STRATIGRAPHIC FRAMEWORK OF SILICIFICATION IN THE MIDDLE SILURIAN CARBONATES OF THE ST. IGNACE DISTRICT OF THE HIAWATHA NATIONAL FOREST, UP MICHIGAN

Sections of the Middle Silurian dolostones in the St. Ignace District of the Hiawatha National Forest (HNF) were measured and sampled in order to determine the mechanism(s) and relative timing of silicification. Silicified features ranged in scale from thickly bedded cherts (several cm’s) down to microscopic replacement. In general, the relative degree of silicification is an important stratigraphic marker for the carbonates in the St. Ignace District of the HNF. Lower portions of the stratigraphic section displayed the most silicified features (as observed in the outcroppings at Scott Quarry), while upper portions of the section displayed little to no chert at the outcrop scale. The most obvious silicified features included silicified fossils, bedded and nodular chert, and megaquartz-filled vugs. Additionally, key stratigraphic sections were measured at scales to resolve chert beds, bands, and nodules.

Hand-sized samples were collected with a hammer when possible, and chainsaw drill cores were obtained in instances where collecting with a hammer was impossible. Samples were thin sectioned and powdered for geochemical analyses (ICP-OES and stable isotopes). Large-scale observations coupled with microscopic relationships help imply the mechanism(s) of silicification and silica source. In addition, microscopic observations reveal relict features that are often not preserved in adjacent dolomitized regions. These relict features (silicified allochems and silica replaced anhydrite), coupled with textures of the silicified material and geochemical and petrographic signature of the dolostone, tease out the mechanism(s) and relative timing of diagenetic events. Interpretations based on previously overlooked silicified material in these carbonates provide a framework to understand the complete diagenetic history of the carbonates and regional stratigraphy of the St. Ignace District of the HNF.