PETROGRAPHIC ANALYSIS OF CHERT WITHIN THE MANISTIQUE GROUP OF THE MICHIGAN BASIN

Samples of chert-bearing dolostone were collected from the Cordell Formation of the Manistique Group at Scott Quarry in the Hiawatha National Forest of Michigan’s Upper Peninsula. Petrographically, two chert textures were identified and they corresponded to stratigraphic position. The lower, older cherts were crystalline with a nodular texture, while the upper, younger chert had a sucrosic texture. Crystalline cherts contained sponge spicules, echinoderms, arthropods, and stromatoporoids that appear as “ghost” grains. Sucrosic chert was associated with dolostone that contained crinoids and stromatoporoids. The origin of the chert in these samples is hypothesized to be from the recrystallization of siliceous allochems. This recrystallization occurs during early burial, when siliceous fluids move upward. The lower sections of the Cordell Formation contain more siliceous organisms, producing larger, crystalline chert nodules. Whereas, the upper sections of the Cordell Formation contain more calcic organisms, with minor silica replacement resulting in a sucrosic texture. There is an abrupt change in chert texture between the two sections, denoting subunits of the Upper Cordell Formation in the Hiawatha National Forest.