THE BURIED BEDROCK TOPOGRAPHY OF NORTH AMERICA REVEALS PRE-PLEISTOCENE RELICT LANDSCAPES AND BROAD-SCALE INFLUENCES OF GLACIAL PROCESSES

The Laurentide Ice Sheet and its meltwater sculpted North America from the western High Plains to the East Coast, leaving vast areas with thick accumulations of glacial sediment overlying scoured bedrock. When combined with ground surface digital elevation models (DEMs), this buried bedrock surface reveals glacial and ice-marginal processes and history. Currently, data on depth to bedrock are disparate and sparse, and interpretations of buried bedrock features have not been updated, in most cases, for decades. Here we create a continental-scale DEM (250-m resolution) of most of glaciated Laurentia by interpolating new bedrock topographic surfaces for Indiana, Michigan, and northeastern Missouri using well log data and combine them with digital contours representing previously mapped bedrock topography. This map reveals the pre-Pleistocene landscape of North America where glacial impacts range from minor alterations in highlands along the boundary of maximum glacial extent to extensive scouring of glacial troughs along features such as the Buffalo Corridor where the James and Des Moines Lobes originated in the North American Interior Plains. Average sediment thickness across the study area is 40 m with thick sequences (>240 m) occurring in places such as the Coteau des Prairies and Lower Peninsula of Michigan. Drainage networks were altered during the Pleistocene by eustatic (Gulf of Mexico) base-level lowering, as indicated by deep stages of bedrock valleys in the lower Mississippi River Basin and by glacial damming on more local scales, as evidenced by ice-marginal valleys that crosscut preexisting valleys with better-developed dendritic drainage patterns.