DISSOLUTIONALLY ENLARGED JOINTS OF THE EASTERN UPPER PENINSULA, MICHIGAN: GENESIS AND ORIENTATION

The Hiawatha Forest (HNF) of Michigan’s Upper Peninsula contains extensive dissolutional features; a subject of study for the last several years. These features form in the Engadine Group (Rockview, Rapson Creek, and Bush Bay Formations) and Manistique Group, which are mid-Silurian dolostones that in part form the Niagara Escarpment.

The research area is on recently exposed bedrock as glacial retreat occurred during the Pleistocene, ~11.7 ka. Glaciation of the area would have destroyed much of the features and weak rock of the area, leaving behind only the mechanically sound sections of dolostone. Therefore, it is probable that many of the dissolutional features are post-glacial as they would not have survived the intense conditions of glaciation. This time frame both helps establish dissolution rates and define surface hydrology.

Data collected over the previous several field seasons has focused on orientation and width of the grikes. These dissolutionally enlarged joints, which drain the deranged drainage of the area, ranged from a few cm across to over half a meter, and vary in depth from cm’s to m’s deep. We expect a dissolution rate of <50mm / 1000yrs if these features are post-glacial based on theoretical dissolution rates.

Two different populations of grike fields have been investigated; one area formed in the Rockview Formation and associated with the lakeshore of glacial lake Algonquin (11.7 ka). The other section formed in the Bush Bay and associated with glacial lake Nipissing’s lakeshore (4 ka). The Algonquin group had an average width of 14.8cm and dissolution rate of 6.72mm / 1000yrs. The Nipissing group had an average width of 14.2cm and a dissolution rate of 17.71mm / 1000yr. Orientation of grikes followed three major trends: ~5°, ~75°, and ~135°. These orientations did not fully match regional joint sets which are: ~1°, ~52°, ~91°, and ~134°.

Grikes and joints within the alvar of Drummond Island were also measured as part of a preliminary investigation into the area. The bedrock here is the Ordovician Queenston Formation. Grike and joint orientation was catalogued in 1m² sections along with width where possible. The width ranges from mm’s to cm’s wide, with an average of ~1cm. Depth was often not in excess of 1 cm. The orientation of the alvar joints was predominately at near ninety-degree angles, ~35° and ~130°.