Geology of the Brooks Range
Geological Provinces
The Brooks range trends E-W, as do the geological provinces comprising the range. These "provinces" more or less correspond to tectonic terranes. That is, the rocks in each geologic province are not necessarily directly continuous with rocks in neighboring provinces. These rocks have been moved significant distances by tectonic processes, as is illustrated on the tectonic evolution pages of this site.
By studying the geology of each province, we have some good guesses about when the rocks formed and in what original environment. Coarse clastic rocks such as conglomerate and poorly sorted sandstone indicates deposition near to a continent or mountain belt with some topographic relief: the source of such sediments. Carbonate often forms in shallow marine environments, although certain carbonates may from in deep water offshore environments. Rocks formed in deep ocean basins typically contain almost no clastic material; instead they form from siliceous ooze which lithifies into chert. Volcanic and magmatic rocks have an obvious source- melted rock- but in what paleoenvironment did the rocks form? This can also be unraveled by geologists. Below is a geological map collage of northern Alaska from Moore et al., 1994 in The Geology of Alaska.
Geological Map of Northern Alaska
Because the quality of the map is less than excellent (but as good as I can get it), following is a key to map colors:
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Unit Color |
Rock Description |
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Green |
Cretaceous sedimentary rocks, primarily syn- and post-tectonic, and primarily in the Colville Basin (North Slope), except for some outcrops along the southern margin of the Range. |
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Yellow/Orange |
Quaternary surficial deposits; to the [north], orange is Tertiary sedimentary rocks- sands, silts, shale, marine seds; to the [south] orange represents the Coldfoot subterrane- Devonian to Proterozoic (?) mica schist and carbonate schist |
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Grey |
Jurassic to Mississippian sedimentary rocks- marine shale, chert, limestone; arkose; black shale; some diabase sills- De Long Mts subterrane. |
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Brown |
[Northeast]: Paleozoic to Proterozoic metamorphic rocks- includes quartzites, schist, marble and limestone. [South and Southeast]: Slate Creek subterrane; phyllite and metasandstone; middle Paleozoic (Devonian?) |
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Blue |
[Northeast]: Hammond subterrane; sandstone, phyllite, carbonates, black shale, conglomerates; Triassic to Mississippian [Central]: Endicott Mountains subterrane; Cretaceous to Upper Devonian; marine shale, sandstone, limestone and chert |
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Purple |
Hammond subterrane; metasedimentary rocks; quartzite, phyllite, schist, marble, carbonate schist, metabasalt, tuff, amphibolite; Devonian to Proterozoic. |
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Pink |
Includes mafic volcanics/deep oceanic sediments and layered ultramafics of the Angayucham terrane (ophiolite), AND two series of intrusive rocks: Devonian granites and Proterozoic granites-gabbro |
Subterrane Map of Northern Alaska
By comparing the geological map, above, and the terrane map below, one can see that "geological provinces" basically correspond to "subterranes", with few exceptions. Rocks exposed in the Brooks Range are divided into a number of subterranes. A terrane is a fault-bounded tectonic package with rocks differing from surrounding terranes. All rocks of the Brooks Range below to either the Arctic Alaska Terrane or the Angayucham Terrane. Following are descriptions of the geology of each Brooks Range subterrane.
Figure from Moore et al., 1994 in The Geology of Alaska
Arctic Alaska Terrane:
The Arctic Alaska Terrane is made up of a number of subterranes which are composed of sedimentary rocks ranging in age from Precambrian to Cenozoic. These rocks have a varying history that tells that tale of the Brooks Range. In various parts of the stratigraphic section we find: rocks that once belonged to the North American craton, passive margin sediments, rift-related sediments, pelagic sediments, volcaniclastics, and foreland-basin deposits. (As mentioned, we use the terms Arctic Alaska Terrane and Arctic Composite Terrane interchangeably.)
The Arctic Alaska Terrane underlies all of the North Slope and most of the Brooks Range. Rocks of this terrane are mostly of continental affinity, indicating North America as the source for most sedimentary material. At its southern outcrop boundary, the Arctic Alaska Terrane is overthrust by the Angayucham Terrane along the Kobuk suture zone. This suture marks the location of the ancient edge of the continent. North-directed thrusting of the Angayucham Terrane over the Arctic Alaska Terrane occurred between 160-140 Ma. In the subsurface, the terrane may extend to the south beneath the Koyukuk basin. To the west, the terrane may extend into Russia; to the east, it extends into Canada and dips beneath Cenozoic sedimentary rocks. The following subterranes and sedimentary assemblage make up the AAT:
Colville Basin: Overlies the North Slope subterrane; foreland basin of Cretaceous-Tertiary age, filled with sediments shed from the rising Brooks Range. Bounded on the north by the Barrow arch, which may have formed by subduction-related buoying (underplating leading to isostatic arching). The Barrow arch is home to the hydrocarbons of the Prudhoe Bay supergiant field. Rocks in the south part of the Colville Basin are gently folded, showing that Brooks Range deformation was actively stepping northward as recently as late Cretaceous time. In the northeast, thrust faulting, folding and erosion have brought pre-Cretaceous Colville basin rocks to the surface (see geological map, top).
North Slope subterrane: Cretaceous and older rocks underlying the Colville basin. These rocks are not deformed by post-Mississippian orogens, except in NW and NE (in the Lisburne Hills and Romanzof Mountains, respectively) where extensive deformation by Cretaceous and Cenozoic contraction has occurred. North Slope subterrane rocks are also exposed in the Mt. Doonerak fenster in the central Brooks Range, providing evidence that the North Slope subterrane underlies much of the Brooks Range. This is a major piece of geological evidence for significant north-transporting thrusts and crustal thickening.
De Long Mountains subterrane: Also comprised of imbricated Paleozoic and Mesozoic rocks but differs in "important stratigraphic aspects" from neighboring subterranes. Variously overlies the Endicott Mountains subterrane.
Endicott Mountains subterrane: Imbricated sedimentary rocks. Comprised of klippe of imbricated mid-Paleozoic to Mesozoic passive margin sediments (see geologic map, top). These rocks structurally overlie the North Slope subterrane.
Together, the De Long and Endicott subterranes make up the Central belt, the geographic core of the Brooks Range. Rocks of the Central Belt are typically ductiley deformed Paleozoic slate, phyllite, schist, carbonates, and orthogneiss
Hammond subterrane: Proterozoic and lower Paleozoic mixed clastics and carbonates. Ductile deformation structures, low greenschist to local blueschist facies metamorphism, but retains relict igneous and sedimentary textures.
Coldfoot subterrane: Quartz-mica schist and carbonate schist, marble; metavolcanics. Also named the "Schist Belt". Mesozoic blueschist partially overprinted by greenschist.
Slate Creek subterrane: Phyllite, metasandstone, rare Devonian fossils
Angayucham Terrane:
The Angayucham Terrane is thought to have once comprised a mostly continuous sheet covering much of the Brooks Range. It makes up the structurally highest (known) klippen of the Brooks Range, since most of the terrane has been removed by erosion. The terrane is 5-10 km thick and composed of Devonian through Jurassic mafic and ultramafic rocks and pelagic rocks. Rocks of the Angayucham Terrane are shown in solid grey on the subterrane map above, and show up as pink on the geologic map at the top.
South of the Angayucham Terrane is the somewhat anomalous Koyukuk basin, filled by Cretaceous sedimentary rocks. This gives the Brooks Range two flanking "foreland basins", which is not observed in most orogens.
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