A mechanism for amalgamation in deposits from shearing flows

Indistinct and gradational ('amalgamated') contacts are commonly reported within the deposits of many geophysical flows, including pyroclastic flows, debris flows and turbidity currents. However, a physical process is rarely offered as an explanation for the observed contact relationships. Herein we present experimental data as a basis for proposing a physically plausible explanation at an appropriate scale for such deposits.

Flume experiments, using a lock-gate release of coloured dry granular bead charges revealed the formation of vortical mixing features between charges and loose substrate materials. Setting and sectioning of the deposits has enabled detailed 3D characterization of the internal architectures formed. Vortical instabilities are captured forming at the contact between the substrate and charge, growing in the vertical plane. The instability growth is inferred to be due to granular shear, in a manner similar to Kelvin-Helmholtz (K-H) instability growth in Newtonian fluids. The highly unsteady and brief nature of the experimental flows has captured these features in the early stages of formation. A sustained current would cause the continued growth and downstream transport of these instabilities, leading to a significant degree of mixing across the basal contact of shearing flows.

The K-H-like instabilities found growing in the vertical plane within shearing dry granular fluids suggests a plausible single mechanism for amalgamation within a wide range of deposits from turbidity currents, debris flows and pyroclastic density currents.