HIGH RESOLUTION MAPPING OF GLACIAL HYDROSTRATIGRAPHY USING LIDAR-BASED HILLSHADE IMAGES, ROTOSONIC CORE, AND GAMMA LOGS

The University of Minnesota’s hydrogeology field camp site is located next to Williams Lake in north-central Minnesota, USA; hosting the field camp since 1995. This location builds on a long baseline of data developed by the USGS. The site represents a glacial cross-road developed in sandy outwash at the intersection of two moraines of Late Wisconsinan age. The Itasca moraine was deposited by the Wadena Lobe from the north, and the north-south trending St. Croix moraine was deposited by the Brainerd Lobe. Rotosonic core shows a 150 meter long sequence of tills and outwash to bedrock. Gamma logs were collected from over thirty monitoring wells within a one square kilometer area at extremely slow logging rates (2 cm/s) and high resolution (164 points/m). While most wells are completed to less than 30 meters depth two wells extend to bedrock. Gamma logs were ground-truthed with several associated rotosonic cores.

LiDAR-based hillshade imagery was combined with gamma logs to build a detailed three-dimensional depositional model within relatively flat outwash and hummocky collapsed-ice terrain of the ice margin. The 30 meter thick surficial outwash deposits contain cyclic medium to coarse sand intervals of 60 to 100 centimeters in an overall fining upward sequence of coarse to fine sand. High gamma kicks within outwash deposits, correlated to pebbly, matrix supported sandy loams in rotosonic core, are interpreted as debris flows covering areas up to 5,000 square meters. Under the surficial sands, 30 meters of sandy loam tills overlay a six meter thick coarse sand forming a confined aquifer. Hydraulic connectivity can be probed with pumping tests conducted in both the surficial and confined aquifer systems. Correlations of more than one km for the buried tills and the confined aquifer delineate earlier glacial advances below the most recent terminal position of the ice.