Abstract East-central Alaska as described in this volume (Fig. 1) is a physiographically diverse region that includes all or parts of the following physiographic divisions (Wahrhaftig, this volume): Northern Foothills (of the Alaska Range), Alaska Range (north of the northernmost strand of the Denali fault system), Tanana- Kuskokwim Lowland, Northway-Tanacross Lowland, and the Yukon-Tanana Upland. The Northern Foothills are largely rolling hills in Pleistocene glacial deposits and dissected Tertiary nonmarine sedimentary rocks. The included part of the Alaska Range is composed of highly dissected terranes of metamorphic rocks that have been intruded by Cretaceous and Tertiary igneous rocks. Mountain peaks reach altitudes as high as 4,000 m, and relief is commonly more than 1,000 m. Glaciers have carved a rugged topography. The Tanana-Kuskokwim Lowland is covered with thick glacial, alluvial, and wind-blown deposits. The Northway-Tanacross Lowland consists of three small basins mantled with outwash gravel, silt, sand, and morainal deposits. The Yukon-Tanana Upland, the largest of the physiographic divisions, consists of maturely dissected hills and mountains with altitudes as high as 1,994 m, and relief ranging from a few to hundreds of meters. Some of the highest areas supported small alpine glaciers during the Pleistocene, and rugged topography resulted locally. With the exception of the Alaska Range, outcrops in eastcentral Alaska are commonly widely scattered and small, due to extensive surficial deposits and vegetation. The vegetation ranges from heavy spruce forests along large streams to tundra at elevations of approximately 1,000 m. The region is largely in the zone of discontinuous

Abstract The basic geologic framework of the Yukon-Tanana upland, Alaska, a mountainous region of about 30,000 sq mi (77,700 sq km) 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 predominantly 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 ultramafic rocks of probably Devonian age, and Permo-Triassic diabase and volcanic rocks are also present. These sedimentary 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 of rocks which range from lower-greenschist to amphibolite facies. Fossils date the parent sediments of some greenschist-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 age 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 ranging in composition from rhyolife to basalt overlie the older rocks in small but significant parts of the eastern upland. Ultramafic intrusions, mostly small and serpentinized, also occur. Work has progressed to the point where the sedimentary rocks in the upland can reasonably be correlated with those in other parts of Alaska, but interregional correlation of the metamorphic terranes must await additional clarification of structural and petrologie relations.