and ammonites) and trace fossils (Rhizocorallium). Condensed Eagle Ford strata represent poorly-oxygenated, low-energy, marine paleoenvironments of deposition. In contrast, overlying regressive Eagle Ford lithofacies accumulated in relatively high-energy, oxygenated, shallow marine environments.