Importance of the Regelation Process to Certain Properties of Basal Tills Deposited by the Laurentide Ice Sheet in Iowa and Illinois, U.S.A

Abstract

Recent stratigraphic studies have characterized the sequence and nature of tills deposited in Iowa and Illinois by Quaternary Laurentide ice sheets. Basal tills within the sequence are each dominated by matrix-size materials and exhibit uniform matrix properties over hundreds or thousands of square kilometers. This uniformity suggests that matrix-size materials were effectively mixed for each basal till. Three mixing processes seem probable: differential shear in debris-rich basal ice; continuous subglacial deformation of the till; and repeated erosion-transportation-deposition of matrix-size materials during recurrent regelation in the basal ice. Shearing and subglacial deformation appear to have been only locally important. Most of the mixing may have occurred during regelation. Particles on the bed may have been frozen into the basal ice during regelation freezing, and transported until deposited during subsequent pressure-melting on a down-glacier obstacle. During recurrent regelation on fine-grained beds such as occurred in this area, particles may have been subject to several periods of erosion-transportation deposition, and mixed through time over long transport distances. This mixing process was certainly not the only process taking place, but it appears to have been the dominant subglacial mixing process.