Stratigraphy and Deformation of Paleozoic Section at Anaktuvuk Pass, Central Brooks Range, Alaska

Abstract

The stratigraphic section at Anaktuvuk Pass consists of more than 13,000 feet of marine and non-marine sedimentary rocks ranging in age from Late Devonian through Permian. Non-marine Upper Devonian rocks include more than 6,300 feet of plant-bearing shale, siltstone, sandstone, and conglomerate, overlying a thick sequence of shale and slate. The Mississippian section, consisting of 4,680 feet of fossiliferous marine sandstone, black shale, dolostone, limestone, and chert, is overlain unconformably by approximately 400 feet of Permian marine shale and siltstone. Post-Paleozoic deformation of these rocks resulted in large-scale imbricate deep-seated thrust faults and large folds overturned toward the north; near the north front of the range the structures grade into broad o en folds associated with shallow thrusts that are limited to Mississippian and younger rocks. Most structural axes, both major and minor, trend east-west, and most thrusts, which tend to be localized along incompetent rock units, dip south, indicating northward release of deformational stresses. The Anaktuvuk thrust, exposed beneath isolated klippen on mountain summits northwest of Anaktuvuk Pass, is a major structural feature with minimum northward displacement of 8 miles. This thrust, confined to Mississippian and younger rocks, is shallow and appears to have moved northward from a structural high that forms the crest of the range. Absence of pre-Upper Devonian strata in exposed deep-seated thrust sheets suggests that the lowest unit of the Paleozoic section at Anaktuvuk Pass, thick in ompetent shale and slate, may constitute the base of a regional decollement in the northern half of the range. Within the area, intensity of deformation decreases gradationally northward as the Paleozoic section thins and as detrital sediments grade into coarser facies.