Mineralogical composition and quartz grain surface textures of Tertiary and Quaternary sediments at Nebraska Sand Hills
Heller, Donald A.
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The stratigraphic column of the Sand Hills is subdivided into three units, the uppermost being dune sand of Holocene Age. Quaternary sand and gravel, of fluvial origin, underlies the erg, and is in turn underlain by finer-grained Tertiary sediments. Modal analysis of the units show that mineralogical composition of the sand is strongly influenced by grain size. Eolian transport has caused dune sand to become more quartzose than underlying Quaternary fluvial sand because of abrasion of feldspar grains and rock fragments* The Quaternary fluvial stratum is more quartzose than the Tertiary sediment, probably because of feldspar and rock fragment abrasion during reworking of Tertiary material by Quaternary streams. S.E.M, study of quartz grains reveals that impact features, considered eolian in origin, are definitely more prevalent on dune sand grains* "Desert dew" processes have caused corrosion along cracks and planes of weakness. Conchoidal fractures are more abundant on fluvial sand grains and were probably caused by collisions during stream transport, which has also produced randomly oriented V-shaped impact pits. Dune sand grains are more rounded than their fluvial counterparts, indicating mechanical rounding by wind. Plots of mean grain sizes of dune samples vs. downwind distance along a transect of the erg shows a distinct break into coarser and finer populations. This appears to he the line of marginal on-lap between encroaching dime sand and Peoria loess south of the dune field. Plots of mineral percentages vs. transect distance show little compositional change in eolian sand throughout the transect. Interdune valley soils have not matured sufficiently to cause uniform depletion of K-feldspar grains with increasing organic matter content in soil profiles. In each test hole, feldspar percentages differed between adjacent beds, apparently because of local abundances of feldspars in underlying fluvial sediments.