Cross-Bedding, Bedforms, and Paleocurrents (Concepts in by David M. Rubin

By David M. Rubin

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Additional info for Cross-Bedding, Bedforms, and Paleocurrents (Concepts in Sedimentology & Paleontology 1)

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If this interpretation is correct, field measurements should show that the cross-beds deposited by the superimposed bedforms dip in slightly different directions from the bounding surfaces scoured by the superimposed bedforms, as illustrated in Figures 65 and 66; cross-beds and bounding surfaces produced by superimposed bedforms can have the same dip direction only if crestlines of the superimposed bedforms exactly parallel the crestline of the main bedform (Fig. 25). 51 CROSS-BEDDING, BEDFORMS, AND PALEOCURRENTS troughs of bedforms where the lee eddy drives superimposed bedforms in an upcurrent direction (Boersma and others, 1968; Dalrymple, 1984).

As the ripples migrated, they left behind crosslaminated beds. The beds have a mean thickness that is equal to approximately half the ripple height, which indicates that during the time that each ripple migrated one wavelength, deposition raised the bed elevation by approximately half the ripple height. In the plane of the photograph, the bounding surfaces that separate the cross-laminated beds dip toward the right or are relatively horizontal, whereas the generalized depositional surface (the bed surface if the ripples were smoothed off) dips at a low angle toward the left.

Plots of the poles of cross-beds in these structures are useful for identifying the bedding as variable two-dimensional cross-bedding, but the structures are so similar to others that the specific depositional processes cannot be determined from the cross-bed dip patterns. ORIGIN: Fluctuations in bedform asymmetry and migration speed are common in reversing flows. The structures shown here-and their three-dimensional counterparts-could be expected to be common in oscillatory flows such as wave-generated flows, tidal flows (Fig.

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