The basic geologic framework of the Yukon-Tanana Upland, Alaska, a mountainous region of about 30,000 sq mi between the Yukon and Tanana Rivers, was delineated primarily by L. M. Prindle and J. B. Mertie, Jr., in the early part of this century. The subsequent recognition of large-scale offset along the Tintina fault, which bounds the eastern upland on the north, has required a reconsideration of the regional stratigraphic and structural relations.

The northwestern part of the upland is dominantly underlain by a sedimentary sequence consisting of rocks which range in age from Cambrian to Mississippian. Cretaceous and Tertiary sedimentary rocks unconformably overlie the older sequence. The Cambrian is apparently underlain by a thick section of grits, quartzites, phyllites, and quartz-mica schists. Pre-Silurian volcanic rocks, mafic and ultra-mafic rocks of probable Devonian age, and Permo-Triassic diabase and volcanic rocks are also present. These sedimentary

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and igneous rocks are cut by granitic plutons of Cretaceous and Tertiary age.

The central and eastern parts of the upland are underlain by a metamorphic complex with rocks that range from lower greenschist to amphibolite facies. Fossils date the parent sediments of some green-schist facies rocks as Paleozoic. Radiometric dates from several localities in the metamorphic complex indicate that Precambrian, Ordovician, and Jurassic-Cretaceous thermal events are recorded in the metamorphic history. Mesozoic granodiorite and quartz monzonite batholiths and smaller granitic plutons of Mesozoic and Tertiary ages intrude the crystalline schists. Locally, unmetamorphosed Cretaceous and/or Tertiary sedimentary rocks are in unconformable or fault contact with the older rocks. Tertiary volcanic rocks which range in composition from rhyolite to basalt overlie the older rocks in small but significant parts of the eastern upland. Ultramafic intrusions, mostly small and serpentinized, also are present.

Work has progressed to the point where the sedimentary rocks in the upland can be correlated reasonably well with those in other parts of Alaska, but interregional correlation of the metamorphic terranes must await additional clarification of structural and petrologic relations.

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