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The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 343-343
Symposium Abstracts: Storm-Dominated Shelves

Laboratory Studies of Oscillatory-Flow Bed Configurations and Their Bearing on Stratification in Shallow-Marine Sands: Abstract

J. B. Southard1

Abstract

Studies using wave tanks, especially those with oscillatory-flow ducts, show that symmetrical oscillatory flows produce several kinds of bed configuration as a function of sediment size, oscillation period, T, and maximum orbital speed, Um (or orbital diameter). Differences in flow in these two kinds of apparatus do not seem to result in fundamental differences in bed configuration. At small to moderate T (8 to 10 s) in very fine to fine sands, vortex ripples arc the stable configuration. These are known from meager but clear laboratory data to be strongly three-dimensional in the upper range of (Um, especially at large T. Systematic field observations on three-dimensional vortex ripples are lacking. The bed geometry is that of hummocks and swales with moderate to negligible anisotropy in plan view, and three-dimensional ripples become less steep and more rounded with increasing Um. The upper limits of Um, T, sediment size, and ripple spacing for three-dimensional vortex ripples is not yet known experimentally, but maximum ripple spacing is at least 1 m (when scaled for cold water). The possibility that the transition from two- to three-dimensional vortex ripples is a consequence of the small size of oscillatory-flow ducts or the presence of side walls seems unlikely, because in the same experimental arrangement, vortex ripples in coarse sands remain two-dimensional for the highest Um and T studied so far. The scarcity of hummocky cross-stratification in sands coarser than 0.25 mm, together with the lack of large-scale ripples in sands finer than about 0.2 mm in dominantly unidirectional flows, suggests that most of such stratification is produced by three-dimensional vortex ripples during deposition from suspension. This interpretation is clouded by the absence of experiments on deposition and the uncertainty about the upper range of conditions for the existence of three-dimensional vortex ripples. Hummocky cross-stratification is likely to be polygenetic in any case; some stratification styles that would be described by that term are probably produced by combinations of oscillatory and unidirectional flow. Experiments on bed configurations in combined flows, especially at high flow velocities and under depositional conditions, are badly needed as a guide to what might be encountered in natural flows.


 

Acknowledgments and Associated Footnotes

1 Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.

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