RELATIVE ABUNDANCE AND LITHOLOGY OF PEBBLES ALONG THE BEACHES OF EASTERN LAKE MICHIGAN

Preliminary examination of fifteen beaches along the eastern Lake Michigan indicates that there are three distinct coastal regions in regards to pebbles’ (8mm-64 mm) relative abundance and their lithology. Beaches in southern coast region, from Indiana border to South Haven, contain relatively abundant pebbles in both backshore and foreshore. Clastic pebbles (mudstones and sandstones) are more abundant (up to 43% of total pebbles) than in the other two regions. Greenish to tan platy clastic pebbles were derived mostly from Ellsworth Shale, while carbonates, chert, and crystalline (igneous and metamorphic) pebbles were derived from the nearby Michigan lobe sediments. Clastic pebbles relative abundance diminishes northward on an account of increase in carbonate pebbles content. There are few anthropogenic pebbles found in this coastal region. In the central coast region, from South Haven to Manistee, pebbles are small and very scarce, both in backshore and foreshore. Irregular, cream to variegated chert and compact to platy carbonates comprise 40%-60%, while crystalline pebbles comprise up to 40% of all the pebbles. Highest abundance of chert in this region and platy shapes of carbonates and crystalline pebbles suggest prolonged weathering, weak littoral currents, and relatively little coarse sediment input into Lake Michigan by Kalamazoo, Grand, and Muskegon Rivers. Despite some plastic debris in backshore storm swashes there are few other anthropogenic pebbles in this coastal region. Northern coast region, from Manistee to Mackinac, contains abundant pebbles, which are predominantly compact to blocky carbonates (60%-90% of total), reflecting their origin from Detroit River Group and Traverse Group limestones and dolomites. There are very few clastic pebbles and almost none anthropogenic pebbles in this coastal region.