MAPPING LATE CRETACEOUS STAGE BOUNDARIES IN THE BOQUILLAS FORMATION, BIG BEND NATIONAL PARK (BBNP), TEXAS: CENOMANIAN-TURONIAN (CE-T); TURONIAN-CONIACIAN (T-CO); AND CONIACIAN-SANTONIAN (CO-S)

Key index fauna and lithostratigraphy were used to map the CE-T, T-CO, and CO-S stage boundaries in the eastern part of BBNP. The Boquillas Formation 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 boundary (~93.5 Ma) is located to within ~1.5m based on contrasting lithostratigraphy 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. Preliminary micropaleontology results indicate the CE-T boundary is in this carbonate mud interval.

The general location of the T-CO boundary (~88.6 Ma) is based on a distinctive mappable Fe-bearing lithostratigraphic unit, Allocrioceras hazzardi Zone (AHZ), that includes the index fauna C. deformis erectus as well as Scaphites semicostatus, Baculites sp., Didymotis costatus and other unidentified invertebrate species. The occurrence of C. deformis erectus has recently been documented (I.W.) to extend ~7m stratigraphically below the AHZ. Underlying layers are characterized by Late Turonian macrofauna including M. herbichi, M. scupini, M. incertus, M. striatoconcentricus, M. mytiloidiformis and D. costatus. Micropaleontology and oxygen isotope analysis of the T-CO boundary interval is ongoing.

The CO-S boundary (~85.8 Ma) 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. Micropaleontology and oxygen isotope analysis of the CO-S boundary interval is also ongoing.