Abstract

Processes responsible for structures in sand dunes consist of (l) primary deposition by saltation and creep and by settling from suspension, (2) redeposition accompanying avalanching, and (3) penecontemporaneous erosion. Characteristics of dune structures were examined in the field by introducing marker beds of magnetite at times of sand deposition, thus recording original surfaces and making possible the determination of subsequent changes. Similar structures were examined in the laboratory by testing processes and comparing the resulting structural forms with corresponding natural features.

Avalanching in sand is of two types: sand flow and slumping. Deformational structures characteristic of each were recorded in the field and were reproduced in the laboratory. Nine varieties of deformational structures are recognized and described. Analysis of these structures suggests criteria for distinguishing compressional types (lower dune slope) from tensional types (upper dune slope).

The analysis of deformational structures also serves to distinguish between forms developed in cohesive sand and those in non-cohesive sand. Since the degree of cohesion is largely a function of the amount of moisture in the sand at the time of avalanching, the deformational structures provide a means for recognizing original dry sand, wet sand, sand crusts, and saturated sand surfaces in ancient deposits. A testing of these criteria was made by comparing laboratory samples with those of dry sand at White Sands, New Mexico, and with those of coastal dunes (probably wet sand) in southern Brazil.

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