THE ORIGIN AND SIGNIFICANCE OF BOXWORK CLAY COATINGS ON DUNE SAND GRAINS FROM THE NEBRASKA SAND HILLS, USA

Aeolian dune sands of Late Pleistocene to late Holocene age in the Neb raska Sand Hills are widely characterized by the development of partia l clay coatings which form connected ridges and ''boxwork'' structures on the grain surfaces. The proportion of sand grains with clay coatin gs typically shows considerable variability with depth in any given se ction. Distinct bands of sand enriched in fines are quite common, espe cially just below the present, or buried, dune surfaces. The morpholog y, texture and mineralogical composition of clay coatings from two sit es, the Merritt Reservoir megabarchan and the Dismal River Ranch secti on, have been investigated using a combination of SEM, X-ray microanal ysis, XRD and Coulter-Multisizer analysis in order to determine their mode of formation and environmental significance. Based on the analyti cal results and field observations it is concluded that the boxwork cl ay coatings formed in the vadose zone mainly as a result of precipitat ion- and surface-runoff induced infiltration of clay and very fine sil t from the dune surface. The main source of fines is concluded to be f ine-grained allochthonous aeolian dust derived principally from the Hi gh Plains or more distal sources.