GEOCHEMICAL DELINEATION OF THE MIDDLE SILURIAN CARBONATES OF THE HIAWATHA NATIONAL FOREST, MICHIGAN

Many Silurian carbonate sequences are dominant within the Michigan Basin. The groups of interest in this study are the Manistique Group and Engadine Group, specifically the Cordell Formation of the Manistique and the Rockview, Rapson Creek, and Bush Bay Formations of the Engadine. All of these formations are characterized by cherty to non-cherty dolostones that are very similar in outcrop appearance. Eight sites were visited within the Hiawatha National Forest (HNF) of Michigan. Petrographic and geochemical data were used to determine the location and nature contacts between these formations. Some contacts were inferred due to the highly vegetated nature of the field location and certain outcrops were placed in their stratigraphic location based on bedding attitude.

This study further delineates the contrast of the two Silurian carbonate groups using stable isotope data, trace element data, mineralogical results, and petrographic with cathodoluminescence (CL) microscopy. The Manistique and Engadine samples plot in two overlapping but distinct fields on ¹³C vs ¹⁸O plots. Specific formations within the Engadine do not break into separate fields on similar plots. Similar trends exist when comparing Fe content and ¹³C for the two groups. These trends suggest that the Manistique and Engadine Groups were dolomitized by either geochemically distinct fluids or a fluid with evolving geochemical characteristics during the dolomitization process. These are preliminary findings based on bulk geochemistry, and high-resolution studies will be needed to determine the exact geochemical nature of these carbonates.