Upper Miocene depositional history of the Central Gulf of Mexico basin
The Upper Miocene (late Middle to early late Miocene) depositional episode, defined by two widespread, transgressive deposits associated with biostratigraphic tops Textularia W (12.0 Ma) and Robulus E (6.2 Ma), records a long-lived family of sediment dispersal systems that persisted for nearly 6 m.y. with little modification. It corresponds to a prolonged lowstand after a dramatic sea-level fall at the end of the middle Miocene and is punctuated by three additional flooding events associated with Cibcides carstensi, Discorbis 12 and Cristellaria K biostratigraphic tops, which subdivide the Upper Miocene depositional episode into four secondary depositional episodes. In the east-central Gulf of Mexico, the Upper Miocene genetic sequence records extensive continental margin offlap, primarily centered on the ancestral Tennessee River and Mississippi River dispersal axes. Thickest sediments were deposited in the Tennessee River delta beneath modern SE Louisiana. The composite fluvial-dominated, wave-modified Tennessee and Mississippi delta system rapidly built beyond the subjacent Middle Miocene shelf margin to construct a sandy delta-fed apron that ultimately spilled distal sandy turbidites onto the adjacent basin floor. Margin outbuilding was locally and briefly interrupted by hyper-subsidence due to salt withdrawal and consequent slope mass wasting, forming numerous depocenters separated by salt massifs and ridges, and various linked structural systems. A broad, but relatively thin, sandy strandplain and clastic shelf succession, supplied by reworking of the deltaic deposits, extends both eastward and westward from the delta system. Abundant strike-reworked sediment locally prograded the strandplain to the shelf edge, with slope offlap exceeding 30 mi (50 km). A large volume of sand continuously bypassed the confined minibasin and upper slope at the flank of active deltaic depocenters into the Mississippi Canyon, Atwater Valley and Green Canyon OCS areas throughout the entire Upper Miocene, forming a linked, long-lived McAVLU intraslope and abyssal fan system in the primary minibasin corridor of the lower slope and basin floor. Sand bypass and subsequent deposition was dominantly through the slope canyon and intraslope fan systems developed in the east-central Gulf. Canyons, fan valleys, leveed channels, gravity mass transport complexes and structural conduits that focused sediment flow provide various sediment transport elements.