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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Petrology
Vol. 56 (1986)No. 3. (May), Pages 387-394

An Experimental Study of Subaqueous Slipface Deposition

Ralph E. Hunter, Gary Kocurek

ABSTRACT

A flume study indicates that grainflow on slipfaces accounts for most cross-strata formed in unidirectional, shallow-water flows. The slipfaces studied were on small megaripples and delta-like steps (0.06-0.28 m high). During intermittent avalanching, at relatively low flow velocities, periods between avalanches were marked by grainfall onto the slipface, the intensity of which was greatest near the brink of the slipface and increased with current velocity. Nearly all grainfall deposits, however, were incorporated into subsequent grainflows. Grain flow cross-strata were made up of relatively distinct layers, at least near the base of the slipface. Continuous avalanching at high flow velocity was marked by a steady stream of grains forming more poorly defined cross-strata. Although the fundamental cause of grain flow is the gradual buildup of sediment on the upper slipface to the angle of initial yield, four other processes were recognized as promoting avalanching: 1) migration of superimposed bedforms to the brink, 2) generation of turbulent pulses upstream of the brink, 3) lee-eddy impingement on the lower slipface, and 4) extension of the lee eddy above the brink. The lee eddy proved very significant in slipface processes by redistributing grainfall sediments and both promoting and impeding grainflow.

Regression analyses showed that the slipface advance per avalanche, Sa, is strongly correlated with the slipface height, H, expressed approximately by Sa = 0.060H. In addition, Sa is a direct function of the rate of slipface advance, Vb. The relationship among Sa, H, and Vb can be expressed as Sa/H = 0.0385[1 - 0.134 (min/cm) Vb]-1. Cross-strata dip angles between 28° and 34° show no systematic relation to H and Vb, but dip angles greater than 34° occurred only when both H and Vb were small, and dip angles less than 28° occurred only when both H and Vb were large.


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