Ponding of Experimental Turbidity Currents: Flow Behaviour and Deposit Implications
The flow behaviour of turbidity currents and the resulting deposits are strongly affected by the topography over and around which they flow. Different types of interaction can be distinguished: 1) flow-substrate interaction; 2) flow diversion and 3) flow ponding. The ponding process occurs when a turbidity current is discharged into a topographic low and part or all the flow is trapped. Flow ponding is of particular interest as it can result in localised and thick sand accumulations that have distinct sedimentary characteristics. Deposits of ponded turbidity currents have been widely recognised in the geological record.
Four different types of ponding can be defined according to flow duration (flow volume) and confinement height (basin volume). Short-lived flows are either partly or fully reflected to form a reverse current whereas sustained currents result in the formation of a thick sediment-bearing cloud (ponded cloud) which may or may not overspill. A series of experiments carried out at the Sorby Lab at the University of Leeds have given new insight into the ponding process in the case of steady long-lived turbidity flows (both partly and fully contained).
The ponded currents are characterised by: 1) interaction between the input flow and the developing suspension cloud; 2) development of a settling interface; 3) multilayered horizontal velocity structure with flow reversals, flow pulses and internal waves; 4) enhanced upward velocity component; 5) unsteady to quasi-steady evolution, and 6) distinctive concentration and grain size gradients.
The evolution of the ponded suspension clouds can be related to the structure of the deposits in natural examples such as turbidite beds from the Tabernas Basin (SE Spain) and the Annot Sandstone (SE France). Flow reversals can explain vertical changes in palaeocurrents. Internal waves can produce vertical textural cyclicity, particularly where density interfaces impinge on surrounding slopes. Collapse of the sediment-bearing cloud can lead to sudden deposition of massive sand and unusual vertical sequences of sedimentary structures. Characteristic grain size and concentration gradients within the suspension can result in thinning and fining up the confining slope, with implications for reservoir quality trends at sand pinch outs.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009