Regional Sequence Stratigraphy, Biostratigraphy, Facies, and Depositional Environments of the Upper Cretaceous Austin Chalk in South and Central Texas

Abstract

The Upper Cretaceous Austin Chalk in south and central Texas is placed in a sequence stratigraphic framework, based on new detailed nannofossil biostratigraphy from three wells across the San Marcos Arch, core and outcrop observations, and well log correlations. The study extends from the Rio Grande River to Austin and from the outcrop belt to the Lower Cretaceous shelf edges. New biostratigraphic control ties the different stratigraphic nomenclature of the outcrop and major producing petroleum fields on either side of the San Marcos Arch. The Upper Turonian-Lower Campanian Austin Chalk is subdivided into three unconformity-bound composite sequences. Sequence boundaries are recognized in core by hardgrounds and firm grounds overlain by transgressive glauconite skeletal packstone that are coincident with biostratigraphic hiatuses. Glauconitic units have correlative log character and overlie angularly truncated beds. The Austin Chalk has unconformities at the top and base and two internal unconformities. Chalk distribution is extremely asymmetric across the San Marcos Arch. The lowest sequence AC–I (Upper Turonian-Middle Coniacian) is very thick west of the San Marcos Arch but is almost completely truncated in the eastern study area (like the underlying Eagle Ford Group). Sequence AC–II (Middle Coniacian-Santonian) is very thick east of the San Marcos Arch but is eroded on the west side of the arch. Sequence AC–III (Upper Santonian-Lower Campanian) thins over the arch, but extends across it. The axis of the San Marcos Arch moved from the eastern study area, to west of San Antonio, to an intermediate position in successive sequences of the Austin Chalk. Downdip, the Austin Chalk thins dramatically downdip toward the relict Lower Cretaceous Edwards margin. This geometry is interpreted as a depositional ramp based on concurrent thickening of the overlying Anacacho Formation and internal facies and markers with the Austin Chalk. Facies maps based on stratigraphic framework show evolution of the shelf during Austin Chalk deposition, with paleobathymetry ranging from subaerial to greater than 1000 feet (300 m).