Genetic Stratigraphy, Stratigraphic Architecture, and Reservoir Stacking Patterns of the Upper Miocene—Lower Pliocene Greater Mars–Ursa Intraslope Basin, Mississippi Canyon, Gulf of Mexico
Lawrence D. Meckel, III, Gustavo A. Ugueto, H. David Lynch, Earl W. Cumming, Ben M. Hewett, Eric J. Bocage, Charlie D. Winker, Brian J. O’Neill, 2002. "Genetic Stratigraphy, Stratigraphic Architecture, and Reservoir Stacking Patterns of the Upper Miocene—Lower Pliocene Greater Mars–Ursa Intraslope Basin, Mississippi Canyon, Gulf of Mexico", Sequence Stratigraphic Models for Exploration and Production: Evolving Methodology, Emerging Models and Application Histories, John M. Armentrout, Norman C. Rosen
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The late Miocene–early Pliocene sediments of the northern greater Mars-Ursa intraslope basin in the Gulf of Mexico record high-resolution (ca. 0.5 Ma) cyclic deposition of couplets of deep water fan lobes (sheet sands) and channelized or amalgamated systems bounded by local to regional condensed sections. Internally, the sheet sand and channelized systems are separated by a transitional surface of bypass, avulsion, and/or erosion. These fourth-order cycles are the building blocks that make up third-order seismic facies assemblages, which in turn record the progressive fill and spill sedimentary dynamics in salt-withdrawal minibasins.
Third-order sequences in the greater Mars-Ursa intraslope basin reflect regional variations in accommodation and sediment supply. The rate of deposition of an older (10.0–7.0 Ma) third-order assemblage is significantly greater than that of a younger (7.0–4.2 Ma) assemblage. The older assemblage has a greater abundance of thick, laterally extensive sheet sands and a higher composite net-to-gross ratio than the younger assemblage, which contains more amalgamated and bypass channels and associated over bank facies, and has a lower composite net-to-gross.
Fourth-order cycles display compensational stacking patterns in which the overlying channel system is best developed where the sheet sand system is thin or not present. The fourth-order stratigraphy of the basin is controlled by high frequency variations in accommodation and sediment supply. Eustatic changes affect the supply of sediment from the shelf to the slope, but they are not a primary control in the formation of fourth-order deepwater sequences.
Condensed sections that bound the fourth-order cycles are deep marine (pelagic) mudstones that drape topography. They occur in association with faunal abundance and diversity peaks and often correspond to abrupt changes in incremental overpressure. Because they represent periods of minimum relative sedimentation rates and maximum relative rates of accommodation creation, basin margins have their highest relief at the ends of these periods. Several of the condensed sections are interpreted to correlate to maximum flooding events on the shelf.
Thick, high net-to-gross fan lobes that occur above the condensed sections have been deposited as sedimentation rates in the basin increased. They fill paleo-topographic lows and onlap abruptly against high-relief basin margins. Deep water faunal abundances in these sections are at relative minima due to the influx of sediment, and in some instances, the sands have a biofacies signature indicative of reworking. However, a lack of corresponding increases in either terrigenous content or reworked Cretaceous material suggests a local source for these reworked sands.
Subtle transitional surfaces separating the sheets and channels are identified by the onset of apparent paleobathymetric deepening at the bases of faunal abundance and diversity peaks. Typically, these surfaces document intra-basinal avulsion or erosion and sediment bypass, caused by infill of available accommodation by sheet sands.
The channelized systems or amalgamated channel/sheet systems that overlie transitional surfaces are associated with decreasing rates of sedimentation and low rates of creation of accommodation. Thin, laterally restricted amalgamated channels and sheet deposits may accumulate in the limited accommodation. If present, erosional channels and bypass scours are filled by sand.