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Few of Alaska's offshore frontier basins have been explored by drilling. However, regional geologic data and tectonic considerations can be used to assess the likelihood that commercial volumes of oil and gas are present in the basins.
Basins of the northern margin of the Gulf of Alaska, and the contiguous Aleutian Ridge on the west, have formed along the Aleutian subduction zone, a tectonic terrane 3,600 km long that separates the Pacific and North American plates. The eastern gulf shelf is underlain by Cenozoic deposits that are as much as 10 km thick, but adequate reservoir beds are thought to be absent in Neogene and younger beds. However, the discovery that potential reservoir and source beds of early Tertiary age underlie the continental slope enhances the oil and gas prospects of the eastern gulf margin. Basins of the central gulf shelf (Kodiak Island area) contain upper Cenozoic beds that are as much as 5 to 6 km thick. These beds are broadly deformed and unconformably overlie more deformed rocks of Paleogen and Cretaceous age. Grabens (unusual for the gulf margin) filled with 6 to 8 km of Neogene and younger beds are present beneath the western gulf shelf (Sanak Island). Publicly available data imply that reservoir and
source beds adequate to form large hydrocarbon deposits are probably absent from the central and western gulf. Farther west the lower Tertiary igneous core of the Aleutian Ridge is overlain by broadly deformed Neogene and younger deposits. These beds are 2 to 4 km thick in summit basins, and probably much thicker below the Aleutian terrace along the ridge's southern flank. Although these basins include diatomaceous and turbidite sequences, the probable abundance of readily altered volcanic detritus cautions against optimistic expectations of large quantities of oil and gas along the Aleutian Ridge.
Five extensive (25,000 sq km) basins filled with as much as 15 km of mostly Cenozoic beds are present beneath the Beringian shelf, and, therefore, north of the Aleutian subduction zone. Except near Siberia, the deposits in these basins are little deformed. Elongate St. George and Navarin basins, along the southern or outer edge of the shelf, have formed on a collapsed foldbelt of miogeoclinal rocks that include beds of Jurassic and Cretaceous age. Subsidence of the foldbelt occurred after subduction of oceanic crust ceased beneath the Beringian margin (60 to 70 m.y. ago) and shifted south to the Aleutian Ridge. In contrast, Norton basin, which underlies the inner or northern edge of the shelf, is floored by Paleozoic and older rocks of Brooks Range affinity that subsided in response t Cenozoic strike-slip faulting in western Alaska. A speculative reading of the geologic history of the Beringian basins implies that some of them could harbor commercial volumes of oil and gas. South of the Beringian margin, the abyssal floor (3 to 4 km) of the Bering Sea basin is underlain by 4 to 10 km of undeformed deposits chiefly of Cenozoic age. Drilling results, and the detection of deep-water bright spots (VAMPs), suggest that hydrocarbon deposits (of unknown volume) occur in the basin. Its basement of Lower (?) Cretaceous oceanic crust was presumably separated from the north Pacific by the formation of the Aleutian Ridge in latest Cretaceous or earliest Tertiary time.
Since early Mesozoic time, the evolution of the structural framework of the north Pacific margin has been controlled by the subduction of more than 10,000 km of oceanic lithosphere. However, recognition that segments of the margin are underlain by deeply submerged miogeoclinal rocks of Mesozoic and early Tertiary age, and the results of DSDP drilling at Pacific margins, attest that the evolution of Alaskan and Bering Sea margins is not adequately described by models of accretionary tectonics or back-arc spreading. Little understood aspects of subduction and post-subduction tectonics that cause and control marginal uplift and subduction are thought to hold important clues to the economic potential of the frontier basins of the north Pacific and Bering Sea regions.
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