Abstract

The Aleutian Ridge consists geomorphically of two provinces: a west-trending, flat-topped (guyot-like) submarine Cordillera, the Aleutian island arc, that rises 4,000 meters above its Pacific and Bering Sea bases; and a 30-90-kilometer-wide forearc province that includes a broad forearc basin, the Aleutian Terrace, and the landward slope of the Aleutian Trench. The ridge’s upper crustal rocks can be grouped into a three-tiered chronostratigraphic sequence: a lower series of mostly Eocene volcanic rocks; a middle series of mostly Oligocene through lower middle Miocene strata that includes large volumes of sedimentary beds; and an upper series of middle Miocene to Holocene deposits that are dominantly sedimentary beds but include the upper Cenozoic eruptive masses of the arc’s modern volcanic centers.

Except for the landward trench slope, the ridge’s upper crustal framework is a massive antiform consisting of a sediment-draped igneous massif of chiefly lower series rocks produced rapidly by voluminous submarine volcanism prior to about 35 million years ago. The massif is at least 200 kilometers wide and extends from the landward trench slope northward beneath the forearc basin and adjacent cordillera of the island arc to the abyssal floor of the Bering Sea. In post-Eocene time, igneous activity effectively ceased over the flanks of the ridge and, except for a pulse in early middle Miocene time, greatly diminished along the arc’s summit region. In Oligocene through early Neogene time, debris eroded from the regionally elevated and volcanically dying crest of the arc accumulated on the submerged (or submerging) flanks and basal regions of the ridge as middle series deposits.

The post-middle Miocene history of the ridge crest records the dominance of extensional collapse and erosion over continuing but localized volcanic buildup of the arc’s igneous massif. Wave-base erosion carved a summit platform across the top of the arc. The truncation was accompanied by the deposition of basinal sequences of upper series beds in summit grabens and perhaps by extensive subsidence of the arc’s sloping flanks. In the more deeply submerged forearc area, relative uplift of the seaward part of the ridge’s igneous basement framed the structural basin of the Aleutian Terrace, which was collaterally filled with 3-5 kilometers of upper series deposits. Also in conjunction with the formation of this basin, a mass of trench deposits, predominantly of Alaskan origin, was compressionally accreted to the lower part of the adjacent landward trench slope.

The rock fabric of the Aleutian Ridge (arc + forearc) records a two-part history of early Tertiary growth and subsequent erosion and structural modification. This formative sequence can be roughly linked to major changes in the plate tectonic setting along the northern rim of the Pacific Basin. Because of the relative dominance of processes of erosion, sedimentation, and deformation during the ridge’s second-stage evolution, its igneous framework is blanketed by Oligocene and younger sedimentary masses that are potential habitats for petroleum accumulations. Petroleum is most likely to occur in the 3-5 kilometer-thick, slightly deformed basinal sections of upper series and perhaps middle series deposits that underlie the Aleutian Terrace and fill summit grabens. Middle and upper Cenozoic sandstone and siltstone beds associated with these sections are little altered and have favorable organic contents and porosity and permeability properties.