CONTRASTING RECORDS AND NEW INTERPRETATIONS OF THE VALPARAISO AND LAKE BORDER MORAINIC SYSTEMS IN BERRIEN COUNTY, MICHIGAN

New geologic mapping of Berrien County, Michigan indicates a major contrast in the sedimentology and morphology of the adjacent Valparaiso and Lake Border morainic systems. Both systems formed at the southeast margin of the Lake Michigan Lobe of the Laurentide ice sheet about 14,000 years BP. The Valparaiso "moraine" in this area is a highly pitted head-of-outwash system composed of sand and gravel, and includes a complex set of multiple glaciodeltaic sequences graded to a series of glacial lakes. In contrast, the Lake Border system consists of low-relief, discontinuous ridges composed mainly of gray silty clay till. The till ridges, which are irregular, branching , and subparallel to the lakeshore, have a discontinuous mantle of eolian sand and are flanked by valleys filled by lacustrine sand. The valleys form a continuous and locally anastamosing pattern between the Lake Border and Valparaiso moraine systems. These contrasting moraine systems do not require separate glacial advances. They may be products of contrasting phases of a single cycle of ice-marginal deposition, recording a change from vigorous proglacial drainage of an active ice lobe to that of a stagnant ice lobe. During the active phase, the sand and gravel of the Valparaiso outwash was deposited; its characteristics indicate derivation and transport from a vigorous, extensive subglacial hydraulic system. The till ridges of the Lake Border system are suggested herein to be have been formed glaciotectonically within a few km of the glacial margin that is marked by a generally coeval head of outwash system. Glaciohydraulic conditions changed as the lobe stagnated, and Valparaiso outwash deposition ceased. The Lake Michigan lobe margin melted back a few kilometers from the Valparaiso morainic system to a position near the Lake Border ridges, where a sediment-poor meltwater system was confined between it and the upland formed by the Valparaiso head-of-outwash. Glaciofluvial erosion accentuated the inner border of the Valparaiso system and formed a throughgoing, valley system.