ALBIAN-CENOMANIAN (CRETACEOUS) SHALE-ON-CARBONATE SEQUENCE BOUNDARIES: TEXAS AND MEXICO

Carbonate sequence boundaries in the Gulf area are key regional correlation horizons and potential indicators of paleoclimate where caliche or paleokarst are preserved. Three lithostratigraphic contacts in the Cretaceous upper Albian to lower Cenomanian marine succession in Texas and northern Mexico are traced across an area of 1.2 million km². On the Comanche Shelf these contacts represent regional sea-level rise and indicate paleoenvironmental change from clear shallower shelf to muddy deeper shelf. In the Chihuahua Trough these contacts indicate increased turbidity in basin water masses. Across these contacts biofacies and paleocommunities changed as water depth and turbidity increased. The lower upper Albian sequence boundary overlies the Dipoloceras cristatum Ammonite Zone and underlies the Hysteroceras inflatum Zone between the Fredericksburg and Washita groups and their regionally correlative formations. The next younger contact at the Albian/Cenomanian boundary between the Drakeoceras drakei and Graysonites adkinsii zones is in the upper part of the Washita Group between the Georgetown and Del Rio formations. The third regional contact at the lower-middle Cenomanian substage boundary separates the Budaiceras hyatti and Acanthoceras wintoni ammonite zones between the Buda Limestone and the Eagle Ford Group.

Carbon isotope chemostratigraphy documents subaerial exposure of these contacts in inner shelf sections. Following exposure the contacts were drowned and became bored hardgrounds. At the shelf margin and in basin sections the contacts are bored submarine hardgrounds or abrupt lithologic changes.