Arguments against Paleoproterozoic glaciation in the Lake Superior Region, U.S.A., Part 3: The Fern Creek Formation

Previous interpretations of the Paleoproterozoic Fern Creek Formation in the southern Lake Superior region (northern Michigan, U.S.A.) have characterized it as a glaciogenic deposit. This interpretation originated from a 1943 study and has been widely accepted in subsequent literature without rigorous re-evaluation. Later studies often presumed a glacial origin, relying on this early interpretation rather than independent sedimentological or stratigraphic evidence. Some researchers proposed a correlation between the Fern Creek Formation and the Huronian Supergroup in Canada; however, this correlation is tenuous due to poorly constrained age data and rests primarily on the assumption of a shared glacial origin. This paper constitutes Part 3 of a three-part series re-examining Paleoproterozoic “glacial” formations in the Lake Superior region; regional geology and classification methodology are summarized in Parts 1 and 2 (Larue, 2025a, 2025b). The present study focuses on a detailed sedimentological analysis of the Fern Creek Formation to evaluate depositional environments and sedimentary processes. Field and petrographic observations indicate that sedimentation occurred through non-glacial alluvial processes. Complementary geochemical analyses, including application of the Chemical Index of Alteration (CIA), reveal that the composition of Fern Creek lutites closely resembles that of lutites from tectonically active Cenozoic basins, further supporting a non-glacial origin. Collectively, these findings eliminate the necessity of invoking glacial processes to explain the genesis of the Fern Creek Formation. Instead, the data support interpretation of the formation as the product of deposition within an alluvial incised-valley system.