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Biostratigraphic age assignment and paleoecological analysis of the sedimentary material recovered on Deep Sea Drilling Project Leg 77 provide a temporal and paleoenvironmental framework for the reconstruction of the Cretaceous and Cenozoic geologic history of the deep southeastern Gulf of Mexico. Planktonic foraminiferal and calcareous nannofossil biostratigraphy, in conjunction with sedimentologic evidence, indicates the sporadic nature of sedimentation during the Türonian through Holocene throughout the study area. This trend is especially well developed within the Upper Cretaceous, which is largely represented only by a thin, condensed section at a single site. The Cenozoic is somewhat better represented, although the thin, unconformity-bound beds characteristic of this interval indicate that sedimentation was still quite sporadic. The record for the Cenomanian and Early Cretaceous is more complete although more variable across the study area. Sites drilled on isolated knolls in the western part of the study area ("basement sites") yield sequences of calcarenites overlain by pelagic sediments. At one site (536), these calcarenites are definitely derived from carbonate platform debris shed into a deeper-water environment. Poor core recovery made it difficult to ascertain whether the calcarenites at the other two basement sites were derived from transported platform debris or in situ outer platform deposition. Two sites drilled on the eastern side of the study area ("basin site") penetrated thick sequences of Cenomanian and Lower Cretaceous hemipelagic deposits. Reworked shallow-water carbonate debris was a consistent and volumetrically important component in these sequences. This fine carbonate debris, probably transported into the basin as distal turbidites, diluted and at times totally overwhelmed the pelagic component of the sediment.