Evolution of the Middle Miocene Submarine Fan East-Central Gulf of Mexico

By

COMBELLAS-BIGOTT, RICARDO I., and GALLOWAY, W.

The University of Texas at Austin, Austin, TX

The Middle Miocene depositional episode (MM depisode) is comprised between the Amphistegina B (15.5 Ma) and Textularia W (12.5 Ma) faunal tops, and records the first major invasion of siliciclastics onto the east-central gulf abyssal plain. A well-organized, sandy submarine fan formed in the modern deepwater east-central Gulf of Mexico during the MM depisode. Eastern displacement of this system from its contemporary deltaic depocenters is remarkable, especially in the early stages of its evolution.

Three genetic sequences bounded by widespread condensed sections record the evolution of the fan. Decreasing percentage of sand and structural control, increasing development of turbidite channel fills, and general westward shift of the sediment dispersal system are characteristic of the MM submarine fan evolution. Initiation of the fan was characterized by bypass of turbidites through submarine canyons located updip of the modern Mississippi canyon. Sediment pathways show preferential WNW-ESE trends controlled by the pre-Louann paleotopography of the basement, preexisting Eocene-Oligocene salt canopies, emplacement of the MM salt canopy in the west, and intraslope minibasins in the east. The fan depositional facies range from sandy, mounded to sheetlike unit.

In the middle stage of the submarine fan evolution, the main sediment dispersal system shifted toward the west. Sediment pathways show preferential NNW-SSE trends. The main depositional facies are erosional-bypass channels, and mixed sand-mud mounded lobes. In the final stage of the submarine fan, sediment trends are N-S. The sediment dispersal system shifted toward the west and east. The bulk of depositional facies are erosional-bypass channels, channel-levee complexes, channelized-mounded lobes, and sandy sheet turbidites.