The Origin of Dark Sand in Eolian Deposits along the Southeastern Shore of Lake Michigan
Dune deposits on Lake Michigan's southeastern shore contain pin stripe laminations: thin (<3 >mm) laterally continuous (10 m) layers of dark sand, with vertical separations of 0.5-100 cm between laminations. On modern dune surfaces concentrations of dark sand are observed in ripple troughs and form continuous sheets that pass under ripple crests. We explore the source of the dark grains and the processes that can concentrate them in layers, using direct field observations, textural analysis, and point counts of sand grain minerals using the energy-dispersive x-ray analysis function on an electron microprobe. Large and medium size fractions of dune sand are dominated by quartz while smaller size fractions contain high proportions of dark heavy minerals (Fe-Ti oxides, garnets, and Fe-Mg silicates). Pin stripe laminations and dark surface patches are enriched in this fine-grained component, suggesting that sorting by grain size is important in their origin. The fine-grained sand fractions of glacial tills have low concentrations of dark heavy minerals. Thus, the high proportions of heavy dark minerals in finer-grained fractions of dune sands developed during postglacial transport. The association of fine dark sands with ripples suggests that they form as translatent stratification when smaller grains preferentially collect in troughs during ripple migration. Dark laminations in dunes seldom display characteristic features of grainflow (upward coarsening in grain size, troughlike cross section), indicating that this mechanism does not account for the majority of pin stripe laminations. Grainfall has been observed creating a patch of dark sand and may account for the formation of some pin stripe laminations. Pin stripe laminations indicate the orientation of past dune surfaces and can help reconstruct past dune geometries and migration histories.
Published in: Journal of Geology, Volume 119, Issue 5, September 1, 2011, pages 487-503. Copyright © 2011 University of Chicago Press, Chicago, IL. The final published version is available at: http://dx.doi.org/10.1086/661192