A seismic stratigraphic analysis of 5,000 n. mi of regional multifold reflection data allows an interpretation of the post-middle Cretaceous geologic history of the deep Gulf Mexico basin. Five horizons that are major unconformities along the southern margin of the basin were mapped throughout the deep Gulf and used to define five depositional sequences (seismic units). Ages for the two youngest horizons were obtained by direct correlation with DSDP holes--Pliocene-Pleistocene and late Miocene. The other horizons are tentatively correlated with proposed major global unconformities and eustatic sea-level changes--middle Oligocene, early Tertiary, and middle Cenomanian. The middle Cenomanian horizon is the most prominent subbottom reflector in the deep Gulf. It represents a striking unconformity on the seismic data along the base of the Campeche and Florida Scarps against which the younger sequences thin, onlap, and pinch out. Isopach maps of the units indicate a combined maximum thickness of 7 to 8 km for the post-middle Cretaceous sediments in the central basin. Variations in individual unit thicknesses and seismic facies reflect regional changes in depositional patterns through time: during the Late Cretaceous through middle Tertiary the main sediment source was in the west, and deposition consisted mainly of homogeneous, probably fine-grained sediments changing upward to alternating sand and mud; in the middle Tertiary there was a major influx of sand and mud from both the north and west; following a late Miocene-Pliocene starved-basin interval, fine-gr ined turbidites from a northern source built the thick Pleistocene-Holocene Mississippi fan.

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