INTERPRETING SUBSURFACE GLACIGENIC SEQUENCES: AN EXAMPLE OF USING EXPOSED ICE-CONTACT SEDIMENTS AS A MODEL FOR MAPPING BURIED ICE-MARGIN DEPOSITS

Key challenges to better understanding buried depositional sequences are mapping and interpreting subsurface geologic data in areas where those data do not provide the detail or resolution of outcrops. Often, interpretation of subsurface data is dependent on the geologist's model of depositional environments. In northeast Illinois, we speculate that styles of deposition along some former ice-margins repeated during overall ice-margin retreat on a local scale for some glacial events. Therefore, in these situations, depositional models of sediment packages that we see at the land surface can be applied to buried sequences, where the subsurface data (such as water-well records) are not as clear as the geology displayed at the outcrop level. The “style” includes the environment of deposition in addition to the scale of individual sediment packages, beds, or units.

Rare exposures of coarse grained ice-contact deposits of the Lake Michigan Lobe are present in some quarries within the Fox River valley, about 30 kilometers west of the Lake Michigan shoreline. These include deltaic, fluvial, and collapsed deposits associated with the retreat of the ice margin during the Woodstock Phase (~16,000 C-14 ybp). East of these quarries, the deposits are buried by younger glacial sediments. Three-dimensional visualization of bulk subsurface data have allowed sediments associated with a buried former ice margin to be mapped while applying a depositional model analogous to the deltaic ice-contact deposits at land surface.