Paleoenvironmental significance of benthic foraminiferal biofacies in the Yegua Formation (Middle Eocene), southeast Texas
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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.