Sequences of regularly spaced, seaward-dipping reflections seen on east-west seismic profiles along the lower slope of the western Gulf of Mexico are interpreted to be prograding clinoforms in the distal parts of large submarine-fan complexes. North-south lines define the lobate nature of the fans, some of which are over 50 km across. Some fans appear to be composites of overlapping, individual fan lobes. DSDP hole 90 recovered upper Miocene laminated, silty mud at the top and bottom of one of the prograding sequences, suggesting that any sand in the system must have been deposited farther upslope in the more proximal parts of the fan. Deposition of these sequences may have occurred during a major late Miocene lowering of sea level.

The late Miocene fans appear to be part of an overall fining-upward sequence along the lower slope beginning with the middle Miocene sandy turbidites and continuing through deposition of late Miocene silty clay laminites into Pleistocene deposition of hemipelagic and pelagic sediments. The sandy middle Miocene turbidites were derived from the Mexican mainland on the west and correspond to a zone on the seismic profiles characterized by chaotic and discontinuous reflections. These reflection patterns suggest a fan environment more proximal than the younger fans above--perhaps a suprafan environment with fan channels.

The overall fining-upward sequence and gradual cessation of turbidite deposition probably were due to the late Tertiary growth of the Mexican Ridges foldbelt, which apparently trapped terrigenous sediment and prevented turbidity currents from reaching the deep sea.

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