Marine geology of abyssal plains in the Gulf of Alaska

Abstract

Acoustic reflection profiles in the Gulf of Alaska afford a new geologic dimension that allows tests of many previous speculations and hypotheses about the marine geology of the three abyssal plains that surround a central area of hills and seamounts. The bulk of sediments forming the two eastern abyssal plains (Alaskan and Tufts) have been transported by turbidity currents from continental sources through many leveed channels. The undulating, partly buried topography beneath the plains is of volcanic origin, and the central hilly area is a high that antedated the deposition of the plains. The moats, or depressions, around hills and seamounts were formed by differential deposition; ponding behind ridges is common. The distinctive features of fourteen channels crossed in the northern Tufts plain are the flat beds within the channel proper, a lower levee on the east side, and a very wide higher levee on the west side (downstream, right-hand side); some channels were originally controlled by lows in the underlying topography; channel migration upward and away from the high-leveed side is apparent. These channels are depositional rather than erosional in origin. Turbidite thicknesses decrease with distance from continental sources. In the southeast part of the Alaskan plain sediment thickness is 680 meters; in the northern Tufts plain maximum thickness is 470 meters, and in the southwest part it is 145 meters thick. The Aleutian abyssal plain, west of the central hilly area, has no present-day source of bottom-transported sediments; it is a fossil turbidite plain (maximum thickness, 282 meters) covered, in the west-central part, by 96 meters of pelagic material. Sediment thicknesses, probable rates of deposition, and known geologic history in adjacent areas, indicate the following geologic history: During Late Cretaceous to middle Tertiary the southward extension of deep-water flysch deposits (from the Alaskan peninsula area) formed the fossil turbidite Aleutian abyssal plain. In early to middle Tertiary, the first down-bowing of the Aleutian trench cut off turbidite deposition, and pelagic deposition began; the East Pacific rise was elevated. In middle Tertiary, the East Pacific rise subsided and great effusions of volcanic material from crestal areas covered the adjacent sea floor; turbidite deposition began over the Alaskan and Tufts plains (formerly the west flank of the rise). Pelagic deposition on the Aleutian plain continued to the present; turbidite deposition continued through the Pleistocene on the Alaskan and Tufts plains. It is concluded that the East Pacific rise in the Gulf of Alaska is in an advanced stage of rise evolution, and that the whole area has been quiescent since middle Tertiary.