A regional stratigraphic and structural framework has been established for the middle Miocene sediment-depositional episode from the shelf through the slope to the basin floor for the east-central Gulf of Mexico. Two widespread, transgressive deposits associated with the faunal tops Amphistegina B (15.5 Ma) and Textularia W (12.1 Ma) define the middle Miocene depositional episode. The middle Miocene episode incorporates four genetic cycles (each ∼1–2 m.y.) bounded by regional maximum flooding surfaces and distal condensed sections.

Two long-lived extrabasinal fluvial-deltaic axes, the ancestral Mississippi and the eastern Tennessee systems, provided the bulk of sediments that infilled the middle Miocene depocenters. Salt-related structural provinces controlled the location and configuration of the depocenters. Linked structural systems, dominated by gravity spreading, and an eastern minibasin province, driven by differential subsidence, were established during this period. Two depositional systems tracts characterize the constructional shelf margin of the middle Miocene: (1) a volumetrically dominant mixed-load fluvial-dominated platform delta, shelf-margin delta, and delta-fed apron systems tract; and (2) a strand-plain, shelf, and muddy slope systems tract. However, the offlapping shelf-margin systems were punctuated by a large-scale slope failure, the Harang collapse system, associated with massive salt withdrawal and retreat of delta systems.

A large volume of sediment, funneled by the Harang collapse system, bypassed the slope, initiating a long-lived submarine-fan system. The fan formed in a minibasin corridor and unconfined abyssal plain, approximately 240 mi (384 km) from the active shelf margins. The fan system evolved from a structurally controlled, elongate, sand-rich to mixed sand and mud fan to a large, radial, mixed sand and mud fan.

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