Geomorphic Characteristics of Lofted Turbidity Current Deposits

Abstract

Hyperpycnal flows are river-derived turbidity currents, which - in the marine realm - commonly contain interstitial fluid that is fresher and therefore less dense than ambient fluid. These flows travel along the seabed surface due to their high suspended sediment concentrations, and their fate depends heavily on the balance between factors that increase bulk flow density, e.g. entrainment of sediment or ambient water, and those that decrease bulk flow density, e.g. deposition of suspended sediment. If suspended sediment is rapidly deposited from the flow, bulk flow density and flow velocity will decrease until it reaches a point of equal density to the ambient fluid through which it is travelling. Once this point is reached, the flow can begin to rise to the water surface or to a depth of neutral buoyancy in a process known as lofting. We ran 21 experimental turbidity currents with varying bulk flow and interstitial fluid densities, across three different basin geometries, in order to characterize the affect on deposit geometry. Our findings show that lofted turbidity currents are width-limited and generate narrower, more elongate deposits than bed-attached flows. We also show that steeper ramp gradients push the lofting point farther out into the basin. We show the effect of variations in bulk flow density, suspended sediment concentration, and fluid density on overall deposit geometry and flow run-out distances. Most importantly, the use of a 3-dimensional experimental tank allows for the first detailed analysis of the lofting process and its effects on length-to-width ratios of turbidite lobes.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016