Although the largest fans may be up to several hundred times the size of the smallest (Plate 1), most modern fans have the same basic facies associations. The general facies descriptions published and based mainly on late Pleistocene deposits of Astoria Fan have not been significantly modified (Nelson and Kulm, 1973); these will be summarized briefly and compared with more recent work on each of the general fan environments and associated facies. The interbedded blanket deposits from hemipelagic deposition and the more recently recognized widespread suprabed or debris-sheet facies also will be outlined. In addition, turbidite muds and proximal to distal thin-bedded-turbidite deposits will be differentiated from bottom-current-deposited contourites and shallow-water graded sands, both deposited by bottom currents.
For each facies association, the general morphology, acoustic character, and seismic stratigraphy will be summarized with the lithologic character to outline the complete facies-association characteristics for each environment.
The lithology of modern near-surface sediment on continental slopes typically is bioturbated or laminated hemipelagic mud with thin silt or very fine sand interbeds (Fig 6-1) (Maloney, 1965; Doyle and Pilkey, 1979). In deeper DSDP drill holes, however, the sediment often may be pure pelagic oozes Cook et al., 1982).
Three major slope sedimentary units made up of this generally monotonous lithology can be recognized: (1) undisturbed, in situ, pelagic and hemipelagic sediment draped on smooth undeformed slopes that exhibit seismic reflectors paralleling the upper slope but often thickening or diverging downslope into sigmoidal configurations (Fig. 6-1), (2) slides and other chaotically deformed