SURFICIAL GEOLOGIC RECONNAISSANCE OF THE ST. IGNACE DISTRICT OF THE HIAWATHA NATIONAL FOREST

Silurian carbonate outcrops within the St. Ignace District of the Hiawatha National Forest were examined to develop a local description of exposed geology. The bedrock geology consists of the Manistique Group (dominantly the Cordell Formation) and the Engadine Group (Rockview Formation, Rapson Creek Formation, and Bush Bay Formation). Previous petrographic and geochemical investigations developed criteria to distinguish between these units; however, there has been little effort to make field differentiations for the area. Building on observations from previous investigations, local descriptions were developed for each outcrop that would be more useful than the general regional descriptions.

The local description of the Cordell Formation is a dark gray to light gray, chert-rich dolostone with abundant fossils. The chert varies from thin (1 – 10 cm) bands, lenses (max of 5 cm thick), and nodules and varies from grayish-blue to black in color. Fossils include corals, stromatoporoids, brachiopods, crinoids, and minor occurrences of bryozoans. Silicification of corals and stromatoporoids are common, especially within the top 5 – 8 meters of the formation.

The local field description of the RockviewFormation is a dark gray to white (weathers to dark gray), chert-poor dolostone that lacks fossils except for one lithofacies. Chert is yellowish-white in color, nodular in shape, and sandy/sucrosic in texture. Bedding features include microbialite structures that appear to be microbial mats; however, no structures remain other than differential erosional features. The fossil-rich lithofacies contains abundant brachiopod molds.

The local description of the Rapson Creek Formation is a white to light gray (weathers to dark gray), chert-free dolostone with minor fossils. Abundant stromatoporoids are found, but they are not replaced by silica. Additional fossils include corals and bryozoa.

The local description of the Bush Bay Formation is white to light reddish-brown (weathers white), chert-free crystalline dolostone with abundant fossils. Fossils include crinoids, brachiopods, and corals. Most of the fossils occur as moldic porosity, and many of the voids are filled by secondary, crystalline dolomite cement. Minor stylolites are found and stained reddish-pink to purple by iron and manganese oxides.