RI-10 Clay Mineralogy, Fabric, and Industrial Uses of the Shale of the Decorah Formation, Southeastern Minnesota

Abstract

The Decorah shale varies both vertically and laterally in its clay mineral assemblages. It is principally an illitic shale in Minnesota, but kaolinite is approximately of equal abundance in the basal part to the southwest, reflecting nearness to the source area, most probably the Transcontinental Arch. Lateral variations in clay mineral assemblages of the Decorah are similar to that of the coextensive older Ordovician Glenwood Formation, suggesting that the source was the same and that both were deposited under similar conditions. The vertical variation in clay mineral assemblages in the Decorah shale, from kaolinite and illite toward the base to only illite in the upper part, reflects the transgressive nature of the Decorah sea. Certain engineering properties of the Decorah shale are related to the orientation of clay minerals, The shale is relatively impermeable perpendicular· to bedding where the clay minerals are arranged with their shortest axis normal to the bedding surface, as in a deck-of-cards arrangement, but is permeable parallel to bedding. In contrast, a random clay mineral arrangement, as in the card-house structure, is permeable in all directions. Both types of clay mineral arrangements are present in the Decorah shale, Maintaining the natural moisture content of the Decorah shale during and after construction aids in stabilizing slopes and foundation bases. The clay mineral data suggest that the Decorah shale may be satisfactory for use in ceramic products such as face brick, sewer pipe, lightweight aggregate, structural tile, and drain tile. Numerous areas along the shale's outcrop belt in southeastern Minnesota would be satisfactory for open pit mining.