THE STRATIGRAPHY OF THE LAKE BORDER MORAINE IN WEXFORD COUNTY, MICHIGAN

Leverett and Taylor (1915) mapped the Lake Border moraine adjacent to the Valparaiso/Charlotte interlobate moraine at the latter’s northwest corner near Harrietta in western Wexford County. They described the moraine as “loose-textured sandy or gravelly drift” and “in places...thinly capped with till but thickly set with bowlders [sic].” Recent mapping and cores of deposits in the Lake Border moraine in Wexford County reveal glaciodeltaic deposits from topset sand and gravel through to bottomset silt and clay rhythmites. Between the village of Mesick and state road M-55, sand and gravel is found at altitudes greater than 400 meters (m) above mean sea level, massive to finely-layered medium sand between 400 and 360 m, massive to finely layered or ripple-drift stratified fine sand (interpreted as lacustrine sand) between 360 and 335 m, and clay/silt rhythmites (interpreted as bottomset varves) are consistently found at altitudes below about 335 m. Diamict in the area has been observed both at the base of the deposit (altitudes around 315 m) as well as on the inner (western) moraine margin at altitudes between 330 and 350 m. Diamict has not yet been mapped on the outer (eastern) flank of the moraine. The diamict in higher positions is vertically and horizontally contained within lacustrine-sand deposits and has erosive lower contacts, draped upper contacts, and internal-flow structures. As such these diamicts have been interpreted to be debris flows. Diamict in the lowest observed positions is regionally continuous and over-consolidated and has therefore been interpreted as a glacial till. This same stratigraphy is found south of state road M-55, albeit at altitudes about 40 m lower. This is interpreted as a response to a lower base level exposed during glacial retreat from the Lake Border phase maximum.

At the maximum extent of the glacier during the Lake Border phase, a high level (~380 m altitude) proglacial lake must have been bounded by the glacier on the west and southwest, older interlobate deposits to the southeast, and probably the Saginaw lobe to the east.