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Abstract

Eolian sandstone generally has large- to medium-scale tabular-planar and wedge-planar cross-beds. Trough cross-bedding is much less common. Cross-beds are composed mostly of steeply dipping laminae which normally are concave upward. In modern dunes the foreset beds near the top of the slip face have steep (29°~34°) dips, but in paleodunes this value is somewhat less (20°-29°), probably because erosion commonly removed the steeper, upper part of each lamina prior to deposition of the overlying set.

Dune cross-bedding is distinguished from other similar structures partly on the basis of more uniform grain size. The nature of the adjacent and/or intercalated beds may help to determine the environment of deposition. Attitude of the bounding surface also is a diagnostic feature.

In the absence of cross-bedding, other criteria are used to identify dune environments. Textural and mineralogical characteristics are not conclusive, and mean grain size seems to be of little value. Although much dune sand is slightly better sorted than other similar sediments, sorting is not distinctive. Positive skewness has been considered as an indication of dune environment; however, negative skewness also has been reported for dune sediments. Dune sand usually, but not always, is more rounded than beach sand.

Dune and beach sediments can be separated on the basis of the heavy-to-light mineral ratio, and the relation between the settling velocities of two or more minerals of different density values.

A combination of criteria, together with the stratigraphic relations of the deposit to adjacent beds, should be used in identifying a dune-sand envirionment.

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