TEXTURE AND COMPOSITION OF BEACH AND DUNE SEDIMENTS IN TWO NATIONAL LAKESHORES OF LAKE MICHIGAN, USA

Indiana Dunes National Lakeshore (INDU) and Sleeping Bear Dunes National Lakeshore (SLBE) are located along Lake Michigan and only 360 km apart, but differ in dune morphologies as well as in texture and composition of beach pebbles and dune sands. Both areas formed as a result of Holocene deglaciation, when glacial drift was washed into Lake Michigan by streams and denudation, reworked and sorted by waves, and finest sediment moved by wind into coastal dunes. Even though Lake Michigan glacial lobe brought erratic lithologies, both areas contain beach pebbles made predominantly of local lithologies, carbonates in SLBE and clastics in INDU. Native beach gravel in INDU is made predominantly of beach shingle, very platy siltstones (70–78%), with secondary crystalline pebbles (15-18%) in the east, and carbonate pebbles (10-12%) in the west. Beach pebbles in SLBE are comprised predominantly of carbonates (70-80%), chert (10-15%), crystalline (8-12%) rocks, and few clastic particles (< 2%).

Very coarse sand (1-2mm), which is found along the beach and on wind ripple crests after the storms, is made predominantly of clastic lithic fragments (35-40%), carbonates (30-35%), chert (15-20%), and crystalline lithic fragments (15-20%) in INDU, while chert (50-55%), crystalline lithic fragments (25-30%), carbonates (15-20%), and clastic lithic fragments (3-5%) characterize SLBE. Most similarities between two areas are with composition and texture of dune sand, where local lithologies are least influential. Sediment samples from INDU foredunes are well sorted, predominantly (75-85%) medium sands with mean size of 0.268-0.280 mm. Dune sand from INDU is also well sorted, predominantly (60-83%) medium sand, and with somewhat smaller size (0.250-0.270 mm) than foredune sand. There is larger discrepancy in sand texture between foredunes and dunes within SLBE. Sediment samples from SLBE foredunes were predominantly (84-93%) medium sand, well sorted, and with mean size of 0.363-0.387 mm. Dune sand is finer, made predominantly (50-80%) of fine sand with mean size of 0.216-0.302 mm. In terms of mineral makeup, both areas are predominantly made of quartz sand (75-85%), however, SLBE dunes and foredunes contain larger amount of carbonates, chert, and feldspars than INDU dunes.