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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. 14.  Structures produced by ripples with a fluctuating positive angle of climb; fluvial beds from the Kayenta Formation (Upper Triassic?) in Zion National Park, Utah.

RECOGNITION: This photograph shows approximately a dozen cycles of fluctuating angle of climb.  Ripple foresets are visible at many locations on the right side of the photograph (small arrow) and indicate that the direction of ripple migration was from right to left.  As illustrated by the computer-generated version of this structure (FIG. 13), increases in the rate of deposition relative to the rate of bedform migration cause the ripples to climb more steeply and to deposit thicker cross-laminated beds; decreases in the angle of climb produce thinner cross-laminated beds with bounding surfaces that more nearly parallel the generalized depositional surface.  The generalized depositional surface (indicated by planes along which adjacent ripples simultaneously changed their angle of climb) dipped toward the left and steepened during deposition of the cyclic beds.  One of the many depositional surfaces recognizable by a change in angle of climb is indicated by the large arrow at the right.

 

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