Summary

Description

The vegetation, topography, and geology of the Johnson River area are representative of the entire eastern interior region of Alaska. This area has a vegetational cover of both boreal forest and upland tundra, has a continental subarctic climate, and is within the zone of discontinuous permafrost. The topographic units include the rolling hills of the YukonTanana Upland, the terraces and moraines on the TananaKuskokwim and NorthwayTanacross Lowlands, and the rugged peaks and deep canyons of the Alaska Range. The bedrock is composed mostly of metamorphic and igneous rocks. The metamorphic rocks, largely schist and gneiss, have been profoundly deformed and are possibly Precambrian in age. Quartzite, hornfels, and amphibolite are minor constituents: of the metamorphic complex. The metamorphic rocks have been invaded by granitic rocks of Cretaceous age, and dikes of several ages cut both the metamorphic and igneous masses. Contacts between the igneous and metamorphic rocks include contact breccias, crosscutting intrusive contacts, and a contact zone of mixed igneous and metamorphic rocks. Although the granitic rocks include granodiorite, granite, quartz monzonite, quartz diorite, and diorite, similarities in mineral and chemical composition of representative rocks and similarity in the absolute age of scattered specimens suggest that they are genetically related and probably part of a single pluton. Leadalpha age determinations were in close agreement and ranged from 90 to 110 10 million years.