The origin of planar lamination in fine-grained sediment deposited by subaqueous sediment gravity flows

Abstract

Planar lamination is a ubiquitous component of modern and ancient fine-grained sediments deposited by subaqueous sediment gravity flows. These sediments commonly exhibit alternating sub-millimetre-thick, sharply bounded silt-rich and clay-rich layers that change little in thickness or sediment texture over lateral distances that range up to at least several tens of metres. Silt-rich layers are moderately to well-sorted and a few tens to hundreds of microns to a single silt-grain thick. In contrast, clay-rich layers are more poorly sorted, and a few tens to hundreds of microns thick. The thickness and regular alternation of these texturally differentiated interlayers, in addition to the absence of features suggesting transport bypass or deposition by migrating rugged bed forms, suggest alternating physical processes and related modes of deposition in the near-bed region immediately above the bed. Previous interpretations have focussed on clay flocculation, which is difficult to reconcile with the high fluid shear conditions in the near-bed region. Here it is suggested that in the millimetre to sub-millimetre-thick viscous sublayer at the base of a hydraulically smooth turbulent flow, a combination of high fluid shear and sediment concentration initially leads to shear thinning and enhanced mobility in the lower part of the flow, and for silt to continue settling to the bed forming a well-sorted silt lamina. As silt settles and clay-size sediment increases in concentration, hydrodynamic lubrication forces strengthen and reduce mobility of the near-bed part of the flow. This condition is then perturbed by a bedward-directed outer flow disturbance that dramatically increases frictional stresses and effective fluid viscosity and ultimately shear jamming that causes gelling and deposition of a poorly sorted clay-rich layer. This process is then repeated multiple times to build up a deposit composed of alternating silt-rich and clay-rich laminae overlain by structureless mud deposited directly from suspension.