The Plio-Pleistocene shelf margins of the U.S. Gulf of Mexico comprise a distinctive array of elements that include shelf margin deltas, break-up complexes, canyons and a variety of features generated by gravity failure, including slide scars and fault scar erosion. Shelf margin failure occurred at all scales, from minor slumps within oversteepened delta fronts to shelf margin break-up complexes, wasting areas up to 25–30 miles wide.
These elements combined in a systematic and predictable manner, giving rise to cyclic stratigraphic records. The stratigraphic architecture varied markedly along strike at any one time and changed significantly through time. These spatial and temporal variations provide important clues as to the evolution of the slope depositional systems. For example, the large-scale shelf margin failure complexes appear to be more significant sources of sand to the slope than entrenched canyons.
Potential reservoirs are located within many of these shelf margin complexes. Hydrocarbons may be trapped within the sandy topsets of shelf margin deltas, individual rotated blocks within break-up complexes, in sandy onlap fill within slump scars or break-up complexes and in sands within canyon fill.
The architecture of the shelf margin is controlled by the complex interaction of sediment influx, eustatic sea level changes, active faulting and salt withdrawal.
The key aims of this paper are to:
Describe the variations in shelf margin observed in the Plio-Pleistocene of the U.S. Gulf of Mexico (study area outlined in Figure 1);
Interpret the depositional processes within the described shelf margin systems and discuss some of the vertical and lateral facies associations;
Evaluate how the shelf margin architecture can be used to predict slope deposition;
Discuss the relationship between observed shelf margin and slope architecture and eustatic sea level changes.
The Plio-Pleistocene in the Gulf of Mexico represents 7 million years of time. The important characteristics of the depositional system during this time are:
Dominantly fine grained sediment was supplied to the Gulf of Mexico through the ancestral Mississippi river.
Sediment input to the shelf was highly focused at any one time. Through the Plio-Pleistocene, the site of the Mississippi delta shifted several times over distances of several tens of miles.
The drainage area of the Mississippi was very large, and so rates of sediment supply to the Gulf of Mexico were phenomenal. The compacted thickness of Plio-Pleistocene sediment exceeds 5 miles in several areas.
The clastic wedge was deposited in a passive margin setting, but onto highly mobile salt. The geometry of sedimentary basins was largely controlled by the amount and behavior of the salt.
The Plio-Pleistocene was characterized bv Iarae and frequent changes in eustatic sea level, These changes increased in frequency and amplitude through the Pleistocene with the onset of major northern hemisphere glaciation.
Shelf margins have long been recognized as a critical interface between shelf and slope deposystems (see Stanley and Moore, 1983). Various aspects of clastic shelf margins within the Gulf of Mexico basin have been investigated by many other authors including Edwards, 1981; Winker and Edwards, 1983; Coleman, et a/, 1983; Suter and Berryhill, 1985; and Pulham, in press.
