PETROGRAPHIC ANALYSIS OF THE BUSH BAY FORMATION, ENGADINE GROUP, HIAWATHA NATIONAL FOREST, UPPER PENINSULA, MI

The mid-Silurian Engadine Group in the Hiawatha National Forest is poorly understood, and comprised of three dolostone formations. The upper-most unit, the Bush Bay Formation, is the focus of this study. The Bush Bay Formation has a negligible dip and a maximum thickness of 20m. A stratigraphic section of the Bush Bay Formation was measured by walking up a slope over a distance of roughly 1.2 kilometers, gaining about 15m of elevation within the Pontchartrain Shores area, east of St. Ignace, MI. An auto-level and stadia rod were used to measure the stratigraphic section along a shallow slope representing the paleo-lakeshore of glacial Lake Nipissing (ca. 7.5ka). Twenty-one hand samples were collected from outcrops which were comprised of slope breaks, reef mounds, grike fields, and alvar.

Hand sample analysis was conducted and showed a general increase in styolite abundance going up section as well as minor variations in allochem variety. Moldic porosity with infilling, and iron staining was present in the majority of samples and there was a general increase of identifiable fossils going up section (i.e. stromatoporids, corals, brachiopods, and echinoderms).

Standard petrographic thin section analysis was then conducted and the results differed slightly from that of the hand sample analysis. Petrographic analysis showed abundant styolites, minor moldic porosity, iron staining in about 50% of the samples, and silica replacement of allochems in about 30% of the samples. Echinoderms were abundant in most samples at the microscopic scale with brachiopods and tabulata coral found in only a few samples (<20%). The Bush Bay Formation on the whole is a dolowackestone, based on petrographic analysis. With the hand sample and thin section data a detailed stratigraphic description of the Bush Bay Formation has been completed and the Engadine Group is closer to being understood.