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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. 53.  Structure produced by a dune with superimposed dunes that migrated obliquely upslope; Navajo Sandstone (Upper Triassic? and Jurassic), Dianah's Throne, near Coral Pink Sand Dunes State Park, Utah.  The larger bushes at the upper left of the photograph are approximately 1 m high.

RECOGNITION: The main dune that deposited this coset of cross-beds migrated toward the right and out of the outcrop, roughly in the direction of dip of the bounding surfaces scoured by the superimposed dunes.  An upslope component of migration of the superimposed bedforms is evident from the onlapping of the foresets immediately to the left of the center of the photograph, but migration directly up the lee slope of the main bedform can be ruled out at that location because the cross-bed traces are horizontal, whereas the bounding-surface traces are inclined.  In contrast, where superimposed bedforms migrate directly upslope, the resulting cross-bed and bounding-surface planes have the same strike, and traces of both planes must be horizontal in the same outcrop plane. Thus, these beds must have been deposited by bedforms that migrated obliquely upslope, as simulated in Figure 46C. This example is more complicated than the simulation in Figure 46C, because the main dune that deposited the beds in this outcrop was not straight-crested.  At the lower left side of the photograph, the dip of simple cross-beds (no compound cross-bedding and therefore no superimposed bedforms) is to the left and toward the viewer; at the right side of the photograph, the dip is to the right and toward the viewer.  Unless the superimposed dunes have the same crestline curvature as the main dune, the migration direction of the superimposed dunes relative to the main dune must vary across the outcrop. 

 

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