Paleozoic Sequence in Eastern Brooks Range, Alaska

Abstract

The sequence described lies in the eastern Brooks Range in northeastern Alaska. Rocks of late Paleozoic age predominate but rocks as young as Jurassic are present. Rocks of Devonian and possibly older age occur in two east-trending belts. In the southern belt the Skajit Limestone, 500-2,600(?) feet thick, formerly believed to be of Silurian age, is now determined to be of Middle(?) and Late Devonian age. An unnamed unit of limestone and siltstone (Upper Devonian), 0-2,500(?) feet thick, rests on the Skajit. The limestone-siltstone unit intertongues with and is overlain by an unnamed unit of slate and sandstone (Upper Devonian) 2,000-4,500 feet thick, which locally rests disconformably on the Skajit. The Kanayut Conglomerate (Upper Devonian), 0-5,000 feet thick, conformably overlies the slate-sandstone unit and probably grades into it laterally. Rocks in the northern belt comprise the four members of the Neruokpuk Formation of Late(?) Devonian or older age; a quartzite-schist member more than 4,000 feet thick, a limestone member 1,000± feet thick, a limestone-phyllite member 1,000± feet thick, and a phyllite-chert member, as much as 3,300 feet thick, which overlies the other members, locally with strong angular unconformity. The Kekiktuk Conglomerate, a new formation of Devonian or Mississippian age, 0-400 feet thick, unconformably overlies the Neruokpuk Formation. The formations of Mississippian age occur in both belts. The Kayak Shale (Lower Mississippian) and Kayak(?) Shale (Lower(?) and Upper Mississippian), 400-1,200 feet thick, rest on the Kanayut and Kekiktuk Conglomerates and, locally, on the Neruokpuk Formation. The Lisburne Group comprises three formations. The Wachsmuth Limestone (Lower Mississippian), 0-950 feet thick, disconformably overlies the Kayak Shale in the western part of the area. The Alapah Limestone (Upper Mississippian), 600-1,950 feet thick, rests disconformably on the Wachsmuth Limestone. Where the Wachsmuth is absent, the Alapah rests conformably on the Kayak(?) Shale. The Wahoo Limestone (a new formation of Pennsylvanian(?) and Permian age), 0-1,400 feet thick, overlies the Alapah with probable disconformity. The Siksikpuk Formation (Lower(?) Permian), is more than 600 feet thick and consists mostly of shale. It rests unconformably on the Wahoo Limestone at Galbraith Lake and on the Alapah Limestone west of Galbraith Lake. East of Galbraith Lake the Sadlerochit Formation (Permian and Triassic) overlies the Wahoo limestone. The Sadlerochit is more than 1,000 feet thick, and consists of sandstone, siltstone, and shale. Unconformities suggest uplift during early Late Devonian time and folding during Late Devonian to Early Mississippian time. The regional unconformity between Mississippian and Permian rocks diminishes eastward and deposition may have been continuous through Pennsylvanian time in part of the eastern Brooks Range.