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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Petrology
Vol. 44 (1974)No. 3. (September), Pages 637-648

Supercritical Previous HitFlowNext Hit in Density Currents

Bryce M. Hand

ABSTRACT

Experiments with saline currents show that most Previous HitflowNext Hit phenomena and sedimentary Previous HitstructuresNext Hit characteristic of supercritical Previous HitflowNext Hit in open channels can be duplicated with density underflows. Antidunes, breaking antidunes, and chutes-and-pools formed spontaneously on an erodible bed at densiometric Froude numbers greater than 1.0. Runs using fixed model antidunes show that equations derived for antidunes and hydraulic jumps in open channels are equally valid for density currents, provided a term is introduced to take account of the reduced density contrast between fluid layers. Antidune wavelength (L) is related to Previous HitflowNext Hit velocity (U) by the equation for internal waves:

U = <rad>gL/2^pgr (^rgr-^rgr^prime)/(^rgr+^rgr^prime)</rad>

where ^rgr and ^rgr^prime are densities of the current and overlying fluid.

Critical slope for large scale density flows should be about 0.001. If so, most natural turbidity currents are supercritical. Wavelength of antidunes formed by density currents must be ^ges12.6 times the effective Previous HitflowNext Hit thickness. For turbidity currents, implied wavelengths are on the order of tens or hundreds of meters. Antidunes of this scale might be manifest as long period pinch-and-swell of individual turbidites, but internal lamination should be essentially parallel to bedding. Under some circumstances, it may be that many successive turbidites comprise the "laminae" of large, composite antidunes.

Density currents provide a range of Previous HitflowNext Hit conditions where "upper Previous HitflowNext Hit regime" bedforms can coexist with "lower Previous HitflowTop regime" forms, pointing up a defect in such classification. Ripple lamination does not in itself imply subcritical conditions.


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