User:Hvanwy1/The North Slope Foreland Basin

The North Slope basin, also known as the Colville basin, is a geologic structural foreland basin that was formed as the result of a collision of the Arctic Alaska Plate with an oceanic island arc during the Mesozoic and Cenozoic eras. The basin covers the entire width of northern Alaska, stretching over 1000 km ranging anywhere from 50 to 350 km wide. Three sedimentary sequences overlay a basement consisting of low grade-metamorphic rocks. A majority of the fill was deposited from the middle Jurassic to the late Tertiary. The North Slope basin is a prolific oil producing basin. It is home to the Prudhoe Bay oil field, the largest in North America. The basin is estimated to contain over 40 billion barrels of conventional oil as well as an estimated 200 trillion cubic feet of gas.



Geologic Setting
The North Slope basin is an asymmetrical trough-shaped basin with its foredeep located near the southern margin. The basin is bounded by several tectonic features. Along the southern edge of the basin lies the Brooks Range, a thrust fold mountain belt associated with the Rocky Mountains of Canada. To the north, the basin is bounded by the Beaufort passive margin, which separates the North Slope basin from the Canada basin to the north. It is bounded to the west by both the Herald arch and the Chuckchi platform. To the east the basin pinches out as it approaches the Alaska-Canada border.

Basin Formation
During the Paleozoic to Mesozoic a passive continental margin formed on the North American plate. Rifting occurred along this margin during the Jurassic and Cretaceous which severed the continental connection and created the Arctic Alaska plate. The Arctic Alaska plate is a continental fragment which currently encompasses all of northern Alaska and extends into both Canada and Siberia. During the Mesozoic to Cenozoic, the Arctic Alaska plate drifted and rotated counterclockwise about 67 degrees away from the North American plate. This produced both the Canada basin as well as the Beaufort passive margin. At about the same time, the southern edge of the Arctic Alaska plate collided with an oceanic island arc which caused the Brooks Range orogeny and formed the North Slope basin.

Stratigraphy
Three sedimentary sequences overlie a basement of Lower Paleozoic low-grade metamorphic rocks. The majority of the fill consists of middle Jurassic to late Tertiary sediments. Older deposits include deep marine turbidites while younger deposits include prograding basinal, basin-slope, shallow-marine, and non-marine deposits including mudstones, sandstones, and conglomerates. These deposits originated from the ancestral Brooks Range as well as the area presently under the Chukchi Sea.

Ellesmerian Sequence
The Ellesmerian sequence overlies the low-grade metamorphic basement rocks. The lower Ellesmerian sequence consists of non-marine clastics as well as shallow marine carbonates and shales of the Endicott and Lisburne groups. The upper Ellesmerian sequence consists of marine and non-marine clastics and marine carbonates of the Sadlerochit group and the Shublik formation as well as organic rich shales and sandstones of the Sag River Formation. These sediments were deposited as a result of passive margin sedimentation of the North American plate. Previous to the formation of the basin, erosion of uplifted material led to removal of much of the upper Ellesmerian sequence in northern Alaska. In the North Slope basin, this sequence thins to the south.

Beaufortian Sequence
The Beaufortian Sequence consists of fine grained pro-delta sediments of the Kingak and Pebble Shales. The appearance of this sequence marks the end of passive margin sedimentation and the beginning of synrift sedimentation. This sequence was deposited in an early Cretaceous basinal trought, called the Collville trough, during active rifting of the North American plate and the Arctic Alaska plate. A period of rift related uplift and erosion produced a regional erosional unconformity called the Lower Cretaceous Unconformity or LCU. Following this uplift and erosion, there was a period of subsidence and marine transgression along the rift margin which created a large marine mudstone/shale which is referred to as the Pebble Shale that marks the end of the Beaufortarian Sequence.

Brookian Sequence
The Brookian Sequence consists of sediments deposited due to the uplift of the Brooks Range from the Cretaceous to the present, infilling the North Slope basin. This sequence contains both source and reservoir rocks of marine shales, marine shale turbidites, and shallow marine and non-marine deposits. Marine shales include the Hue Shale, an important source rock. Marine shale turbidites include the Torok, Seabee, and Canning Formations. Finally, shallow-marine and non-marine deposits include the Nanushuk, Tuluvak, Schrader Bluff, Prince Creek, and Sagavanirktok Formations.



Basin Evolution
Following deposition of the Ellesmerian and Beaufortarian Sequences, uplift of the Brooks Range was followed by mass wasting of sediments of the Brookian Sequence due to over steepening of the thrust wedge. This filled in the North Slope basin and provided the tectonic load that deepened the Colville trough through subsidence. The Brooks Range thrust wedge extends to the south more than 160 km in some parts of the basin. On the northern edge of the basin, rifting along the Beaufort margin along with the tectonic load from the Brooks Range thrust wedge caused the Barrow Arch to uplift. This uplift resulted in the erosion of over 1,000 m of thick foreland deposits that were overlaying the arch. The thrust wedge continued to extend north from the Brooks Range, further deepening the Colville trough to its present depth of over 10 km. Continued rifting along the Beaufort margin caused sagging and eventual collapse of the continental margin along the north side of the Barrow Arch, giving it its present shape today.



Barrow Arch
The Barrow Arch is a prominent anticlinal structural feature of the North Slope basin. It is a rift shoulder of the Beaufort passive margin and was formed due to tectonic loading from the Brooks Range orogeny and rifting of the Beaufort margin. Faulting, erosion, and marine transgression has created structural-stratigraphic hydrocarbon traps that are associated with this feature resulting in major hydrocarbon discoveries.



Hydrocarbon Resources
The North Slope basin is a prolific oil and gas producing basin. The basin is estimated to contain over 40 billion barrels of conventional oil as well as an estimated 200 trillion cubic feet of gas. See Alaska North Slope and Prudhoe Bay Oil Field for additional information on hydrocarbon resources of the North Slope basin.