The Yukon-Koyukuk province covers 100,000 sq mi (259,000 sq km) along the west coast of Alaska, extending from the Brooks Range to the Yukon delta, but excluding the Seward Peninsula. It is essentially a vast tract of Cretaceous rocks locally veneered by Quaternary alluviated beds and volcanic rocks. The Cretaceous section contains a large volume of volcanic rocks, and most of the sedimentary sequence consists of first- and second-cycle volcanic debris. Above basal andesitic rocks is a 10,000-ft (3,048 m) section of graywacke and mudstone of Albian age, overlain by 10,000 ft or more of shallow-marine and nonmarine paralic sandstone, shale, conglomerate, and coal--dated as late Early and early Late Cretaceous. This part of the stratigraphic section may contain hydrocarbons but severe structural complications limit the possibilities to two belts, one in the north along the Kobuk River and the other a narrow northeast-southwest band extending 300 mi (483 km) from the Koyukuk Flats to the Yukon-Kuskokwim lowland.
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Petroleum possibilities of Yukon-Koyukuk Province, Alaska: region 1