THE BURIED BEDROCK SURFACE AND PLIOCENE TOPOGRAPHY OF THE NORTH AMERICAN LAURENTIDE REGION

Continental ice sheets crossed North America from the Canadian Shield to the Great Plains, Midwest, and New England of the United States, eroding cratonal basement rock and depositing glacially-derived sediment in a ring hundreds of kilometers wide at the ice marginal region. Continental glacial deposits form the low-relief and agriculturally-productive landscape of the north-central USA and southern Canada, but buried beneath them lies a hidden topography of river gorges, drainage divides, and glacially-scoured bedrock that records the past 2.6 million years of landscape evolution. We digitally removed this sedimentary cover to reveal the buried bedrock surface of North America. We then extracted preglacial features from this surface and used them to reconstruct Pliocene paleotopography. The resulting digital elevation models (250-m resolution) illuminate the transition of the central North American landscape and its rivers because of the onset of Quaternary glaciations, demonstrate the transition from the erosional ice-sheet interior to its depositional margins, and provide an upper limit for Quaternary aquifer thickness from Alberta, Canada, to Ohio, USA. By subtracting the Pliocene surface from the current bedrock surface, we found that an average of 68 m (median 46 m) of material was removed by the Laurentide Ice Sheet. Erosion and deposition were not spatially uniform but occurred within clearly defined nested zones of maximum erosion and deposition relative to the ice sheet center at Hudson Bay. The data synthesized in this work provide a new understanding of the continental-scale modification of Late Cenozoic fluvial networks via Pleistocene glaciation that erased some river valleys while deepening and blanketing others. Juxtaposing the reconstructed relict landscape with current bedrock topography demonstrates the ability of continental ice sheets to reshape land masses by redistributing rock and sediment, whereas previous work has focused mainly on continental denudation by glacial erosion.