LAUSR

Abstract The Albian–lower Cenomanian Comanchean Series on the Comanche Shelf, Texas and northern Mexico is a laboratory to calibrate the numerical ages of mixed carbonate-siliciclastic cycles. The stratigraphic contacts of the long-term cycles formed during subaerial exposure followed by marine drowning. Correlation of these cycle contacts in different basins tests their synchroneity and the hypothesis of eustasy. Petrographic and geochemical evidence of Early Cretaceous meteoric diagenesis differs across the shelf because the Comanche Shelf spanned the subtropical arid belt and the mid-latitude warm humid zone. The contact between the Fredericksburg and Washita groups is Al SB Wa1, which is a composite subaerial unconformity and transgressive drowning surface that is traced from southeastern Oklahoma to El Paso and down-dip to the shelf margin. Chronostratigraphy of the Comanchean Series Fredericksburg and Washita groups is constrained by multiple biostratigraphic data of ammonites, caprinid rudists, and foraminifers. The lower upper Albian Al SB Wa1 is dated by the Dipoloceras cristatum Zone below and the Mortoniceras pricei Zone above. Caprinid zones in carbonate facies are integrated with ammonites and foraminifers to provide a complementary correlation criterion. This contact is coeval with a condensed interval in Europe and flooding sequences in western Asia and northern Africa. Depleted carbon isotope ratios indicate subaerial exposure below sequence boundaries at base of Washita Group, at base of the lower Cenomanian Del Rio Formation and base of the Cenomanian Woodbine-Eagle Ford groups. These contacts are coeval with sequences in Europe, Africa, Asia, and South America. Late Albian–early Cenomanian sea-level changes provide a chronologic scale for significant oceanic plate reorganization dated at about 100 Ma.