Meckel, Lawrence D.1
(1) Shell Exploration & Production Co, New Orleans, LA
ABSTRACT: Supply-Driven Cyclicity in the Deepwater Gulf of Mexico
Regional shifts in the location of the proto-Mississippi River deltaic depocenter caused temporal and spatial fluxes in sediment supply that exerted a significant control on the third-order stratigraphy of late Miocene - Pliocene intraslope basins of the deepwater Gulf of Mexico. The late Miocene – early Pliocene Mars-Ursa and late Pliocene Auger-Macaroni intraslope basins are more than 200 miles (320 km) apart, yet each displays a characteristic up-section transition from more sheet-like deposits to more channelized deposits that can be directly related to the changing position of the deltaic depocenter (and its associated increase in sediment supply to the slope with respect to accommodation), independent of fluctuations in eustasy.
The more continuous, higher net-to-gross sheet sands were deposited during initial phases of basin fill, when the proto-Mississippi delta was in a proximal position with respect to each basin and sediment supply to the slope was at a maximum (11.8 – 7.5 Ma in the Mars-Ursa area; 4 – 3 Ma in the Auger-Macaroni area). As the basins filled, less accommodation was present on the slope, and the delta shifted locations (7.5 – 4 Ma in the Mars-Ursa area; 3 - 2 Ma in the Auger-Macaroni area). In both basins, this phase is dominated by less continuous, lower net-to-gross channels and overbank deposits. The duration of each cycle is thus a function of the establishment and subsequent abandonment of the associated deltaic depocenter.
Interestingly, the two basins show additional similarities. Updip locations in each basin, near sediment entry points, are characterized by higher-frequency (fourth- to fifth-order) alternations of sheets and channels bounded by marine condensed sections. Downdip locations adjacent to local ‘backstops’ have more sheet sands, and higher net-to-gross ratios. Although these similarities may be coincidental, it is more probable that they reflect similar styles of sequential fill associated with short-term fluctuations in relative sea-level and topography within an overall supply-dominated succession.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.