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The geologic evolution of the Gulf of Alaska sedimentary province includes active trench deposition during Mesozoic-early Tertiary time and prograding shelf and glacial sedimentation during the remainder of the Cenozoic. Through interaction between bounding oceanic- and continental-plate movements, these beds have been deformed by repeated high-angle reverse and transcurrent faulting and folding. The stratigraphic succession within the province includes uppermost Mesozoic and a composite total of 40,000 ft (12,000 m) of Tertiary rocks. Major rock types are clastic sedimentary and volcanic rocks.
Compressional forces and wrench-faulting tectonics have generated sharp, elongate, structural folding of the Tertiary sediments in the Yakataga area (Kayak Island to Icy Bay). These structures generally trend either northeast-southwest or east-west and are bounded on the southeast or south by high-angle reverse faulting. Three apparent ages have been observed in these structures: early Miocene (post-Poul Creek-pre-lower Yakataga), late Miocene (middle Yakataga) and early Pliocene.
Post-Paleocene sediments in the Gulf of Alaska were derived from a Mesozoic arc complex and are generally poor in quartz and rich in feldspars and rock fragments. The sandstones are texturally immature and mineralogically unstable. Diagenetic alterations of framework grains resulted in the formation of (1) early calcite or zeolite cement, (2) clay rims and coats, (3) zeolite and
phyllosilicate pore fill, (4) late calcite and siliceous overgrowths and formation of prehnite and epidote.
The Poul Creek Formation is the richest source rock present in the Gulf of Alaska. It has acted as the source of most of the oil seeps in the area. The lower Tertiary section has been a minor source section for oil and gas in the area. The Yakataga Formation is mainly glacial in nature and is a very poor source rock. Organic maturity parameters indicate all formations except the Yakataga have experienced sufficient time-temperature conditions to be fully mature and in a generative hydrocarbon stage. Normalized interval-velocity analysis of offshore geophysical data suggests likelihood of poor source and reservoir rocks over much of the Gulf of Alaska basin.
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