A Tool to Interpret High-Density Turbidity Current Processes from High-Density Turbidite Lithofacies
Cartigny, Matthieu; Eggenhuisen, Joris T.; Postma, George
Many deposits in deepwater depositional systems are described as "High-Density Turbidites", without proper reference to the variability in lithofacies, grain fabrics, and therefore reservoir quality that may be included in deposits from high-concentration suspensions. In this contribution, we use observations and quantifications of experimental high-density turbidity currents to construct an interpretative tool that can be used for inversion of HDTC flow processes from HDT lithofacies.
High-density turbidity currents exhibit internal density stratifications. In the basal part of these stratified flows, high sediment concentrations cause rheological deviations from the Newtonian turbulent flow that dominates clear water and low density conditions. Previous field studies have classically linked crude stratification bands, spaced laminations, and internal erosion surfaces to high-density turbidity currents. Various mechanisms have been proposed for these variable depositional characteristics; however, none of these propositions has been thoroughly tested by experiments or theory.
We present experiments of quasi steady high-density turbidity currents varying in initial sediment concentration between 9-26 v% by volume.
Three distinct internal flow layers were distinguished. Type I layers have lower sediment concentrations and show strong turbulent behaviour in both observations and velocity measurements. At higher sediment concentrations an additional type II layer forms below the type I layer. This layer has a higher concentration, while turbulence levels are low compared to those of type I layers. At even higher sediment concentrations turbulence levels are dampened further, and a type III stratification develops.
Small-scale fluctuations (0.2-2 seconds) in the otherwise steady flows were observed to control depositional behaviour. However, the influence of these fluctuations gradually decreases with increasing sediment concentrations as function of the different types of basal flow layers.
By combining these observations with theoretical grain size sorting mechanism and previous experimental results that link depositional expressions to flows of different sediment concentrations, the different layer types observed here are linked to distinct depositional expressions within the family of high-density turbidity current deposits known from the rock record.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013