Pleistocene shelf-margin delta: Intradeltaic deformation and sediment bypass, northern Gulf of Mexico

Recent studies of shelf-margin deltas in the Gulf of Mexico lack three-dimensional (3-D) seismic interpretation that allows prediction of lithology distribution, deformation features, and overall reservoir heterogeneity, and only a few 3-D studies of shelf-margin deltas in other parts of the world exist. This paper examines the 3-D geomorphology and internal facies architecture within a sequence-stratigraphic framework of a shelf-margin delta in a salt dome minibasin, within the Gulf of Mexico, offshore Louisiana, using 3-D seismic data. The 3-D data also allow an evaluation of the potential pathways for sediment bypass from delta front to slope and deep-water depositional environments.

The delta consists of a series of offlapping sandy clinoforms, interpreted as being associated with a prolonged forced regression and ensuing lowstand of eustatic sea level, related to oxygen isotope stage 6, which lasted for about 45 k.y. The lowstand delta is onlapped by a transgressive mud wedge and is underlain and capped by regionally persistent highstand mudstones. The central part of the delta shows intradeltaic syndepositional growth faults, likely triggered by deposition of heavy delta-front sands over mobile prodeltaic muds, reflecting the river-dominated nature of the delta. A postdepositional shallow-water mass transport complex (MTC), which moved almost perpendicular to the direction of the regional delta progradation, remobilizes the deltaic deposits. The MTC was likely induced by uplift of the adjacent western salt dome. We hypothesize that the high degree of syndepositional and postdepositional deformation is related to the prolonged nature of the eustatic lowstand, as well as the salt tectonics.

Tributive submarine slope channels form downdip of the delta front and may also reflect incision during the prolonged eustatic lowstand. The salt tectonics caused focusing of the dispersed delta-front sediments, mostly originating from terminal distributary channels, into the convergent tributive slope-channel system. Although this is an unstable supply-dominated shelf-margin delta, the salt tectonics likely prevented the delta from avulsing once it reached the shelf edge and also caused the focusing of dispersed sediment effectively into a point source for delivery to deep-water systems. This contrasts with less confined high-sediment supply delta systems farther along the coast, which lack linked submarine deposits because they reach the shelf margin before maximum eustatic lowstand and avulse before building significant deep-water systems.