NEARSHORE GRAVEL COMPOSITION IN SOUTHERN LAKE MICHIGAN

This study examines the shape and lithology of nearshore pebbles (> 8 mm) along 70 km long Indiana’s coast of Lake Michigan. Four major coastal structures - Michigan City pier and breakwater, Burns Harbor pier, Gary Works pier, and Indiana Harbor peninsula – altered natural littoral drift creating four independent cells, between which no transfers of coarse sediment occur. Downdrift from the coastal structures severe beach erosion has prompted extensive beach nourishment with nonnative sediment. Also, lake fill (imported for development of Indiana Harbor and Gary factories); industrial byproducts (slag, hollowed limestone, coal); and anthropogenic sediment (concrete, brick, glass, plastic, etc.) was introduced by growing NW Indiana communities. Natural gravel, collected from Nippissing phase beach terrace, contains predominantly platy clastic (mudstone and sandstone) lithologies (87%) with some chert and carbonates (12%), and negligible crystalline (1%) components. Northeastern sector of the shoreline contains compact carbonates (47%), crystalline pebbles (24.5%), and platy clastic pebbles (24.1%). At the western end of NE sector platy clastic pebbles (48.9%) predominate over compact carbonate grains (26.1%), reflecting the dilution of nourished sediment in St. Joseph area to the east. Eastern sector contains carbonates (39%), crystalline (25%) and clastic (25%) pebbles, which toward the western end of the sector comprise clastics (62%), carbonates (15%), and crystalline pebbles (13%), reflecting the dilution of nourished sediment in Mt. Baldy area near Michigan City. Eastern part of Central sector contains platy clastic pebbles (72%), compact carbonates (13%), and compact crystalline pebbles (10%), most similar composition to native gravel. However, in the western part of the central sector, platy clastic pebbles diminish to 37% of total, while anthropogenic pebbles reach 33% of total, and chert (20%) outnumber carbonates (8%). Western sector contains large amount (50-70%) of chert pebbles, followed by carbonates (12-20%), and anthropogenic pebbles (5-10%). In streams that drain to southern Lake Michigan, carbonates comprise 50-75% of total, while anthropogenic pebbles make up 16-37% of total. Most carbonates have angular edges suggesting crushed rocks derived from road fill material.