Site hosted by Angelfire.com: Build your free website today!

 

Flow Efficiency Controls on the Evolution of Composite Sand Bodies Deposited From Turbidity Currents

O.S. Al-Ja’aidi, W. D. McCaffrey, and B. C. Kneller

Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, West Yorkshire, UK.

(e-mail : ossj99@yahoo.com)

Turbidity current flow efficiency (sensu Mutti) affects the ability of flows to transport sand, and therefore plays a major role in governing the location and geometry of turbidite sandstones. In turbidity currents, the principal controls on flow efficiency are the grainsize distribution (i.e. sand-rich vs. mud-rich grain flows), density, and overall volume. In flows of higher efficiency the transport phase is prolonged, thus elongating the deposit or displacing it distally. To enhance our understanding of this phenomenon we performed a set of experiments in which we examined these controls. Turbidity currents were released via a lock exchange mechanism into a square tank. Flow efficiency was varied by systematically varying 1) the proportion of fines, 2) the flow density (i.e. mass fraction of suspended sediment) and 3) the volume of the turbidity current. One aim of this work is to improve determinations of palaeo-flow efficiency from core. In turn this may improve our ability to prediction sand distributions in the subsurface.

Another control on the depositional behaviour of flows is the topography generated by deposition from earlier flows. Here we set out to investigate the role of flow efficiency in governing the stacking patterns of composite sedimentary deposits, and in particular the interplay between bypass and lateral offset stacking. In these experiments, a series of identical unconfined flows was generated, in which each flow over-ran the deposits of its predecessors. The flows scale to natural-scale turbidity currents bearing/depositing coarse/very-coarse sand. The geometry of the deposit was measured after each run, enabling both the evolving shape of the composite deposit and also the isopach distribution of individual events to be shown. These can be used to develop an insight into the behaviour of real systems both at outcrop and in the subsurface.

Keywords: Flow efficiency, compensation, topography