The upper 6-7 m of sediment of the eastern Mississippi cone consists of a repetitious vertical succession of gray silt and silty clay intercalated with a few layers of fine sand and topped by a 20-50-cm layer of yellowish-brown foraminiferal clay. Disequilibrium age determinations indicate that the lower silty layers, representing the deposits of latest low sea-level stand, were deposited more rapidly than the upper foraminiferal clay. These sedimentation rates, which depend primarily on the rate of the detrital influx and sea-level change, average about 30 cm/1,000 years.

Sedimentary processes on the deep-sea fan are interpreted from sedimentary structures, textures, and compositions, as well as from bathymetry, bottom photographs, and continuous seismic profiles. The more than 20 varieties of minor sedimentary structures recognized from X-ray radiographs are grouped into five varieties that correlate closely with sediment type. None of the structures is typical of vertical "turbidity sequences." On the contrary, the evidence suggests that the primary mechanisms of sediment transport are differential pelagic settling and low-flow-regime bottom currents, with mass movements by sliding or slumping common in channel and slope areas. Statistical evaluation of the occurrence and distribution of minor structures indicates that (1) most of the structures asso iated with coarser materials are analogous to structures formed by traction transport or by ripple migration in shallow water, and (2) the distribution of both bottom-current intensity and internal waves that create small-scale ripples is local. Photographs of the present bottom support this conclusion.

The importance of diagenetic solution of carbonate, mostly planktonic foraminifers and pteropods, as verified by laboratory experimentation, is related to the degradation of organic matter in the sediments. The most active solution occurs near the boundary between the upper foraminiferal clay and the lower silty layers and is partly responsible for (1) the abrupt decrease of carbonate downward in the cores, (2) the rearrangement of clay particles into secondary thin laminae, and (3) the shortening of the distance between noncarbonate silt and sand layers or laminae. These results, combined with compaction, accentuate the uniformity of layering.

The upper cone is indented by digitate leveed valleys and canyons cut by transverse ridges, whereas the lower section is characteristically smooth. The bathymetry of the cone reflects its underlying structure. Continuous seismic profiles show that the lower cone is composed of relatively uniform flat-lying beds, representing at least five major depositional cycles since Plio-Miocene time and as many as 14 since Late Cretaceous time. In contrast, the upper cone has many internal irregularities, probably caused by gravity sliding, folding, and slumping contemporaneous with deposition, and by diapiric salt intrusion. The cone's depocenter has shifted continuously basinward as the Mississippi delta has prograded gulfward since Late Cretaceous time.