GLACIAL GEOLOGIC FRAMEWORK OF THE WOLF POINT QUADRANGLE, NORTHEASTERN MONTANA, BASED ON HYDROGEOLOGIC AND GEOPHYSICAL STUDIES

The transborder Poplar River is located in southern Saskatchewan, Canada and in the United States on the Fort Peck Indian Reservation. It flows into the Missouri River near the City of Poplar. The modern drainage is in a geologically important area at the western edge of the Laurentide ice sheets. In the unglaciated terrain west of the Poplar River, the Cretaceous Hell Creek Formation and Bearpaw Shale are exposed with the Tertiary Fort Union Formation. On the east side of the Poplar River, shallow aquifers are composed of sands and gravels that are mainly 1) early to middle Pleistocene pre-glacial and interglacial 2) middle to late Pleistocene glacial outwash, 3) late Pleistocene ice-contact deposits, 4) Holocene post-glacial alluvium and 5) Holocene alluvium-colluvium. There have been at least two known major advances of Pleistocene continental ice-sheets in this area. The most recent advances occurred during the Illinoian and the Wisconsin Stage. The Wisconsin ice sheets advanced westward up the Missouri River Valley, and locally had a smaller lobe of ice that pushed northward into the Poplar River Valley. The electrically very conductive (low resistivity; 10 ohm meters) Bearpaw Shale is the regional basement unit for glacial deposits and generally is a confining unit. The area has also been the site of oil production in the East Poplar oil field since the early 1950s. In the 1980s, contamination from oil production was documented in the shallow aquifers.

Airborne, ground, and borehole electrical geophysical studies in addition to detailed hydrogeologic studies of the fluvioglacial stratigraphic units have been conducted to map the shallow aquifer system. The glacial deposits range in composition from electrically conductive silts to coarse low conductivity channel fill gravels. Maps and conductivity depth sections from the airborne electromagnetic (AEM) survey show local surface and subsurface glacial features such as silt-rich lake sediments and different lithologies or terrains that can be correlated with the glacial history. The depth sections can be correlated with the elevation of the top of the Bearpaw Shale determined from drilling to refine the geometry of the paleo-surface. In some areas the AEM signature of buried channel deposits are enhanced by the presence of saline waters from co-produced waters.