MAGNETITE LAYERS IN A MASSIVE PARABOLIC DUNE, SOUTHEASTERN SHORE OF LAKE MICHIGAN

After scouring by strong fall and winter winds, thin (<0.2 cm) black layers enriched in magnetite are exposed along the interior walls of Green Mountain Beach Dune, a 40 m high actively migrating parabolic dune 10 km southwest of Holland, Michigan.  A total of 310 magnetite layers were encountered in 5 traverses down the walls of the dune with 26% occurring in sets of 2 to 4 closely spaced layers.  The average separation (perpendicular to layering) between sets is 21 cm; the layers dip at an average of 27°.  Strikes change from parallel to the dune limbs high on the walls to nearly perpendicular to the dune axis at the base of the walls.  These relationships suggest that the layers formed on the lee slope of the depositional lobe of the migrating dune.

Thin sheets of magnetite in broad patches were observed 10-15 meters down the lee slope of the active depositional lobe in April and May, 2002.  A trench dug on the lee slope along the dune axis 12 meters below the crest revealed 5 sets of well-developed layers in the upper 40 cm (sand deposited from January-June, 2002) but no well-developed layers in the lower 60 cm (sand deposited from September-December, 2001).  Layers became less frequent and more poorly developed further down the slope and were absent at the base.

Magnetite sand occurs on the beach as well as the dunes and probably is derived from glacially transported material eroded from iron formations further north.  Radiocarbon dates of buried paleosols within the dune show that the oldest magnetite layers formed > 2250 ± 100 YBP while the youngest formed < 395 ± 105 YBP.  Textural analysis of 3 layers shows that the magnetite grains are well sorted (uniformity coefficient of 1.35) and are finer grained (average median grain size 0.15 mm) than either immediately adjacent quartz sand (0.22 mm) or typical mixed dune sand (0.25 mm).

Concentrations of fine-grained magnetite on the lee slopes of dunes could form by sorting during either air fall or grain flow.  The thin, sheet-like form of the deposits tends to support the air fall hypothesis.  Confirming the mechanism that forms these layers may shed light on the origin of more cryptic laminations in dune sands with less visually distinct constituent mineral grains.