Lower Paleozoic carbonate rocks of the Baird Mountains quadrangle, western Brooks Range, Alaska

Abstract

Lower Paleozoic carbonate rocks in the Baird Mountains quadrangle form a relatively thin, chiefly shallow-water succession that has been thrust-faulted and metamorphosed to blueschist and greenschist facies. Although this succession was thought to be mostly Devonian until recently, a large part of it is in fact pre-Silurian in age. Middle and Upper Cambrian rocks - the first confirmed in the western Brooks Range - occur in the northeastern quarter of the quadrangle, south of Mt. Angayukaqsraq. These rocks consist of massive marble that grades upward into thin-bedded metalimestone/dolostone couplets and contain pelagiellid mollusks, acrotretid brachiopods, and agnostids. Sedimentologic features and the Pefagiellas indicate a shallow-water depositional environment. Overlying these Cambrian rocks is a thin sequence of Lower arid Middle Ordovician metalimestone and phyllite containing graptolites and cool-water, mid-shelf to basinal conodonts. Upper Ordovician rocks in the Mt. Angayukaqsraq area are bioturbated to laminated dolostone containing conodonts of warm-, shallow-water biofacies. In the Omar and Squirrel Rivers area to the west, the Lower Ordovician carbonate rocks are thicker and quite different in lithofacies and biofacies. These rocks are mainly dolostone with locally well-developed fenestral fabric and evaporite molds, and bioturbated to laminated orange- and gray-weathering dolomitic marble and metalimestone. Conodonts and sedimentary structures indicate deposition in restricted to normal marine, shallow to very shallow water platform environments. Exposures of Upper Silurian rocks occur near Mi. Angayukaqsraq and on the middle fork of the Squirrel River, and consist mostly of thinly laminated dolomitic mudstones. Conodonts in these rocks indicate deposition in a somewhat restricted, shallow-water environment. Devonian carbonate rocks are widely distributed in the western Baird Mountains quadrangle; at least two distinct sequences have been identified. In the Omar and Squirrel Rivers area, Lower and Middle Devonian dolostone, metalimestone and marble are locally cherty and rich in megafossils. To the north, in the Nakolik River area, Middle and Upper Devonian marble and metalimestone are interlayered with planar- to cross-laminated quartz-carbonate metasandstone and phyllite. Baird Mountains carbonate rocks show some striking similarities in biofacies and lithofacies to lower Paleozoic carbonate rocks of the Seward Peninsula to the southwest and the central Brooks Range to the east.