STRATIGRAPHY AND STRUCTURE OF THE NORTHEASTERN DOONERAK WINDOW AREA, CENTRAL BROOKS RANGE, NORTHERN ALASKA

At the eastern terminus of the Doonerak Window four fault-bounded tectono-stratigraphic assemblages are recognized; they are from bottom to top: (1) lower Paleozoic phyllites, slates, and volcanic rocks (Apoon assemblage); (2) Mississippian imbricates consisting of conglomerates, shales, and limestones (Blarney Creek assemblage); (3) Endicott Mountains assemblage consisting of a lower sequence of calcareous clastic rocks and an upper sequence of noncalcareous clastic rocks; and (4) a lower Paleozoic assemblage composed of marbles, phyllites, and slates (Skajit assemblage). Previous workers have correlated the upper sequence of the Endicott with the Upper Devonian Hunt Fork Shale. The contact between the lower sequence and the Hunt Fork is gradational and interfingering. Limestones in the lower sequence contain Middle to Upper Devonian corals. Thus, the lower sequence can be correlated with the Beaucoup Formation. The Apoon/Blarney Creek contact was previously thought to be an angular unconformity. Relationships at the contact demonstrate that for the most part it is, instead, a tectonic contact. Where the contact is depositional, it is a disconformity. Three penetrative phases of folding (D1b, D2, and D3) are recognized in all four assemblages; an older phase (D1a) is extremely rare, but was observed in the Hunt Fork. No pre-Mississippian structures were encountered. The D1 and D2 structures are related to north-directed thrusting during the Jurassic-Cretaceous, while D3 may be related to late-stage left-lateral strike-slip displacement along EW-striking faults. A "breach" fault truncates the Blarney Creek assemblage on the southeastern flank of the Doonerak Window, and juxtaposes the two different facies of the Endicott Mountains assemblage on the eastern flank. The basal thrust of the Endicott Mountains assemblage is continuous, demonstrating that the Doonerak Window is an actual window. The observed structures can only be explained by a duplex model. The amount of shortening in the central Brooks Range is greater than 350 kilometers as estimated from balanced cross-sections