Transport dynamics of mass failures along weakly cohesive clinoform foresets

Abstract

This study addresses the initiation mechanisms of mass failures on clinoform foresets. Previous studies have created mass flows by releasing dense watersediment mixtures into standing water, thus imposing the initial conditions for the mass failures rather than allowing them to form on their own. Parameters such as the density, composition and initial momentum of the failures are pre-determined, precluding observation of the factors that set them initially. This study uses a new experimental method that allows a range of mass failures to self-generate. Building a clinoform using a cohesive mixture of walnut-shell sand and kaolinite allows the foreset to build up and fail episodically, generating mass failures. Slopes undergo a series of morphological changes prior to failure, creating a concave shape that becomes exaggerated as deposition continues. This morphology leaves the slope in a metastable state. Once the slope is destabilised, failure is initiated. This study investigates the effect of clinoform progradation rates on failure size and frequency by conducting experiments over a range of water and sediment discharge rates. Neither failure size nor failure frequency changes with discharge rate; instead, increases in sediment supply are taken up by changes in the partitioning of sediment between the steep upper foreset and the more gradual delta-front apron (toeset) below. Sediment is delivered to the delta-front apron by a form of semi-continuous slow creep along the foreset. This slow creep is a failure mode that has not received sufficient attention in the submarine mass-flow literature. The independence of failure size and frequency from sediment supply rate suggests that the presence of mass-failure deposits does not provide information on the rate of sediment delivery. If these relations hold at field scales, this would imply that individual mass failures are relatively insensitive to changes in water and sediment supply.

Fault‐controlled base‐of‐scarp deposits

“…(a) Flume experiment showing the accumulation of what here is defined base‐of‐scarp deposits during the early depositional phases. The base‐of‐scarp deposits subsequently evolve to a Gilbert‐type delta system prograding above an unconformity separating the unsteady to the steady‐state phases (modified after Abeyta & Paola, 2015). The unconformity can be referred to as the S 2 surface recognised in the studied examples.…”

Sink‐to‐source: Influence of offshore dynamics on upstream processes

“…Mass failure events, which occur along the foreset, highlight the effects of removing or altering keystone points along the foreset. Experiments conducted by Abeyta and Paola () generated self‐organized mass failure events, including intermittent large events that substantially altered the local foreset geometry. The experimental mix used included clay, which altered foreset mass‐flow dynamics away from the fairly regular small avalanches that characterize noncohesive granular flows to larger, more irregular mass failures.…”

“…A sediment mix that accommodates a range of mass-flows along the foreset allows us to explore the impact of events that alter the keystone point along the forest. To understand how such complex foreset dynamics affects overall deltaic behaviour and morphology, we conducted experiments similar to those of Abeyta and Paola (2014) using the same sediment mixture, but in two dimensions, to observe how offshore events affected the fluvial morphodynamics of the delta. In this case, the "sink" control operates within the distribution of mass between the foreset and bottomset, (b) C silt = 2%; insufficient reduction in foreset slope to "un-choke" system.…”

“…Using the experimental mix developed by Abeyta and Paola (), a 1:1 ratio mix by volume of walnut‐shell sand and kaolinite clay, we conducted a 2‐D delta experiment in the Saint Anthony Falls Laboratory Delta Basin. The Delta Basin is 5 m square and 0.6 m deep basin with a fixed horizontal floor.…”

Fault-controlled base-of-scarp deposits

“…Figure 8 -A) Flume experiment showing the development of base-of-scarp deposits during the early phases.Base-of-scarp deposits evolving to a Gilbert-type delta system on top of an unconformity separating the unsteady to the steady-state phases (modified afterAbeyta and Paola, 2015). B) Profile of Gilbert-type deltas evolving above depositional surface characterised by different depositional angles (modified afterLai et al, 2017).…”