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Pacific Coastal & Marine Science Center

Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”

Cross-Bedding, Bedforms, and Paleocurrents

Photographs

Photo of rock or sand showing pertinent structure or structures; see caption below.

FIG. 33.  Structure produced by a sinuous, relatively transverse dune with a constant elevation along its trough; eolian deposits in the Navajo Sandstone (Upper Triassic? and Jurassic), Navajo National Monument, Arizona.

RECOGNITION: The cross-stratified bed that is the subject of this example occurs just below center in the photograph.  The bed is several meters thick and contains a preserved downwind convexity (convex-up bedding indicated by arrow) and an adjacent downwind concavity (concave-up bedding to the left).  Despite having a three-dimensional lee slope, the dune must have had a nearly constant elevation along its trough because the base of the set is planar.  The set boundary is even more planar than that shown in Figure 32, which suggests that either the dune's plan-form sinuosities were less extreme than those in Figure 32 or that the dune climbed at a lower angle than simulated in Figure 32.  The sediment transport direction must have been parallel to the axis of the downwind convexity, because deposition was relatively symmetrical on its two opposing flanks.  Deposition on the downwind convexity alternated from the left-facing side to the right-facing side, as indicated by the vertically zig-zagging path of the crest of the convex-up structure.  The origin of such zig-zagging is simulated in Figure 59.

 

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