SURFICIAL GEOLOGY OF A COMPLEX INTERLOBATE AREA, CASS COUNTY, MICHIGAN

Cass County, Michigan lies within an interlobate area between the Lake Michigan lobe (LML) and the Saginaw lobe (SL) of the Laurentide Ice Sheet. In 2016, the Michigan Geological Survey mapped the Jones and Vandalia 7.5 minute quadrangles as part of a USGS Great Lakes Geologic Mapping Coalition project. Surficial mapping, greatly aided by newly acquired LiDAR elevation data, suggests that these quads have been overridden or affected at least twice by both of these lobes during the last glaciation. The two earliest events include an advance of the LML to some distance east of these quads, identified by east-west trending tunnel valleys in the quads, and an advance of the SL lobe south of the two quads, identified by a large north-south oriented tunnel valley in the Mottville quad south of the Jones and Vandalia quads. The relative and absolute ages of these early events have not been determined. The next event was a re-advance of the SL to the position of the Sturgis moraine, which continues east of the study area across the St. Joseph meltwater sluiceway. The topography of this moraine in the study area, which is hummocky and extremely high relief, suggests that stagnant or buried ice from one of the previous advances might have been present at the time of moraine formation. A large south trending outwash fan supports the SL origin of this high relief moraine. The final event was a re-advance of the LML to the Kalamazoo moraine just west of the Vandalia quad. Meltwater fans grade west across the quads from this event.

Two deep borings drilled by the wire line coring method reached Coldwater Shale bedrock at 455 and 320 ft. The deeper borehole, located hear the high point of the Sturgis moraine, was capped by a till/lacustrine section 240 ft. thick. Leached, reddish sand below it may represent the Sangamon (?) geosol. Another primarily lacustrine section with a till at the base at 400 ft. underlies the upper sequence. Sand and gravel complete the sequence to bedrock. The shallower boring has, in descending order, an outwash fan sequence with a till at the base (76’), a thick lacustrine interval with a till at the base (255’), an oxidized sand and gravel to 295’ and an extremely hard dense red-to gray till to bedrock at 310’.