BERRIASIAN TO THE HOLOCENE PROVENANCE CHANGES IN THE COLVILLE FORELAND BASIN, NORTHERN ALASKA: DETRITAL ZIRCON U-PB RESULTS FROM THE DALTON HIGHWAY CORRIDOR

Detrital zircon ages from 30 sandstones spanning the width and depositional history of the Colville Basin reveal a succession of provenance changes that reflect tectonic events in Arctic Alaska. The oldest sample analyzed, from a sandy lag on an unconformity above the Blankenship Member of the Shublik Formation, yields broadly distributed zircons ranging from 445 Ma to 2.5 Ga that likely reflect detritus derived from underlying Mississippian to Jurassic passive-margin deposits. Also in this sample are peaks in zircon probability at ~167 and ~140 Ma that represent detritus derived from the upper plate of the north-directed Brooks Range thrust belt and Koyukuk arc to the south. These deposits predate the Valanginian coquinoid limestone, indicating Brookian deposition probably began during the Berrisian in this area. Samples from overlying Valanginian to Albian syntectonic, wedgetop, and foredeep sandstones along the south flank of the basin (Okpikruak, Kongakut, Fortress Mountain Formations) contain large numbers of 200-500 Ma zircons. Zircons crystallized at ~200 to 350 Ma cannot have originated in the Arctic Alaska terrane because it lacks magmatic and metamorphic rocks of this age. This leads to the conclusion that the detritus was derived by erosion of Triassic strata with similar zircon assemblages in the Russian Far East and Chukchi Sea and transported longitudinally over 800 km eastward down the Colville basin. Albian and Upper Cretaceous deposits in the axial part of the Colville Basin (Nanushuk to Prince Creek Formations) in contrast, display large numbers of 360-, 390-, and ~420-Ma zircons derived by deep denudation of the nearby foreland and hinterland parts of the Brooks Range, probably during the regional extensional exhumation of the orogen that began in the Albian. Abundant 90-Ma zircons in the Turonian-Coniacian Seabee Formation reflect deposition of thick bentonitic ash beds during this time interval. The 90-Ma zircons are also present in strata above the Paleogene unconformity from the base of the Cenozoic Sagavanirkoktok Formation to the modern Sagavanirkoktok River delta. The zircons in the Cenozoic strata probably were derived from erosion of exhumed Seabee and older strata during renewed north-directed Brooks Range thrusting that took place in the Paleocene.