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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. 44.  Structure formed by a dune with a sinuous lee slope but without scour pits in the trough; Navajo Sandstone (Upper Triassic? and Jurassic), Zion National Park, Utah.

RECOGNITION: Bedforms with sinuous crestlines commonly have scour pits in their troughs, but this example demonstrates that such scour pits may be absent.  Sinuosity of the lee slope of the dune that deposited this set of cross-beds is demonstrated by the differences in dip direction of the cross-beds in the set.  The dip direction is in general toward the viewer but was measured to have a spread of roughly 90 degrees.  The concave-up beds in the center of the photograph were deposited in a lee-slope concavity that migrated toward the viewer; the convex-up beds were deposited on a migrating convexity.  A uniform-elevation trough profile (rather than a trough with scour pits and intervening spurs) is demonstrated by the planar bounding surface at the base of the set.  It is not known whether the uniform elevation of the trough resulted from cohesion of sediment exposed in the dune trough or from in-phase crestlines as shown in Figure 42.   While the dune migrated toward the viewer, the lee-slope sinuosities also migrated from left to right through the outcrop plane.  This behavior is recognizable from the asymmetry of the cross-bedding (FIG. 42A).  Many of the cross-beds in this structure are themselves cross-stratified; these beds were deposited by superimposed bedforms that migrated across the sinuous lee slope of the main dune.  Like the larger lee-slope sinuosities, these superimposed bedforms migrated with a preferred left-to-right component through the outcrop plane (demonstrated by the preferred dip direction of cross-beds deposited by the superimposed dunes). Such migration in a preferred direction across an outcrop plane can result from either the bedform being oblique to the sediment transport direction or from the bedform being oblique to the outcrop. In this example, the dune is suspected to have been oblique to the direction of sediment transport, because the outcrop is more similar in appearance to the oblique-bedform simulation (FIG. 42A) than to the transverse-bedform oblique-outcrop simulation (FIG. 32B).

 

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