Biostratigraphy and geochronology of the Eagle Ford shale, Austin chalk, and lower Taylor marl in Texas based on calcareous nannofossils

Abstract

Three hundred and twenty-one (321) samples from 15 type and reference sections of strata of the Eagle Ford, Austin, and lower Taylor (Cenomanian to early Campanian) in central, north-central, and northern Texas were examined for calcareous nannofossils. The nannofloras were compared with those of European Cretaceous stage stratotypes. The result confirms that, with minor modifications, Sissingh's (1977) zonation can best tit the floral succession in Texas. The Cenomanian/Turonian boundary coincides with the base of the Quadrum gartneri Zone (zone 11) defined as the first occurrence of Quadrum gartneri. It also approximates the simultaneous disappearance of Axopodorhabdus albianus, Helenea chiastia. Cretarhabdus loriei. Lithraphidites acutum. Lithraphidites eccentricum and Rhagodiscus asper. The boundary is recognized in the Eagle Ford. It is disconformable and is precisely located at 1.5 ft below the top of the Bouldin Member in central Texas, 3.5 ft below the top of the flaggy Cloice in north-central Texas, and 10-15 ft below the top of the Britton Clay in northern Texas. The Turonian/Coniacian boundary is above the entry of Marthasterites furcauts. The Coniacian/Santonian boundary coincides with the top of the Micula decussata Zone (zone 14) redefined as the last occurrence of Lithastrinus septenaruis, which in Texas is the nearest datum to the entry of the Santonian ammonite Texanites reported from the basal Vinson of the Austin Group. The Santonian/Campanian boundary is below the entry of Aspidolithus parcus parcus at the base of the Phanulithus obscurus Zone (zone 17), which in central Texas is within the upper Dessau of the Austin Group. With precise age-controls from calcareous nannofossils, integrated with lithology depositional sequences in the studied interval also can be delineated and which can then be compared with eustatic cycles of Haq et al (1987). The result shows that the Eagle Ford is correlated with Cycles 2.5, 2.6, and 2.7 of Supercycle UZA-2, the Austin with Cycles 3.1, 3.2, 3.3, 3.4, and 3.5 of Supercycle UZA-3, and the lower Taylor with Cycle of 4.1 of Supercycle UZA-4...