Reconnaissance geology of the northern Yukon-Koyukuk Province, Alaska

Abstract

The northern part of the Yukon-Koyukuk province is a broad wedge-shaped depression of Cretaceous and Tertiary volcanic and sedimentary rocks bordered on the west, north, and southeast by a metamorphic complex, chiefly of Paleozoic age. The province is not a simple sedimentary basin, as formerly believed, but a highly mobile tract subjected to repeated volcanism and plutonism during Cretaceous and Tertiary times. The oldest rocks in the province are a thick sequence of marine andesitic volcanic rocks of earliest Cretaceous (Neocomian) age. These are overlain by terrigenous sedimentary rocks, largely of Early and Late Cretaceous (Albian and Cenomanian) age, which were deposited in two deep troughs separated by a broad volcanic and plutonic high. The sedimentary sequence, which locally may be as much as 25,000 feet thick, is made up chiefly of marine volcanic graywacke and mudstone but includes coalbearing paralic deposits in the upper part. Subaerial felsic volcanic rocks of Late Cretaceous and early Tertiary age are widespread in the eastern part of the province, and two broad basalt lava fields of late Cenozoic age occur in the western part of the province.