Hamilton, T.D., 1986, Late Cenozoic glaciation of the central Brooks Range, in Hamilton, T.D., Reed, K.M., and Thorson, R.M., eds., Glaciation in Alaska: The geologic record: Alaska Geological Society, p. 9-50.
Four major glacial episodes are recognized in the central Brooks Range. From oldest to youngest these are the Gunsight Mountain episode and the Anaktuvuk River, Sagavanirktok River, and Itkillik glaciations. Minor advances of cirque glaciers took place during the late Holocene Neoglacial interval.
Drift of Gunsight Mountain age has been strongly modified by subsequent stream erosion and pedimentation. North of the Brooks Range, it consists primarily of boulders of the Kanayut Conglomerate redeposited in younger terrace gravel. Probably correlative drift forms broad moraines that are deeply buried by loess, glacial-lake sediments, and alluvium in topographic basins south of the range. Drift of the subsequent Anaktuvuk River glaciation forms the outer component of massive compound lateral moraine complexes that extend north from the Brooks Range. Correlative moraines farther south outline piedmont lobes that extended into the Koyukuk and Chandalar valleys and that filled the Kobuk valley as far west as Kotzebue Sound. The Anaktuvuk River glaciation was followed by a long interval of stream erosion and downcutting during which valley systems of the Brooks Range attained their present configurations. Drift of the succeeding Sagavanirktok River glaciation forms the oldest and outermost glacial deposits on the floors of modern valleys. Distributions of glaciers indicate that by Sagavanirktok River time the Noatak and Kobuk Rivers had captured their present upper valleys, that tectonic troughs had formed or were continuing to develop south of the Brooks Range, and that tectonism was active in the eastern Brooks Range. Drift of Itkillik age is divisible into three or four components in most valleys. The oldest glacial deposits, of Itkillik I age, were formed by a single recognizable glacial advance in some valleys but by two distinct advances in others; drift of Itkillik II age forms well preserved end moraines and related deposits. Near the close of Itkillik II time, glaciers readvanced to the north flank of the Brooks Range and, south of the Continental Divide, terminated in upper parts of mountain valleys.
Glacial deposits of Gunsight Mountain advances are considered late Tertiary in age because of their extreme modification by stream erosion and pedimentation and because they resemble deposits assigned to the Tertiary elsewhere in Alaska. The Anaktuvuk River glaciation is considered to be early Pleistocene in age because correlative deposits on the Seward Peninsula are overlapped by an 800,000-year-old lava flow. The Sagavanirktok River glaciation, represented by one drift in some valleys and two in others, is considered broadly middle Pleistocene in age. Glacier advances of Itkillik I age probably occurred about 110-60 ka based on tephrochronology and on correlations with the glacial record of northwest Canada. The subsequent Itkillik II phase began about 25 ka, and a late readvance of Itkillik II age is radiocarbon dated at 13 to 11.5 ka.
The glacial record provides important insights into the tectonic, drainage, and permafrost history of northern Alaska. Distribution of the drift sheets and of associated outwash trains indicates that the major valley systems of the central Brooks Range were in place by late Tertiary time but that significant evolution continued to take place during the Quaternary. Tectonism has been minor in the northwestern part of the study area but may have remained active throughout Quaternary time to the east, as well as in troughs that parallel the south flank of the range. The oldest record of permafrost is in deposits of the early Pleistocene Anaktuvuk River glaciation, and relict glacier ice may still be present as a form of permafrost within many drift sheets of Itkillik age.
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This publication is part of a larger work. Please see Hamilton, T.D. and others, 1986 for more information.
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