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William D. McCaffrey1,
Benjamin C. Kneller2,
Omar Al-Ja'aidi2
(1) Leeds University, Leeds, United Kingdom
(2) Leeds University
Abstract: 
Flow
efficiency controls on the geometry and location
of turbidite sheet systems
Flow efficiency gives a relative
measure of the distance a flow may travel. It is a characteristic of the flow,
and is therefore independent of topographic effects. In flows of higher
efficiency, the transport phase is prolonged, thus elongating the deposit, or
displacing it distally. Although early workers stressed the importance of the
fine-grained component of the sediment budget on flow efficiency, new work
outlined here illustrates that this is only one of a number of linked controls,
which include: 1. the suspended sediment grainsize distribution, 2. suspension
density and 3. flow volume. The presence of a significant fine-tail to the
sediment budget controls flow efficiency through the mechanisms of buoyancy
enhancement for the coarser-grained suspended sediment component, reduced rates
of momentum loss due to low mud deposition rates and friction reduction;
suspension density affects flow efficiency principally through its impact upon
the flow momentum, whilst suspension volume controls flow efficiency
principally by affecting the shear velocities. We present experimental data on
scaled, sediment-bearing turbidity currents which model the deposition of
basin-floor turbidite sheet sands. These illustrate that each of the controls
outlined above has a characteristically different effect upon the volume and
geometry of the resultant deposit. Determining paleo-flow efficiency can be
important when predicting the loci of sand deposition, and we discuss the
pitfalls of making this assessment from core. Recent work has illustrated that
cycles in the calibre of sediment delivered to the shelf edge may be related to
sea-level, and we evaluate the effects of corresponding allocyclic cyclicity in
flow
efficiency upon large-scale system development.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana