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The Pleistocene Alaminos Fan in the northwestern deep Gulf of Mexico is a large submarine fan located at the base of the continental slope. High-resolution, near-seafloor 3D seismic data were interpreted to study the evolution of two Pleistocene shallow sequences. Results indicate that there can be considerable variability in the evolution of deepwater systems in the same overall setting, and the significance that seafloor topography and gradient changes can make in the overall development of different architectural elements. Seismic facies indicate that the sequence consists primarily of mass-transport deposits (MTDs), channel-fill sediments, and sheet deposits in the lower sequence, whereas the upper sequence consists of a single channel.

The older sequence consists of two distinctly different deepwater systems. To the east, a prominent channel system overlies basal MTDs. The channel system consists of one upfan channel that bifurcates downfan to at least six discrete channels flanked by levees. The evolution of the downdip distributary channel system is the result of deposition in the unconfined setting. To the west, an updip channel fed sheet deposits that developed in the back limb of a fold in a confined setting. The sheet, in turn, is overlain by a channel–levee system. This vertical change is likely the results of the fill-and-spill of sediments in the area.

In the near-floor sequence, a single deepwater channel is flanked by low-amplitude levee reflections. Quantifying the dimensions of this younger channel shows that it evolved from a wider, straight channel at the base of the sequence to a relatively narrow sinuous channel with increasing sinuosity upward.

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