Traverse Rocks of Thunder Bay Region, Michigan

Abstract

The Thunder Bay region of Michigan contains the only complete outcrop section of the Devonian Traverse rocks, an assembly described in detail for the first time in this paper. The lower part of the section contains much shale and alternations of shale and limestone. These comprise, in ascending order, the Bell shale, Rockport Quarry limestone, Ferron Point shale, and Genshaw formation. Reefs and biostromes are prominent in the middle of the section. The formations resulting from this type of deposition are the Newton Creek limestone, Alpena limestone, and Four Mile Dam formation. Following the reef stage, shales and impure limestones were deposited to form the Norway Point, Potter Farm, Thunder Bay, and Squaw Bay formations. Older parts of the group are Middle Devonian, but the three last-named formations are Upper Devonian in age. Unconformities are present in this area at the base and top of the Traverse, at the top of the Alpena limestone, and probably at the top of the Norway Point beds.