Sequence Stratigraphy and Depositional Controls on the Upper Cretaceous Austin Chalk across the San Marcos Arch in South and Central Texas

Austin Chalk correlations have long been challenging due to extreme thinning across the San Marcos Arch. Regional well-log correlations, outcrop and core descriptions, and detailed nannofossil biostratigraphy from three wells across the San Marcos Arch indicate the Austin Chalk in south and central Texas can be subdivided into three asymmetric composite sequences. Sequence boundaries are recognized by hardgrounds and firmgrounds, overlain by transgressive glauconite skeletal packstone beds coinciding with biostratigraphic hiatuses, and underlain by angularly truncated beds. The lowest sequence is very thick west of the San Marcos Arch but almost completely eroded to the east (like the underlying Eagle Ford Formation). The upper two sequences are very thick east of the arch, eroded over the arch, and remain thin on the west side. Several million-year hiatuses separate sequences.

Facies indicate increased oxygen and water circulation updip. Downdip, the Austin Chalk thins dramatically toward the relict Lower Cretaceous Edwards margin. This geometry is interpreted as a depositional ramp based on (1) the coincident thickening of the overlying Anacacho Formation, (2) Austin Chalk sequences with organic-rich facies at the base that become more oxygenated upsection, and (3) limited seismic data. The depositional ramp was formed through sediment redistribution by bottom currents on relict topography associated with continuous Gulf-ward subsidence. Texas, as the gateway to the Western Interior Seaway, reflects far field effects of Cordilleran tectonics. New correlations indicate water circulation entered through south Texas through the late Coniacian (~86 MY), but switched to pass through east Texas afterward.