GEOMORPHIC ASSESSMENT OF GLACIAL LAKE AGASSIZ STRANDLINES IN NORTHWEST MINNESOTA

The study examines the shoreline history in the southern basin of glacial Lake Agassiz by assessing the geomorphology of strandlines in an area of NW Minnesota. Study components include: 1) identifying strandlines up to 15 km long, including beach ridges, escarpments, and spits, and comparing their elevation changes using LiDAR-based DEMs, 2) measuring shoreline tilts that reflect glacioisostatic adjustment (GIA), and 3) analysis of the internal structure and thickness of strandlines resting on clay-rich till using ground penetrating radar (GPR) and field mapping.

DEM analyses and sediment samples for a surficial geologic map (USGS EDMAP) resulted in a map that, in general, shows ~50 strandlines organized into strandline groups (SGs), rather than individually named beaches. Eleven SGs are mapped, each characterized by one or two well-developed ridges to the north that branch southwards into 3-10 ridges. At the southern end of an individual SG, the landward (older) ridge is higher in elevation than the lakeward (younger) ridge by as much as 10 m. Adjacent SGs are up to 3 m different in elevation. Tentative interpretations are that ridges within SGs, and the SGs themselves, record stable lake levels. The 1-3 m differences in elevation between successive ridges record many, low magnitude drops in lake level. Relative age of the strandlines is recorded by the southerly tilt of a ridge’s long axis. Highest elevation ridges have the highest gradients (1.1 m/km) while basinward lower elevation ridges have the lowest gradients (0.19 m/km). Higher gradients are assumed to represent higher rates of GIA earlier in deglaciation.

GPR data reveal that shoreline thickness ranges from ~1–7 m beneath ridge crests and ~2–6 m near the base of slopes. Most GPR profiles of beach ridges reveal an erosional contact with till, lakeward progradation of littoral sediment in the nearshore zone, and overwash deposits near the crest and along the backshore side of ridges. The presence of pebble and cobble gravel along ridge crests suggests formation in high wave-energy environments. Collectively, the data suggests that strandlines develop within an overall regressive system, but with sufficient durations of lake level stability for ridges to develop before the next lake level fall results in a new ridge forming within the same SG or as the start of a new SG.