CRETACEOUS-EARLY EOCENE TWO STAGE BASIN DEVELOPMENT, YUKON FLATS BASIN, NORTH-CENTRAL ALASKA

The Yukon Flats basin of north-central Alaska is a 22,000 km² active basin that is located to the north of the Tintina fault and contains as much as 4.5 km of Cenozoic strata based on seismic data. New sedimentological and palynological data collected from three outcrop belts along the southern border of the Yukon Flats basin provide an opportunity to study the evolution of this basin system. New palynological data indicates that the oldest of these outcrop belts, located along Cheyenne Creek, was deposited during the Late Cretaceous (Maastrichtian). The outcrop belts exposed near the village of Rampart, and along the Yukon River near the Drew Mine, are both Paleocene-early Eocene. Our new data indicate the following two stages of basin development: (1) Maastrichtian: Conglomerate, coarse sandstone, and mudstone were deposited by braided fluvial systems that flowed to the west/northwest. This outcrop belt is enriched in quartz and chert. These strata are interpreted to have been derived from an Aptian/Albian thrust belt located to the east/southeast of the basin. (2) Paleocene- Late Eocene: Mudstone, coal, fine sandstone and conglomerate were deposited by lacustrine systems and eastward-flowing, braided fluvial systems. This petrofacies, dominated by greenstone and chert, is interpreted to have been derived from the Mississippian-Triassic Rampart Group that is presently exposed on the west and northwest flank of the Yukon Flats basin.

Maastrichtian strata may be related to extension and thrust reversal caused by the initiation of strike-slip displacement on the Tintina fault. These basinal strata, and the thrust belt from which they are derived, are proximal to the Charley River thrust belt when ~400 km of post-Paleocene dextral displacement is restored to the Tintina fault. The second phase of deposition, represented by the Paleocene-early Eocene strata, may be the product of extension related to early Tertiary strike-slip displacement on the Tintina fault. Basin development was coeval with bimodal volcanism, granite emplacement, and the cooling of metamorphic rocks to the south of the basin. This stage of basin development is inferred to represent regional tectonic subsidence that led to the development of poorly organized, internally drained watersheds with intermittent ponding of water within the basin.