Turbidity Current Flow Structure Linked to Vertical Textural Changes Within a ‘Turbidite’ Deposit: An Experimental Approach

Simon P. Barker¹, William M. McCaffrey², Peter D. W. Haughton¹, Jaco Baas², and Caroline Choux^2
1 University College Dublin, Dublin, Ireland
² University of Leeds, Leeds, United Kingdom

Despite the abundance of studies into turbidity currents and their deposits, internal flow structure, sediment transport mechanisms and controls on sediment deposition remain poorly constrained. This study presents the results of an experimental program designed to quantify the spatio-temporal evolution of mean flow properties and turbulence structure within quasi-steady, particulate turbidity currents, and relate temporal evolution of internal flow structure to vertical textural changes within the associated ‘turbidite' deposit (itself essentially a time series record of temporal fluctuations in base-of-flow grain size and concentration).

Results indicate flows were highly non-uniform in terms of mean flow properties and turbulence, demonstrating the difficulty of interpreting instantaneous flow structure from time series data. Base-of-flow time series data recorded the periodic passage of turbulent eddies, impacting near-bed grain size and concentration. Increased turbulence during eddy passage ejected sediment upward as a turbulent ‘burst' resulting in a transient decrease in mean grain size and concentration due to enhanced mixing at the base of the flow. Between eddies, the associated drop in turbulence led to a loss of competence and sediment fell more rapidly from suspension, coarsest sediment followed by finer sediment, resulting in periodic peaks in near-bed grain size and concentration. Image analysis of the underlying deposit identified the cyclic repetition of coarse, poorly-sorted horizons grading into fine, moderately sorted horizons. These textural changes within the deposit reflect the fluctuations in near-bed grain size and concentration due to the passage of eddies. This may account for the development of spaced lamination within natural turbidites.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005