Tricks of turbidity currents and confined turbidite basins: a review

Abstract

Turbidity currents are able to travel hundreds of kilometres on the basin plain. However, the margins and floors of oceans, seas or lakes are rarely smooth. The topography can significantly alter the route of turbidity currents and thus modify the deposition of beds; this is termed as confinement. The earlier concepts of ponding and containment refer to turbidity currents completely trapped within a basin. Smaller alteration on turbidity current route also fits into the term of confinement. In general, confined turbidite beds are thicker and cover smaller areas than unconfined turbidites. The major characteristics of individual confined turbidite beds are defined from outcrops and cores. Palaeocurrent indicators showing different flow directions, renewed normal grading and thick mudstone caps are common features. Five types of reflected turbidite beds can be determined. Their characteristics mainly depend on the volume of the flow, the gradient
of the slope and the relative position of the log and the slope. Turbidites and analog models indicate that turbidity currents can respond to a confining slope in different ways. They are able to travel upslope if the slope is not steep and high. Otherwise the flow will be reflected or deflected. Density stratified currents are prone to decoupling into a lower dense and an upper dilute part when meeting an obstacle. The lower part is deflected, the upper part is reflected or it surmounts the confining slope. Confined turbidites build specific turbidite systems. In general, the sedimentary fill of confined basins is cha racterized by upward increasing sand/mud ratio, decreasing rate of confinement and transition from lobe to channel deposits. The evolution of confined basins includes ponding, fill and spill — in other words, flow stripping —; bypass and erosion. Slope or basin floor topography is the most important factor influencing the evolution of confined basins. Since topography is developed in response to structural deformation, the examples of confined basins are described here according to their plate tectonic setting.