Anatomy of Drift Ridges Revealed by Shallow Seismic Shear Wave Profiling

Ridges, up to 30 m high and generally oriented NE-SW across the Illinois Episode drift plain in southern Illinois, USA, have been variously interpreted as eskers, crevasse fills, moraines, and kames. The ice contact diamictons and sorted sediments that occur in these ridges are typically Illinois Episode in age and likely record the final melting of the Laurentide Ice Sheet near its southernmost extent in the continental U.S. Shallow shear wave seismic profiles across several of these ridges help reveal their complex origins. Borehole control includes sediment cores with shear wave and natural gamma logs. The shear wave profiles reveal m-scale features of drift and bedrock over a depth range of 1 up to 100 m. Terrapin Ridge overlies a bedrock valley with drift up to 70 m thick. Dipping seismic reflectors on the stoss side are interpreted as imbricated till sheets, whereas horizontal reflectors on the lee side are interpreted as mainly outwash sediments over basal till and glacilacustrine sediment. Although most ridges were probably formed during the Illinois Episode, based on current data, the core of this particular ridge may be a remnant moraine from a pre-Illinois Episode glaciation.