ABSTRACT

The Upper Miocene depositional episode (UM depisode) records a long-lived family of sediment dispersal systems that persisted with little modification for nearly 6.5 m.y. In the central Gulf of Mexico Basin, this depisode records extensive margin offlap, primarily centered on the paleo-Tennessee and Mississippi River dispersal axes, that began immediately following the Textularia W/Textularia stapperi flooding and was terminated by a regional flooding event associated with the Robulus E biostratigraphic top. Using four additional flooding events associated with Cibicides carstensi, Discorbis 12, Cristellaria K and Bigenerina A biostratigraphic tops, the UM genetic sequence can be subdivided into five genetic subsequences, which display an overall progradational stacking pattern. Thickest sediments were deposited in the paleo-Tennessee River delta beneath the modern SE Louisiana, where three major depocenters were recognized. These depocenters migrated in both strike and dip directions, and margin progradation was very prominent. There were also sediments continuously bypassed into the Mississippi Canyon and Atwater Valley OCS area throughout the entire UM, forming a secondary depocenter at the base of the paleo-continental slope. The composite fluvial-dominated paleo-Tennessee delta system rapidly built beyond the subjacent middle Miocene shelf margin to construct a sandy delta-fed apron. Margin outbuilding was locally and briefly interrupted by hypersubsidence due to salt withdrawal and consequent slope mass wasting. Adjacent deep Gulf floor deposition was dominated by culmination of the east Gulf McAVLU submarine fan system, which spread beneath the Atwater Valley OCS area. A broad, but relatively thin, sandy shore zone and clastic shelf succession, supplied by reworking of the deltaic deposits, extends eastward from the delta system. In the west-central Gulf, adjacent to the paleo-Mississippi delta system, abundant strike-reworked sediment locally prograded the strand plain to the shelf edge, with slope offlap exceeding 30 mi.