PETROGRAPHIC ANALYSIS OF SILICIFIED CORAL FROM THE MANISTIQUE GROUP OF THE MICHIGAN BASIN

Samples of silicified coral were collected from outcrops of the Cordell Formation from the Manistique Group located in the Hiawatha National Forest, Upper Peninsula Michigan. The silicified coral were analyzed petrographically using standard petrographic microscopy and cathodoluminescence to determine extend and nature of silicification. Petrographically, the silicified sections of coral consisted of chalcedony and megaquartz filled septa. The surrounding dolomitized sections of the rock were dolowackestones with very poor luminescence. Abundant pyrite is associated with the megaquartz, and the texture of the chalcedony suggests replacement of anhydrite. Several hypotheses about the relative timing between silicification and dolomitization exist. The observations of this study places dolomitization prior to silicification. The silicification here is dependent on evaporite emplacement, and occurs after deposition of the overlying Salina Group. Initial dolomitization and silicification are early diagenesis products, while dolomite cementation occurs during late burial. Further geochemical and mineralogic analyses will continue to refine the exact timing of these events and place the extensive chert elsewhere in the Manistique Group within context.