Print this pageFlow Behaviour of Ponded Turbidity Currents: From Experimental Study
Marco Patacci1, William M. McCaffrey2, Peter Haughton1, Jaco H. Baas3, and Gareth Keevil2
1School of Geological Sciences, University College Dublin, Dublin, Ireland
2Institute of Geological Sciences, The University of Leeds, Leeds, United Kingdom
3School of Ocean Sciences, University of Wales, Bangor, United Kingdom
Interaction between turbidity currents and topography plays an important role in defining temporal and lateral changes in flow behaviour. Sedimentary structures such as banding and cyclic grain size trends from single event beds have been recognised as the result of such modification. Despite being widely recognized, the dynamics of confined turbidity flows and of the suspension clouds they create is still poorly known. Building on a pilot study, results from new experiments are presented that constrain the behaviour of a turbidity current ponded against a frontal slope.
A series of experiments using scaled steady turbidity flows were run in a special designed tank with a variable geometry, allowing full and partial ponding. Ultrasonic transducers were employed for non-intrusive 3D measurement of the flow velocity. Close-up video recording of the aggrading deposit constrained the sedimentation rates. Sampling of the suspension was used to calculate temporal and spatial variation of the flow concentration and grain size fractionation. The deposit grain size was measured using a combination of SEM imaging and laser grain sizer analysis.
Velocity and concentration data reveal the pattern of growth, the internal circulation and a system of internal waves within a sustained suspension cloud reflecting initial rebound, multiple reflections and the end of the flow input. The change in grain size stratification after the passage of the head and the first reflection reflects the diminished mixing capability of the flow. The grain size trends in the resulting deposit appear closely tied to the grain size stratification of the flow. The results can help improve the understanding of ponded turbidites and prediction of bed geometry, sedimentary structures and textural trends as they approach confining slopes.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas