Sediments originally deposited on the Alaskan Abyssal Plain have been depressed to form the eastern Aleutian Trench. Simultaneously, a wedge of horizontally bedded sediments, about 1 km thick at its axis, has been deposited in the trench. The time-transgressive facies change between this wedge of sediment and the abyssal-plain sediment sequence shows up as a discontinuity on seismic records. Sediment is being deposited up to 10 times faster in the trench than on the abyssal plain. Ninety percent of the sediment in the trench arrived there by moving down the trench wall as slumps or in turbidity currents, and has been partly redistributed by turbidity currents flowing in a channel along the trench axis; 7 percent has come from the Surveyor Channel distribution system on the abyssal plain; and 3 percent is pelagic (mainly ice rafted). Sediments pass from a sandy facies near the trench wall to a silt-mud facies on the abyssal plain. If a similar trench were exposed on land, the stratigraphic sequence would consist of sandstone grading transitionally downward into shale, and then into an abyssal-plain sequence. Sediment fill in the Aleutian Trench is abnormally thick as a result of Pleistocene glaciation; Neogene trench-fill sediments would be less than 800 m thick.
