Geology of Mackinac Straits in Relation to Mackinac Bridge

Abstract

The construction of Mackinac Straits bridge has provided new data on the Paleozoic and Pleistocene history of the Mackinac region. The bedrock includes carbonates, chert, shale, and gypsum of late Silurian and middle Devonian age. Leaching of salt in the Pointe Aux Chenes (Upper Silurian) formation permitted the collapse of overlying rocks and produced the Mackinac breccia, but brecciation was not so intense or of such great magnitude as suggested in earlier studies. The breccia has been reconsolidated by loading under younger sedimentary rocks, recemented by secondary calcite, and perhaps additionally compacted by loading under glacial ice. The breccia has a bearing capacity considerably in excess of the load imposed by the bridge. Glacial deposits above bedrock are till and lacustrine clays of the Valders glacial substage, although older glacial deposits occur on the northern sides of the submerged Mackinac River gorge and a tributary channel. A buried sea stack encountered in probing indicates a level ...