Cook Inlet Basin
The Cook Inlet Basin is a northeast-trending collisional forearc basin that stretches from the Gulf of Alaska into South central Alaska, just east of the Matanuska Valley. It is located in the arc-trench gap between the Alaska-Aleutian Range batholith and contains roughly 80,000 cubic miles of sedimentary rocks. These sediments are mainly derived from Triassic, Jurassic and Cretaceous sediments.
The region is heavily influenced by two major tectonic elements which are still active in the area today. The western side of the basin lies directly above the Aleutian subduction zone where the Pacific Plate is subducting beneath the North American Plate. However the eastern side of the basin overlays the subduction of the Yakutat microplate beneath the North American plate. Active subduction along various sides of the basin produce regional compression that lead to major folding, faulting and the formation of anticline structures within the sediments. Anticline structures provide ideal hydrocarbon traps, and so the Cook Inlet Basin is widely known for its hydrocarbon accumulations and its overall production of oil and gas.
Regional tectonism
The geology of Alaska is characterized by the collision and accretion of terranes over the last 100 Ma and its features formed as a response to plate convergence and subduction.Pacific Plate megathrust
Alaskan tectonism is mainly dominated by the subduction of the Pacific Plate beneath the North American Plate. The subduction boundary is marked by a 4,000 km long trench known as the Aleutian Trench, where seismic activity is common and the volcanic arc produced is part of the Pacific Ring of Fire. The initial subduction of the Pacific Plate triggered the formation of the Bruin Bay Fault system, which is responsible for northeast trending faults throughout the western portion of the Cook Inlet Basin. Thrust type shearing is present along this active margin and it causes the sediments to form anticline structures as a response to regional compression.Yakutat Microplate
The collision of the Yakutat microplate and the Alaskan superterrane is believed to have happened sometime during the Miocene epoch, after the subduction of the Pacific Plate had already begun. The Yakutat microplate is characterized by a basement of oceanic crust overlain by continental crust. Throughout the basin, Tertiary non-marine stratigraphy have been deformed into northeast-trending, discontinuous folds which partially extend into Mesozoic strata. The ongoing collision is causing the forarc region to collapse into itself, similar to the way a zipper collapses into itself when being zipped shut. This collision is also causing the buildup of an accretionary prism known as the Kenai Mountains, which borders the southeast region of the Cook Inlet basin.Structural cross section
Major faults
Bruin bay Fault System
The Bruin Bay fault system is system of steeply dipping, northeast-striking faults which extend roughly 498 km along the western region of the basin. It extends from the south of Becharof Lake on the Alaskan Peninsula up to its termination point against the Castle-Mountain Lake fault system. The system separates Mesozoic and Cenozoic volcanic sediments and Upper Triassic and Lower Jurassic volcanic rocks of the arc from Mesozoic marine and non-marine strata within the forearc basin. Faults within the system are generally high angle and westward dipping, but due to the accumulation of sediments on the surface over time, the Burin Bay Fault system is present predominantly within the subsurface. The fault system was believed to have been active during Naknek deposition, and intrusive volcanic plutons indicate fault activity occurring before the Oligocene epoch.Border Ranges Fault System
The Border Ranges Fault system is a normal-oblique fault and stretches 1,500 mi on the eastern side of the basin. It separates the forearc basin from the subduction complex as well as deformed metamorphic rocks from the subduction complex. The Fault system originated during the pre-miocene period as a result of megathrust subduction and it was subsequently filled with turbidity deposits. Since then, it underwent contractional deformation in the Cretaceous and Paleocene-Eocene due to strike-slip movements.Castle Mountain Fault System
This system of northeast striking faults extends roughly 200 km long and is the only fault in the region with Holocene faulting present at the surface. Motion along this fault dates as far back as 47 Million years ago and was supposedly active during the Jurassic Period, where it experienced right lateral strike-slip motion. It is still partially active today.Stratigraphy
- Tuxendi Formation
- * Siltstone, clay-rich, some organic material
- Hemlock Formation
- * Sandstone, conglomerate sandstone, conglomerates
- Tyonek Formation
- * Massively bedded sandstones with siltstone and coal
- West Foreland Formation
- * Conglomerates, sandstone, siltstone
- Sterling Formation
- * Generally massively stacked fluvial channels, massive sandstone/conglomerate interbedded mudstones, siltstones and thin coals
- Beluga Formation
- * Siltstone with fluvial sandstone, thin layers of discontinuous coal, volcanic tuffs
Depositional history
Late Triassic and Early Jurassic (237 Ma–174 Ma)
The Upper Triassic lithology, known as the Kamishak Formation, and the Lower Jurassic lithology, known as the Talkeetna Formation, are both part of an oceanic island arc. These formations are present along the East-West margin of the Bruin Bay Fault zone and are intruded by igneous plutons originating from dehydration melt produced by Pacific Plate subduction. The Kamishak Formation is reflective of a shallow reef environment that graded into deeper marine sediments that were deposited during a transgressive ocean sequence. During the late Triassic, the pacific plate was subducting beneath the North American Plate causing plutons, dikes and sills to intrude into the country rock, especially near the Burin Bay Fault zone. Triassic sediments within the Kamishak sit atop a 7775 ft layer of Permian volcanic rocks.The Jurassic Talkeetna formation sits unconformably on top of the Kamishak formation, although some disconformities are seen throughout the peninsula. The Talkeetna is composed mostly of volcanic material interbedded with characteristic lava flows and tuffs. Geochemical analysis of the lava flows indicate the formation was once at great depths within the ocean. The formation records periods of shallow, intermediate and deep crustal levels originating in an oceanic island arc environment.