STRATIGRAPHIC EXAMINATION OF THE MCKAY BAY MEMBER OF THE BUSH BAY FORMATION, ENGADINE GROUP, HIAWATHA NATIONAL FOREST, UPPER PENINSULA, MI

This study focuses on the poorly understood, yet complex, Engadine Group located in the Hiawatha National Forest of the Upper Peninsula of Michigan. This mid-Silurian unit is situated above the Manistique Group and below the Salina Group. An attempt to better understand this group and its three formations has been made recently through a series of field and lab investigations. The uppermost unit of the Engadine Group, the Bush Bay Formation, is divided into three members and the uppermost member, the McKay Bay Member (MBM), was examined during this study. The MBM has a negligible dip and a maximum thickness of 20m. Consequently, a stratigraphic section was measured along a slope over ~1.2 kilometers, gaining ~15m of elevation within the Pontchartrain Shores area, east of St. Ignace, MI. An auto-level and stadia rod were used to measure this section along the shallow slope representing the paleo-lakeshore of glacial Lake Nipissing (ca. 4.5ka). Twenty-one hand samples were collected from outcrops comprised of slope breaks, bioherms, grike fields, and alvar to delineate and describe the MBM.

Hand sample analysis showed a general increase in styolites going up section as well as variances in texture and allochems. Moldic porosity with infilling was consistent among most hand samples, and there were many identifiable fossils going up section including echinoderms, brachiopods, stromatoporoids, and corals. Echinoderms (crinoids) and styolites were abundant in most samples at the microscopic scale. Overall, the majority of the MBM is made out of dolowackestones with minor dolopackstones. Allochem abundance varying up section may indicate separate reefal and interreefal facies. Using these data, a detailed stratigraphic column of the MBM has been completed, and the Engadine Group is closer to being understood.