Information Circular 27. Minnesota Kaolin Clay Deposits: A Subsurface Study in Selected Areas of Southwestern and East-Central Minnesota

Abstract

Large deposits of kaolin-the residual products of the weathering of igneous and metamorphic rocks-are common in southwestern and east-central Minnesota. Thin and discontinuous beds of sedimentary kaolin (resulting from the reworking of the residuum) are less common and volumetrically less significant. The weathering occurred prior to deposition of Late Cretaceous sedimentary strata. Because climatic conditions were uniform within the study area, the differences in the thickness and composition of the residuum are attributable to the mineralogic and hydrologic properties of the parent rock (protolith). Goldich (1938) and others have shown that weathering and the products of weathering are strongly controlled by the mineralogic composition of the protolith. Rocks rich in mafic minerals and plagioclase are generally more extensively weathered, and their residuum is richer in kaolinite. This susceptibility to weathering is evident in the mineralogic composition of the residuum and is reflected in the concentration of kaolin near the top, and in the addition of other minerals with increasing depth. The weathering process is also dependent on the movement of water and other fluids; therefore, permeability is a second important control on the production of kaolin. In igneous and metamorphic rocks, fractures in the form of joints or faults are the primary paths of fluid movement. Weathering follows these paths, and the resulting kaolin deposits mimic the shape and orientation of the structural features. Preservation is the final factor controlling the distribution of kaolin clays. The upper part of the residuum contains the largest amount of clay, but it also is the most vulnerable to erosion. Glacial erosion has had a profound effect on the distribution and thickness of kaolin clay deposits in the state. The thickest deposits are found where Late Cretaceous sediments overlie the kaolin and indicate total preservation of the weathered products.