Tectonic framework of petroliferous rocks in Alaska

Abstract

Alaska, comprising 3.6 X 10 6 sq km (about 28 percent) of the land, shelf, and upper continental slope of the United States, has been estimated by the U.S. Geological Survey (1974) to contain about 25 percent of the Nation's petroleum resources. Some 11 billion barrels of petroleum liquids and 31 trillion cubic feet of natural gas have been announced as discovered to date. In northern Alaska, Paleozoic and Mesozoic shelf and slope deposits of the Brooks Range orogen were thrust relatively northward over the depressed south margin of the Paleozoic and Mesozoic Arctic platform, upon which a foredeep (the Colville geosyncline) developed in earliest Cretaceous time. Detritus from the Brooks Range filled the foredeep and pro-graded northwest and northeast to fill the Cretaceous and Tertiary North Chukchi and Umiat-Camden basins and form the Beaufort shelf. In southern Alaska, a series of arc-trench systems developed on oceanic rocks during the Jurassic and Cretaceous. Between the arcs and the metamorphic (continental) terranes of east-central and northern Alaska, large back-arc and arc-trench gap basins received thick volcanic and detrital deposits. These deposits were extensively deformed and disrupted by mid-Jurassic to Tertiary plutonism, Laramide oroclinal bending, wrench faulting, and arc-related compression. The Laramide events 'continentalized' the late Mesozoic back-arc basin deposits and welded them to the older continental terranes to the north and east. Subsequent sedimentation was localized and nonmarine except in onshore and offshore coastal basins, where thick mixed marine and nonmarine sections were deposited. The Aleutian arc and associated Queen Charlotte transform fault system have dominated structural and depositional patterns in southern Alaska since the early Cenozoic. The largest petroleum reserves in Alaska (the Prudhoe Bay and associated fields) and the best prospects for additional large discoveries are in northern Alaska, where an extensive terrane is underlain by Upper Paleozoic to Tertiary carbonate and shelf, slope and delta clastic deposits. The pre-Tertiary back-arc and arc-trench gap basins in southern and interior Alaska are too intensely deformed or too low in porosity (because of diagenetic mobilization of labile constituents) to offer more than modest local prospects. The Tertiary coastal basins do, however, offer large tracts of thick marine and nonmarine clastic rocks and in some areas many large folds to exploration. Such basins are known to be petroliferous on Bristol Bay and the Gulf of Alaska and to contain major accumulations of oil and gas at Cook Inlet, but they are relatively little explored.