INFLUENCE OF PALEO-CHANNEL DEPOSITS ON COASTAL BLUFF FAILURES AT NORTHEASTERN LAKE MICHIGAN

A 30-m high coastal bluff at Sleeping Bear Dunes National Lakeshore has failed several times over the last 100 years, each time along the same stretch of coast. Adjacent coastal segments, although similar in morphology and sediment type, have remained stable. The most recent landslide (1995) removed 60 m of bluff along about 500 m of coastline, depositing approximately 1 million m3 of sediment on the lake floor. Based on data from high-resolution seismic reflection and ground-penetrating radar, we attribute the episodic failures to a locally high sediment supply from longshore transport and reworking by two regressions and a transgression, which have produced a complex stratigraphy. The highly deformed landslide deposits, up to 18 m thick, extend 3-4 km offshore and unconformably overlie well-stratified glacial and lacustrine sediment. The landslide-prone bluff is underlain by sandy channel-fill deposits that are oriented nearly perpendicular to the shoreline. The paleochannel is approximately 10 m deep and 400 m wide and probably represents stream incision during a lake-level lowstand about 10.3 ka B.P. The channels filled with sediment during the subsequent transgression and lake-level highstand, which climaxed about 4.5 ka B.P. As lake level fell from the highstand, the formation of beach ridges and sand dunes apparently sealed off the channel and isolated a small inland lake (Glen Lake). This small lake lies 5 m above the level of Lake Michigan and may be a source of piped groundwater. No other paleochannels were recognized in adjacent areas, nor have those areas experienced large landslides. Our hypothesis is that the paleochannel acts as a conduit for pore water flow, and thereby locally reduces soil strength and promotes slope failure.