2023 GCAGS Journal, Vol. 12
TITLE
Regional Sequence Stratigraphy, Biostratigraphy, and Facies of the Upper Cretaceous Austin Chalk in South and Central Texas
AUTHOR(S)
Christine Griffith, James Pospichal, Eric de Kaenel, Michael Pope, and Arthur Donovan
ABSTRACT
This regional study constructs a sequence stratigraphic framework for the Upper Cretaceous Austin Chalk in South and Central Texas, based on new nannofossil biostratigraphy, core descriptions, correlations of wireline logs, and synthesis of outcrop, core, and paleontological descriptions from literature. The study area extends along strike from the Rio Grande to Austin, and along dip from the outcrop belt to the limit of downdip well control at the Lower Cretaceous shelf edges. New biostratigraphic control enables correlations across the region and ties the stratigraphic nomenclature from the outcrop to the major producing petroleum fields on either side of the San Marcos Arch. The upper Turonian–lower Campanian Austin Chalk consists of three unconformity-bounded sequences, with a basal unconformity, two internal unconformities, and an overlying unconformity. Sequence boundaries are recognized by nannofossil biostratigraphic hiatuses, which coincide with hardgrounds, Glossifungites ichnofacies, and coarse-grained glauconitic and phosphatic skeletal lags in cores and outcrops. Glauconite-rich beds have regionally extensive log character that overlie angularly truncated section. Facies within sequences can be distinguished as inner ramp (light colored skeletal wackestone, with diverse burrowers and macrofossils), middle ramp (medium gray horizontally burrowed skeletal wackestone), and outer ramp (interbedded dark horizontally burrowed and laminated skeletal wackestone), based on a depositional model of increasing oxygen content in shallower water. Regional cross sections show that the three sequences (AC–I to AC–III) in the Austin Chalk have an extremely asymmetric distribution across the San Marcos Arch. The upper Turonian–lower Coniacian section (AC–I) is very thick west of the San Marcos Arch but is almost completely eroded in the east. The upper Coniacian–Santonian section (AC–II) is very thick east of the San Marcos Arch but is almost completely eroded over the San Marcos arch, and is thin on the west side. The upper Santonian–lower Campanian sequence (AC–III) is more balanced, but similar to the middle sequence. Downdip, the Austin Chalk thins dramatically toward the relict Lower Cretaceous Edwards margin. The Austin Chalk was deposited as a downdip thinning wedge, due to greater carbonate productivity updip and non-deposition or erosion downdip. This interpretation is based on the geometry of the overlying Anacacho Formation, which thickens as the Austin Chalk thins, and internal facies relationships and markers within the Austin Chalk. Petroleum reservoirs in the Austin Chalk occur across a range of inner to outer ramp depositional environments but generally are older on the west side of the San Marcos Arch compared to the east side.
PAGE(S)
45-75
DOI
https://doi.org/10.62371/MBXQ1033
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