Foraminiferal data analysis and lithofacies analysis of a three-well transect through the Middle Eocene Yegua Formation in southeast Texas provide insights into the depositional and paleoenvironmental history of the Gulf of Mexico Basin. Vertical and downdip changes in the lithology of the Yegua Formation in the three wells represents the depositional environments of a delta system that prograded onto the continental shelf, updip from the shelf margin. Two progradational episodes and two marine transgressions of the Yegua delta system occurred within this interval of the Yegua Formation in southeast Texas. Factor analysis of benthic foraminiferal census data reveals five major recurring assemblages of benthic foraminifera. These assemblages, or biofacies, occupied environments ranging from marginal marine to normal marine, middle-to-outer shelf environments. The stratigraphic relationships of the five biofacies show paleoenvironmental complexities that are not readily apparent from the lithofacies analysis. Integration of lithologic data and nonforaminiferal paleontologic data with the foraminiferal data produces a detailed paleoenvironmental reconstruction of the Yegua shelf in dip direction. Comparison of the foraminiferal data from the Yegua Formation with modern foraminiferal data from the Gulf of Mexico indicates that several properties of modern foraminiferal assemblages are similar to the foraminiferal assemblages of the Yegua Formation. Generic predominance, species diversity, and planktic to benthic ratios of modem foraminiferal assemblages can be used to help determine the paleoenvironmental significance of the Yegua foraminiferal assemblages. These properties of modern foraminiferal assemblages are not exact analogs for Middle Eocene assemblages and should be applied with caution. / text

Within Big Bend National Park, the unconformable contact between the Buda Limestone and the overlying Boquillas Formation represents the Commanchean-Gulfian boundary. Previous studies of the geochronology of this interval have relied primarily on provincial ammonite faunas rather than foraminifera, and place the Buda and basal Boquillas in the Lower Cenomanian. Because of its indurated nature, a comprehensive foraminiferal biozonation has not been acquired for the Buda Limestone. Recent revisions to Cretaceous foraminiferal biozonations and taxonomies necessitates a new biostratigraphic study of the Buda - Boquillas interval. The overlapping ranges of F. washitensis, G. bentonensis, G. caseyi, P. appenninica, P. delrioensis, P. stephani, and R. montsalvensis place the Buda within the upper portion of the Early to Middle Cenomanian Th. globotruncanoides Zone. Microkarst found on the surface of the Buda Limestone has been interpreted as representing a subaerial exposure and sequence boundary. However, microkarst-like features can result from subaqueous or intrastratal processes. Carbon and oxygen stable isotope analysis of the lower and middle Buda has indicated a mean δ13C value of 1.73‰ VPDB, which is in line with other values reported from the Lower Cenomanian. The top 2.6m of Buda contains a 0.62‰ negative δ13C shift from 1.88‰ VPDB to 1.26‰ VDPB in a 40 cm interval, expected if subaerial exposure occurred. Higher variation in measured carbon isotope values beneath the contact also lend evidence for meteoric alteration. The standard deviation in δ13C values from the top 2.8 m of the Buda is 0.207, which is 2.16 times larger than the rest of the studied section at 0.096. The Buda contains a shallow pelagic-dominated fauna of heterohelicids (45-90%), globigerinellids (3-37%), and hedbergellids (4-22%). Intermediate-depth globigerinellids display an initial increase followed by a marked decrease in abundance upsection, interpreted as sea level transgression and regression, respectively. The lower contact of the Buda with the Del Rio Clay has been previously interpreted as a subaerial exposure, and a P:B break from ~0% planktonics in the upper Del Rio to ~80% in the Buda supports this claim. This study therefore interprets both the upper and lower contacts of the Buda as sequence boundaries. The overlying 1.2 m Boquillas is nearly devoid of benthics and represents a deeper assemblage including the double-keeled Dicarinella sp., as well as several Upper Cenomanian (D. algeriana Subzone) species. Based on foraminiferal data, the duration of the Buda - Boquillas unconformity is roughly equivalent to the missing Th. reicheli and Th. greenhornensis Biozones, or a sizable portion of the Middle Cenomanian. / Systematic paleontology -- Biostratigraphy of the Buda Limestone -- Biostratigraphy of the lowermost Boquillas Formation -- Stable isotope geochemistry. / Department of Geological Sciences