ABSTRACT

The Mississippi Fan is a lobe-shaped prism of muddy Quaternary sediments extending for about 600 km. from near the present Mississippi River Delta to the Sigsbee Abyssal Plain. The Mississippi Trough notches the shelf edge near the fan apex. The trough apparently channeled detritus across the outer shelf and upper slope and acted as a point source of fan sediment. Upper, middle, and lower regions of the fan are identified on the basis of sea floor gradient and relative smoothness, seismic character, and influence of salt structures. The upper fan occurs in water depths from about 1,300 to 2,500 meters and has irregular, hummocky topography with an average surface gradient of about 1°. Structural and topographic complexities in this part of the fan have resulted from salt diapirism, slumping and possibly current scour. Core lithologies consist of clay; silt; and scattered, very fine-grained sand.

The middle part of the fan is characterized by a low gradient (<0.25°), moderately smooth surface, and complex internal structures as shown by seismic profiles. Mid-fan deposits thin from 1,300 to 800 meters toward the southwest and southeast, and near-surface sediments are clays and silts. Widespread intervals of chaotic seismic response--interpreted as slump and slide deposits--occur adjacent to the depositional loci of the fan. Interbedded units of continuous reflectors probably are turbidites, hemipelagites, and possibly contourites.