Abstract

Epiguruk, a prominent bluff along the Kobuk River in northwestern Alaska, exposes a rich depositional record of Quaternary eolian and fluvial sand, with associated loess, paleosols, and periglacial features. Three major complexes of alluvial and eolian deposits are separated by two conspicuous organic-rich paleosols which formed during cool-moist interstadial intervals. Sediments between the two paleosols include eolian, channel, and flood-plain deposits that formed during alluviation of the Kobuk River to a height of about 12 m above the present level. The youngest depositional complex, which overlies the upper paleosol, is divisible into late Wisconsinan and Holocene components and into fluvial-channel, flood-plain, eolian-dune, sand-sheet, loess, and pond facies. Eolian sand from the active Kobuk sand sea overloaded the river during late Wisconsinan time, causing it to alluviate to about 13 m above its modern level. The Holocene record reflects erosion and deposition by a small southern tributary to the Kobuk River, downcutting by the Kobuk River toward its modern level, and subsequent erosion across a meander belt nearly 8 km wide.

Sixty-six radiocarbon ages, many from rooted shrubs, provide a firm chronology for the past 35 k.y. at Epiguruk. The Kobuk River incised to near its present level by about 35 ka, and the upper paleosol began forming near that level about 33 ka, coincident with the Fox termal event of central Alaska. Although higher-standing sandy facies of this paleosol began forming earlier, thick peat beds did not accumulate until about 33 ka at those sites. Late Wisconsinan loess influx and alluviation began about 24 ka, synchronous with the Itkillik II glaciation of the Brooks Range. The river overlapped progressively higher surfaces of the upper paleosol, attaining its maximum height of about 13 m above modern river level between 20 and 19 ka. It downcut about 18.5 ka and subsequently migrated northward. Deposits at the north end of the bluff show that the river had begun building its modern flood plain by 8.6 ka, and it has varied little in elevation since that time.

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