OUTBURST ORIGIN FOR THE CENTRAL KALAMAZOO RIVER VALLEY? GEOPHYSICAL TRANSECT RESULTS

The central reach of the Kalamazoo River Valley (between Battle Creek and Plainwell, MI) is about a mile wide and contains up to 160 feet of glacio-fluvial deposits. South of Plainwell (north of Kalamazoo), this valley cuts through the Kalamazoo Moraine which extends an additional 300 feet above the present day valley floor. This Wisconsinian moraine along the east edge of the Lake Michigan ice lobe was apparently first breached by meltwater flowing SSE near the present-day valley. Subsequent to the retreat of that ice lobe, there were apparently outburst events from meltwater derived from the Saginaw lobe whose NNW-flowing waters incised the valley 40-160 feet below the present valley level. Other glacial advances, retreats and fluvial events left a complex mix of sands, lag gravels which coarsen to boulders, and till filling the valley. These lithologies and depths have been verified by borehole investigations. At some locations the valley appears to follow the course of an older bedrock valley. Geophysical investigations have been concentrated along a transverse profile about 4 miles north of Kalamazoo. They consist of vertical resistivity soundings, shallow reflection seismology, and GPR with 35 MHz and 100 MHz antennae. The resistivity increases with grain size and the Mississippian Coldwater Shale is an excellent low-resistivity marker for the base of the valley fill. The seismic reflection data do not define as many continuous layers as does the resistivity inversion, but the Coldwater Shale does have a very good internal marker bed whose reflection can be assumed horizontal, thus constraining velocities in all overlying valley units. The GPR has been used for long reconnaissance profiles, as well as for detailed 3-D volume investigations in selected areas. Penetration with the 100 MHz antennae exceeds 250 nsec (about 28 ft), and more than 400 nsec (about 40 ft) with the 35 MHz antennae. In the western side of the valley the GPR shows a continuous unconformity reflector at 16 feet, and another at about 30 feet. Between these reflectors are large-scale foreset structures with northerly dips. The upper reflector clearly shows an incised channel at one point. 3-D GPR rectangular volumes on specific features greatly assist in determining local transport directions.