We investigated the mineralogy of concentrations of dark sand evident in layers and surface patches on beaches and large (up to 60 m) parabolic dunes on the eastern coast of Lake Michigan.  Trenches in both the beach and dunes reveal thin dark layers (TDLs) up to 4 mm thick.  Dune TDLs are planar or curved concave-upward and dip steeply (up to 32º) parallel to dune foresets.  Post-depositional deformation is common.  Laminae occur singly or in sets of 2 to 5 spaced 0.5 to 4 cm apart.  TDLs overlie both reverse-graded grain flow and wind-ripple laminated strata either concordantly or truncating them at low angles.

We sampled TDLs and patches, paired with “plain” sand beneath them, in 2 back beach and 4 dune sites.  We took multiple traverses across polished grain mounts of 0.355, 0.25, 0.15 and 0.125 mm sieve fractions on an electron microprobe and identified the mineralogy of 131 to 234 grains per sample by their energy dispersive spectra.  Dark layers from the dune are finer grained (median 0.164 mm) than dark sand from the beach (0.238 mm) or light sand from either setting (0.290 mm).  The coarser (0.355 and 0.25) sand fractions of both dark and light sands are nearly identical, dominated by quartz (81-92%) and feldspar (3-13%).  The 0.125 mm fractions for dark and light dune sands are similar and are mainly dark oxides and ferromagnesian silicates (Table).  Dolomite is the dominant carbonate.  Sphene, epidote, apatite, and zircon are minor components.  Dark dune sands can be formed by concentration of the finer-grained portions of the light sands; this size sorting is primarily responsible for their dark color.  The heterogeneous composition of the finer grains reflects the mineralogy of glacial source sediments.  Compositional differences between dark beach and dune sands probably reflect selective loss of denser oxide minerals during eolian transport.

% grains in 0.125 sieve:   Qtz      Fel     Fe-Ti Oxide      Fe-Mg Silicate   Carbonate  Other

Dark Dune Sand              15.6    4.8          43.7               14.2                    9.4         12.3

Light Dune Sand              16.7    6.3          41.6               15.3                  11.6           8.5

Dark Beach Sand              1.1    0             91.1                 5.2                    0              2.6

Light Beach Sand            11.6    7.8          60.3               12.8                    0.6           6.9