Abstract

A regional sequence stratigraphic study, based on well-log correlations, outcrop and core observations, and new detailed nannofossil biostratigraphy from three wells across the San Marcos Arch, indicates that the Austin Chalk consists of three composite sequences whose distributions and thicknesses vary greatly across the study area. This sequence stratigraphic framework provides a ‘Rosetta Stone’ to tie the various existing Austin Chalk stratigraphic nomenclature in outcrop and the major producing petroleum fields on either side of the San Marcos Arch. Sequence boundaries are recognized in core by hardgrounds, firm grounds, and lithoclasts, overlain by transgressive glauconitic skeletal packstone beds that coincide with biostratigraphic hiatuses. Glauconitic units have distinctive character in well logs, can be widely correlated and overlie angularly truncated beds. Unconformities at the top and base, as well as two internal unconformities, define the three composite sequences of the Austin Chalk. These stratigraphic sequences are 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 (similar to the underlying Eagle Ford Group). Sequence AC-II (upper Coniacian-upper Santonian) is very thick east of the San Marcos Arch but is eroded in the center and on the west side of the arch. Sequence AC-III (upper Santonian-lower Campanian) thins over the arch and is thicker on the east side. The study also shows that the axis of the San Marcos Arch moved from the eastern study area to west of San Antonio and then to an intermediate position in successive sequences of the Austin Chalk. Down-dip, the Austin Chalk thins dramatically toward the relict Lower Cretaceous Edwards margin. This geometry is interpreted as a depositional ramp based on the coincident thickening of the overlying Anacacho Formation and internal facies and markers within the Austin Chalk. Petroleum reservoirs occur across multiple depositional settings but typically are older west of the San Marcos Arch than to the east.