Depositional and Tectonic Evolution of the Middle Miocene Depositional Episode, East-Central Gulf of Mexico

Ricardo Combellas-Bigott¹ and William Galloway^2
1 ChevronTexaco, Bellaire, TX
² University of Texas at Austin, Austin, TX

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 (As the mm depisode is not coincident with the formal time-rock or time unit, I think that middle should not be capitalized) depositional episode. The middle Miocene episode incorporates four genetic cycles (~1 to 2 Ma) bounded by regional maximum flooding surfaces and distal condensed sections. Two long-lived extrabasinal fluvial/delta 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/delta-fed apron systems tract; and (2) a strandplain/shelf/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/mud fan to a large, radial, mixed sand/mud fan. Significant untapped middle Miocene hydrocarbon resources remain in the deep Harang collapse system and sand-rich ponded facies assemblages of the fan system.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005