Incipient granular mass flows at the base of sediment-laden floods, and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands

Abstract

Extraordinary floods commonly produce thick deposits as a result of the entrainment and transport, often at high concentrations, of large volumes of clastic detritus. This paper describes the sedimentology of a stratified bouldery sand deposit generated by a 20-km 3-breakout flood from intracaldera Lake Taupo following the 1.8-ka Taupo eruption. The proximal deposit is characterised by a strongly bimodal grain-size distribution, with downstream-fining boulders enclosed in a poorly sorted and weakly bedded coarse sand to gravel matrix. This combination of characteristics is inferred to reflect simultaneous deposition of small and large particles by two distinct mechanisms: (i) accretion of the sandy matrix from an incipient basal granular mass flow as the flood lost capacity, and (ii) progressive emplacement of the boulders as the flood lost the competence to roll them along the channel floor. The first mechanism is a product of a nonlinear particle concentration gradient in the sediment-laden flow, which caused it to segregate into a basal incipient granular mass flow that formed the transient depositional system indicated by the lithofacies. The second mechanism is a function of the superjacent, turbulent, turbid flow that acted as the primary transporting system. This pairing of transport and depositional systems is likely to characterise many sediment-laden aqueous flows with clay mineral concentrations too low to develop non-Newtonian rheologies, such as jökulhlaups and lahars that entrain unweathered pyroclastic material. Qualitative and quantitative differences in the way bedload is transported in the presence of this basal granular mass flow, as compared to simple water, make impossible accurate estimates of flow competence based on Shields entrainment functions for beds of both uniform and mixed grain size distribution.