Lower Paleozoic or Proterozoic basement rocks occur in windows and thrust plates in several areas of the Brooks Range. Uranium-lead radiometric analyses of highly metamorphosed rocks from the Baird Mountains and Ernie Lake area have yielded Proterozoic ages. Structural, stratigraphic, petrologic, and isotopic evidence exists for Proterozoic(?) rocks in the schist belt; around the Chandalar, Arrigetch, and Igikpak plutons; and in the Cosmos Hills window. Fossiliferous, lower Paleozoic, low-grade metasedimentary rocks occur in the Romanzof Mountains, Doonerak window, and Baird Mountains, and may also surround the Chandalar plutons. Locally, the Lower Paleozoic rocks are unconformably overlain by Devonian to Mississippian metasediments and may stratigraphically overlie older higher grade metamorphic rocks. Similarities in the stratigraphic settings and lithologies and in fossil ages and affinities allow correlation of the lower Paleozoic rocks in the southern Brooks Range.

Correlation of lower Paleozoic rocks exposed beneath the Endicott allochthon at the Doonerak fenster with coeval rocks in an overlying thrust plate to the south at Snowden Mountain is especially significant. A west-trending thrust fault, which is rooted in lower Paleozoic basement, along the north side of Snowden Mountain is postulated to account for these relationships. Apparently, the Endicott allochthon roots beneath the Snowden Mountain thrust fault. Evidence from conodont samples currently being studied by A. Harris may bear on the extent of the lower Paleozoic rocks in the upper plate of the Snowden Mountain thrust and in the Chandalar area.

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