Flow dynamics as Froude-supercritical turbidity currents encounter metre-scale slope minibasin topography

Abstract

Seafloor topography can affect turbidity current dynamics on deep-water slopes, significantly influencing the dispersal of sediment. Despite the common occurrence of topographic complexity, there are few detailed investigations of topographic interactions and their effect on downslope flow evolution in intraslope environments. In this study, the sedimentology and architecture of an Upper Cretaceous intraslope fan succession deposited within an extensional, fault-bound minibasin are described from a rare, well-exposed, near-continuous, oblique depositional-dip outcrop of the Tres Pasos Formation, Chile. The 2 to 8 m thick studied interval transitions downslope from high-energy heterolithic strata, including metre-scale steep-faced scours, to non-amalgamated thick-bedded sandstones. Abrupt increases in sandstone percentage, sandstone bed thickness and grain size occur on the hangingwall blocks of south-east and north-east-dipping normal faults that bound the minibasin. Sandstone beds are dominated by backset or wavy low-angle stratification proximally, contain compositional banding near faults, and are characterised by increased proportions of planar laminated and structureless turbidite divisions downslope along the transect. Experimental observations of turbidity current interactions with topography are synthesised into a qualitative framework, which is used to interpret flow processes and characteristics from deposit trends. The results reconstruct the response of Froude-supercritical, stratified turbidity currents with denser basal layers when encountering metre-scale fault scarps. The analysis shows that metre-scale topographic features can substantially alter the flow properties of stratified turbidity currents, and their downslope flow evolution to include the development of transitional, depositional and flow-stripped sediment gravity currents. However, in comparison to base-of-slope settings, overall flow conditions are interpreted to be more uniform over slope breaks and zones of flow expansion in a partially confined intraslope environment. These findings have considerable implications for understanding flow response to similar scale morphological features on the seafloor and the potential for flow transformations in intraslope settings.