The layered rocks in this area originated as continental shelf sediments overlain by volcanic arc successions. Now called Yukon-Tanana Terrane, they tectonically over-rode the western edge of ancient North America beginning in Middle Jurassic time.
Yukon-Tanana Terrane
Three elements are present in the map area. The west half comprises the Big Salmon Complex; the east half is a separate, in part contemporaneous succession composed of the Dorsey Complex and Swift River Group. Unconformably overlying both these elements are less metamorphosed Klinkit Group and Triassic sediments that are here interpreted as overlap assemblages. The unexposed contact between Big Salmon Complex and Swift River Group is inferred to be an east-side-down normal fault.