NEW 1:250,000 GEOLOGIC MAP OF THE WESTERN ALASKA RANGE: PORTIONS OF THE LIME HILLS, MCGRATH, TALKEETNA AND TYONEK QUADRANGLES

The Western Alaska Range is bounded on the north by the Denali-Farewell fault, on the east by the Susitna lowland, on the south by the Cook Inlet lowland and Merrill Pass, and on the west by the Swift-Big River lowland. The Farewell terrane is exposed in the NW third of the map area. It consists of widespread non-Laurentian Cambrian-Devonian deep-water carbonate and siliciclastic strata (Dillinger sequence) and more sporadic exposures of younger strata (Mystic sequence). Mystic units consist of diverse Devonian lithofacies including shallow marine carbonates, upper Paleozoic deepwater clastics, Permian and Triassic conglomerates with Dillinger clasts of variable age and thermal index, Late Triassic-Early Jurassic lavas and dikes with rift chemistry, Lower Jurassic phosphatic and quartz-rich clastics, and rare Lower Cretaceous coquinid limestones. Early SW-verging folds (Permian?) are overprinted by Late Cretaceous NW-verging folds that also deform the NW part of the adjacent Cretaceous Kahiltna basin.

Kahiltna strata underlie the central third of the map area and are divisible into an Upper Cretaceous quartz-rich petrofacies on the NW and an Upper Jurassic to Upper Cretaceous volcaniclastic petrofacies in the SE, separated by a sharp contact interpreted as a fault. South of a NW band of NW-verging folds, Kahiltna strata are deformed in SE-verging folds. These strata have detrital zircon peaks as young as 85 Ma and zircon fission track ages near 80 Ma, and are cut by post-deformational plutons as old as 77 Ma, constraining deformation and fault juxtaposition of the two petrofacies to a narrow 85-77 Ma time window.

The southern third of the map area consists predominantly of Cretaceous to Quaternary plutonic and volcanic rocks of the Alaska Peninsula magmatic arc intruding or overlying Kahitna strata. 104-85 Ma plutons are restricted to the Peninsular-Wrangellia terrane, 80-75 Ma plutons extend north into the central Kahiltna basin, while 72-67 Ma plutons extend across the entire map area. 72-67 and 44-37 Ma units are elongate north-south, while most 63-55 Ma plutons and volcanic fields are more equant. 31-25 Ma plutons and <1 Ma volcanic rocks (Spurr and Hayes volcanoes) are more localized. Cu-Au deposits are associated with 77-76 and 72-67 Ma magmas, while REE prospects are associated with 63-57 and 31-25 Ma plutons.