THE TEXTURE AND LITHOLOGY OF BEACH GRANULES AND FINE PEBBLES ALONG THE SOUTHERN SHORES OF LAKE MICHIGAN

Texture and lithology of beach granules (2-4mm) and fine pebbles (4-8mm) is compared to coarse gravel (>8mm) lithology collected from the beaches of Lake Michigan in Indiana. From Michigan border to Gary (Northeastern, Eastern, and Central sectors of the shoreline) most of beach sediment is natural, but was recently affected by addition of nourished sediment in several erosion prone areas. In this area most of the granules and fine gravel are clastic particles similar in lithology to coarse gravel. In all three sectors of the shoreline clastic particle relative abundance diminishes eastward in all three size categories. Also, as the grain size diminishes the clastic particle relative abundance diminishes from coarse gravel (78%) to fine gravel (65%) and granule fraction (40%). Carbonates were not abundant in natural sediments and their relative abundance diminishes westward in all three grain size categories, reflecting its origin in nourished sediments. Most of nourished carbonate was in coarse gravel fraction (26%), as its relative abundance diminishes in fine gravel (15%) and granule (15%) fractions. On the other hand chert relative abundance increases westward in all grain size categories, coarse gravel (3% to 25%), fine gravel (14% to 22%) and granules (9% to 19%) and in all three sectors. Relative increase in chert abundance with diminishing grain size can be attributed to long term weathering and removal of less resistant lithologies. Crystalline particles are second most abundant particle in fine gravel (23%) and granule fractions (28%), and their relative abundance increases with diminishing grain size. Relative abundance of anthropogenic particles is largest in granules (7-12%) and decreases in fine gravel (0-1%) and coarse gravel (0.5-3%) fractions. Relatively larger amount of anthropogenic particles in granule and fine gravel than in coarse gravel likely reflects more rapid weathering of glass, brick, concrete, and slag, released by industrial and urban areas along the shoreline. From Gary to Illinois border (Western sector) is man-made shoreline that contains very little natural sediment. Chert is most abundant lithology in all three grain size categories. However, chert’s relative abundance decreases from 65% in coarse gravel, to 50% in fine gravel and 26% in granule size fractions.