EPIGURUK - A LATE QUATERNARY ENVIRONMENTAL RECORD FROM NORTHWESTERN ALASKA

Epiguruk, a prominent bluff along the Kobuk River in northwestern Alas ka, exposes a rich depositional record of Quaternary eolian and fluvia l sand, with associated loess, paleosols, and periglacial features. Th ree major complexes of alluvial and eolian deposits are separated by t wo conspicuous organic-rich paleosols which formed during cool-moist i nterstadial intervals. Sediments between the two paleosols include eol ian, channel, and floodplain deposits that formed during alluviation o f the Kobuk River to a height of about 12 m above the present level. T he youngest depositional complex, which overlies the upper paleosol, i s divisible into late Wisconsinan and Holocene components and into flu vial-channel, flood-plain, eolian-dune, sand-sheet, loess, and pond fa cies. Eolian sand from the active Kobuk sand sea overloaded the river during late Wisconsinan time, causing it to alluviate to about 13 m ab ove its modern level. The Holocene record reflects erosion and deposit ion by a small southern tributary to the Kobuk River, downcutting by t he 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 ky. at Epigu ruk. 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, coi ncident with the Fox termal event of central Alaska. Although higher-s tanding sandy facies of this paleosol began forming earlier, thick pea t beds did not accumulate until about 33 ka at those sites. Late Wisco nsinan loess influx and alluviation began about 24 ka, synchronous wit h the Itkillik II glaciation of the Brooks Range. The river overlapped progressively higher surfaces of the upper paleosol, attaining its ma ximum height of about 13 m above modern river level between 20 and 19 ka. It downcut about 18.5 ka and subsequently migrated northward. Depo sits at the north end of the bluff show that the river had begun build ing its modern flood plain by 8.6 ka, and it has varied little in elev ation since that time.