USING GSSP GUIDELINES FOR THE CENOMANIAN-TURONIAN (CE-T); TURONIAN-CONIACIAN (T-CO); AND CONIACIAN-SANTONIAN (CO-S) STAGE BOUNDARIES, BOQUILLAS FORMATION, BIG BEND NATIONAL PARK (BBNP), TX

The Late Cretaceous Boquillas Formation in BBNP consists of marine limestone and carbonate mud/shale deposited in the southern part of the Western Interior Seaway from the late Early Cenomanian to Middle Santonian. The CE-T, T-CO, and CO-S stage boundaries were identified based on regional and detailed geologic mapping, globally recognized index macrofauna, distinctive lithostratigraphic units and comparison to established and proposed GSSP.

The CE-T boundary is located to within ~1.5m in BBNP based on contrasting lithostratigraphic intervals and macrofauna. The lower Late Cenomanian (I. pictus and I. ginterensis) interval is separated from a limestone layer with Early Turonian (M. puebloensis and M. goppelnensis) fauna by ~3m of carbonate mud with no distinguishing macrofauna. Preliminary micropaleontology results indicate the CE-T boundary is in this carbonate mud interval.

The T-CO boundary is based on a distinctive mappable Fe-bearing lithostratigraphic unit and the abrupt appearance of the index fauna C. deformis erectus and heteromorphic ammonite species including Allocrioceras hazzardi. Underlying layers are characterized by Late Turonian macrofauna including M. scupini, M. herbichi, M. incertus, and M. striatoconcentricus. The Allocrioceras hazzardi Zone, within the Fe-bearing lithostratigraphic interval, is a distinctive ~1.27m thick interval with a unique faunal assemblage that includes A. hazzardi, C. deformis erectus, Scaphites semicostatus, Baculites sp., several Belemnite sp., Didymotis variabilis as well as other invertebrate species. We proposed this interval as a candidate for the T-CO GSSP in 2008 to the International Union of Geological Sciences Subcommission on Cretaceous Stratigraphy as described by I. Walaszczyk and C.J. Wood at the 33^rdIGC Meeting in Oslo, Norway.

The CO-S boundary is based on three lithostratigraphic layers and the macrofauna associated with two of these layers. The lower layer (uppermost Late Coniacian) is a distinctive mappable white limestone with P. cycloides, M. complicatus, I. anomalous, and P. americanus. It is overlain by ~1m of carbonate mud with no identifiable macrofauna which is overlain by a 10-20cm thick mappable limestone layer packed with the Early Santonian index fauna C. undulatoplicatus as well as a Sphenoceramus sp.