STRUCTURES OF DUNES AT WHITE SANDS NATIONAL MONUMENT,NEW MEXICO (AND A COMPARISON WITH STRUCTURES OF DUNES FROM OTHER SELECTED AREAS)1

The type, scale, and relative abundance of sedimentary structures in four kinds of dunes at White Sands National Monument, New Mexico, were determined by examination of vertical sections on walls of trenches cut through the dunes both in a windward direction and at right angles to this direction. Analysis of cross-stratification in all dunes examined indicated certain common features: sets of cross-strata mostly are medium- to large-scale; nearly all laminae dip downwind at high angles (not uncommonly at 30°-34°); most bounding surfaces between sets of cross-strata are nearly horizontal on the upwind side, but have progressively steeper dips to lee, downwind; and individual sets of cross-strata tend to be thinner and the laminae flatter near the top than at the bottom of a dune in vertical section. Sparse but distinctive structural features that are characteristic of the four types of dunes are varieties of contorted bedding, rare ripple laminae, and either local scour-and-fill bedding, or festoon bedding. Other structures, apparently limited to either one or two types of dunes, are the concave-downward foresets in some parabolic dunes; the low-angle reverse dips of upwind strata on high transverse dunes; and the almost horizontal laminae which represent apparent dip in sections normal to wind direction in dome-shaped and transverse dunes. Describing cross-stratification in terms of three dimensions, dune structure at White Sands consists dominantly of the tabular planar sets, with units thickest near the dune base, thinner above. To a lesser extent the sets are of simple (non-erosion) tabular form and relatively uncommonly, of the trough type. Wedge planar forms are scarce. The planar forms characteristically are of two classes in nearly equal proportions: those in which bounding surfaces are virtually horizontal and those in which they dip at moderate to high degree. A brief comparison is made between the structures of dunes that are characteristic of one effective wind direction, as at White Sands, and certain others formed by winds of two or more directions. Seif dunes of Libya, reversing dunes of the San Luis Valley, Colorado, and star dunes in Saudi Arabia are discussed as examples of complex dunes formed by multi-directional winds.