One of the most important criteria for recognizing wind-laid deposits is based on their sedimentary structure. Eolian sandstone generally has large- and medium-scale cross-beds of the tabular-planar and wedge-planar types. Trough-type cross-beds are less abundant. The cross-beds commonly are composed 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°) because of erosion which precedes deposition of the overlying set.

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

In the absence of cross-beds, other criteria are used to identify dune environments. Textural and mineralogical characteristics are not sufficiently conclusive. The mean grain size seems to be of little use. Although dune sand is slightly better sorted than other 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 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 velocity of two or more minerals of different density values.

Several criteria, together with the stratigraphic relations of the deposit to adjacent beds, should be used to identify the dune-sand environment.

End_of_Article - Last_Page 707------------