Behaviour of Fully Ponded Turbidity Currents in Confined Basins: An Experimental Study

Patacci, Marco¹, William. D. McCaffrey², Peter D.W. Haughton³, Lucie Baudouy⁴ (1) University College Dublin, N/A, Ireland (2) University of Leeds, Leeds, United Kingdom (3) University College Dublin, Dublin, Ireland (4) University College Dublin, Dublin 4, Ireland

Tectonic deformation of the deep sea floor can produce local accommodation filled by contained turbidites. Flow ponding can result in unusually thick sandstones with distinctive structures that onlap surrounding slopes. Despite being widely recognized, the dynamics of confined turbidity flows and the suspension clouds they create is still poorly known. Key issues are what controls the wide variety of deposit structures observed in confined basins, and the nature of lateral and vertical textural trends within ponded sandstones.

A series of experiments using scaled steady turbidity flows were run in a special designed tank with a steep slope at its end allowing full 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 textural fractionation. After each experiment, the deposit was fixed, sampled and imaged by SEM. Image analysis was used to map lateral and vertical grain size distributions.

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 results help explain the complex vertically tiered structure of ponded sandstones in areas of deep bathymetry, and lateral changes as the surrounding onlap slopes are approached. Deposition from suspension clouds such as these will be determined by the ratio of basin length to flow length.