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Abstract

The Mississippi Fan, located in the northeastern part of the Gult of Mexico, is a gentle, arcuate pile of clastic sediments derived primarily from the ancestral Mississippi River drainage basin. The fan is confined by the Florida Escarpment on the east and the Texas-Louisiana-Florida continental slope on the north and west. On the south the fan abuts the Yucatan Escarpment and merges to the southeast with the Florida Plain (3,300 m) and to the west with the Sigsbee Plain (3,500 m). The radius of the fan from its apex (at a water depth of 1,200 m) to the plains is about 350 km. The fan has a surface area of about 170,000 sqkm, slightly smaller than New England.

The upper fan contains a partly leveed channel cut into older fan sediments. The channel is filled with late Pleistocene (Wisconsin) fine clastic sediments. The middle part of the fan, typical of many, is composed of a massive complex of fan channels that has constructed a crown or suprafan upto 500 ma bove the surrounding fan surface. The lower fan is characterized by smooth, gentle slopes that contain depositional distributary channels.

Reflection seismic profiles from the 1969 USNS Kane Gulf survey show sufficient continuity of seismic character to permit subdividing the upper part of the sedimentary section into three seismic stratigraphic units. Unit A, the youngest, consists of aproximal facies of disrupted seismic zones and a distal facies of generally parallel reflections separated by transparent zones. The former is interpreted to be channel, slump, anddebris-flow deposits, and the latter is turbidite flows interbedded with hemipelagic sediments. Unit A is correlated with the Sigsbee (Pleistocene) seismic unit; the fan formed during the time represented by unit A.

Two older intervals were mapped to show the relation present in this region before fan development. Unit B contains three or four bands of generally continuous reflectors that gradually converge down the lower fan and on the plain. These are inferred to be turbidite sequences. Unit C is largely an acoustically transparent layer, indicative of homogeneous hemipelagic sediments.

Computer-generated isopach maps of each unit allow study of the Neogene-Quaternary regional depositional patterns. Sediments comprising unit A exceed 3 km in thickness on the middle fan. Unit B thickens rather uniformly to the north and northwest, which suggests that the source was from that directional so. The oldest section mapped, unit C, is distributed more uniformly over the area.

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