Geology of the Skagway B-3 and B-4 quadrangles, southeastern Alaska

Abstract

The Skagway B-3 and B-4 quadrangles are near the northern extremity of southeastern Alaska in a mountainous region characterized by abundant glacier-related erosional and depositional features. The Chilkat River fault, a segment of the Chatham Strait fault which is a major tectonic element in southeastern Alaska, underlies the markedly linear Chilkat River valley and separates the quadrangles into two distinctive geologic terranes. The terrane east of the fault is dominated by igneous rocks of Cretaceous and earl y Tertiary age. These rocks consist of an older mafic and ultramafic assemblage that includes metamorphosed lavas, gabbro and diorite, and pyroxenite and the younger quartz diorite and related rocks of the Coast Range batholith. Structural and topographic trends east of the fault have a markedly linear northwestward orientation, which is typical of large parts of southeastern Alaska.