DEPOSITIONAL ORIGIN OF LAMINATED LAYERS IN FAIR OAKS DUNES OF NORTHWEST INDIANA

The Fair Oaks Dune field in northwest Indiana covers about 1,100 square miles south of the Kankakee River. The stabilized dunes are generally 30 to 45 feet in elevation. Original dune forms are interpreted as megabarchans produced by north-easterly winds. The megabarchans extend >5 miles in length, with widths of 1-3 miles, and spacing 3-5 miles. The modern forms are nested compound parabolic dunes with lee slopes oriented in northeast and southeast directions. Alternating red and tan layers (2-4 in thick) are observed within the internal structure of dune deposits which are underlain by gray fluvial/lacustrine sands. The reddish layers are more resistant to weathering than the tan layers. Their upper boundary is irregular but sharp while the lower boundary is diffuse. Thin sections show that the upper boundaries of reddish layers consist of 2-3 mm laminae made of very fine sand and silt with clay sized particles filling interstitial spaces. There is no evidence of cross-cutting but laminae occasionally bifurcate and join the underlying reddish laminae. Similar structures have been described as ‘silt-clay lamellae', ‘clay lamellae' or ‘textural subsoil lamellae' and are mainly interpreted as a product of pedogenesis. We interpret these reddish laminae as pin stripe laminations, a distinctive feature of modern and ancient eolian sediments. Depositional in origin, pin stripe laminations represent settling of silt and very fine sand from suspension within moving avalanche on dune slip faces. A sample from eolian sediment 7 ft below the surface yields an OSL age of 7290 ± 540 yr. We are currently conducting OSL dating of sand samples that will distinguish the relation between the timing of glaciofluvial/lacustrine sand deposition and subsequent eolian activity in the Fair Oaks Dune field.