Abstract: Foraminiferal Biofacies, Local Zonation and Paleobathymetry of the Hackberry Sequence (Middle Oligocene Frio) of Southwestern Louisiana 

Abstract

ABSTRACT The Hackberry Sequence is a Middle Oligocene (Frio) wedge of deepwater sediments characterized by the Hackberry foraminiferal assemblage. Reevaluation of foraminiferal faunas through high-resolution biostratigraphic studies has increased the known diversity to over 200 species, one of the richest microfaunas in the Gulf Coast Paleogene. Careful evaluation of these diverse assemblages allows recognition of ten foraminiferal biofacies within the Hackberry Sequence. Each biofacies, characterized by distinctive assemblages of key and secondary foraminiferal taxa, has been recognized from recent work in the downdip portion of the Hackberry in Cameron and Calcasieu parishes of southwestern Louisiana. The time of Hackberry deposition also serves as the point of origin or arrival for many long-ranging foraminifera within the Gulf Coast Basin fossil record. Hackberry foraminiferal biofacies are useful for subdividing this sequence into a local zonation scheme, each indicative of a particular paleobathymetric regime. Analysis suggests that the basal to lower biofacies were deposited in depths ranging from lower to middle bathyal. Transported downdip shelf sands in the lower Hackberry are marked by influxes of inner shelf taxa, especially Amphistegina floridana and Elphidium rota. This suggests a correlation of the lower Hackberry to the updip Chickasawhay Formation. Middle and upper Hackberry biofacies range from middle bathyal to deep middle neritic, indicating a gradual in-filling and environmental shallowing throughout deposition of the Middle Oligocene Hackberry Sequence. The biostratigraphic and paleoenvironmental nature of the Pre-Hackberry unconformity is also addressed.