BIOSTRATIGRAPHIC AGE CONSTRAINTS OF THE BUDA LIMESTONE, BIG BEND NATIONAL PARK, TRANS-PECOS TEXAS

The Buda limestone is an extensive formation spanning much of Texas and southward through Coahuila, Mexico. The Buda limestone lies within Lower Cenomanian ammonite sequences (Cobban et al., 2008). Lock et al. (2007) interpreted the lower contact of the Buda limestone with the Del Rio clay as a subaerially exposed unconformity based on the presence of Del Rio clasts in the basal Buda. The contact between the Buda and the overlying Boquillas fm. is interpreted as a subaerially exposed unconformity based on the presence of a pitted surface at the top of the Buda limestone. This feature is referred to as a microkarst surface (Lock and Peschier, 2006; Lock et al., 2007). It is probable that the Buda limestone is bounded by subaerial unconformities, and that it represents its own depositional sequence; however, ammonites belonging to both the late Lower Cenomanian and the Upper Cenomanian are found at the lowermost Boquillas fm., implying that the Lower Cenomanian fauna are reworked into Upper Cenomanian sediment (Young 1958). Cooper et al. (2008) places the lowermost Boquillas fm. in the late Lower Cenomanian based on ammonites. This work is in agreement with the foraminiferal zonation by Frush and Eicher (1975). The lower Boquillas fm. is thought to have been deposited in anoxic conditions (Frush and Eicher, 1975; Lock and Peschier, 2006). Chronologically correlative sections in Europe show similar evidence of anoxia, and an anoxic Mid-Cenomanian Event (MCE; ca 92ma) has been described, characterized by the onset of deposition of black shale layers, a shift to a more eutrophic planktonic foraminiferal assemblage, and a positive 13C excursion (Coccioni and Galeotti, 2003). This study will identify the planktonic foraminiferal biozones present within the Buda limestone and lowermost Boquillas fm., correlate these units with the timescale of Gradstein et al. (2004), and evaluate the Buda – Boquillas contact, using carbon isotope evidence for the MCE. This study will evaluate the possibility of the Buda – Boquillas unconformity forming by extreme sediment starvation and the formation of hardgrounds, with earlier fauna later being reworked into the basal Boquillas fm. This model allows the Buda limestone and Boquillas fm. to be related in terms of sequence stratigraphy, since eustatic sea level need not have produced an unconformity.