Abstract

Turbidite beds and pyroclastic density current deposits commonly show abrupt internal changes in grain size with stepwise upward fining. Grain-size breaks divide a turbidite bed into a lower sandy zone, a middle silty zone, and an upper muddy zone. Zone boundaries need not correspond with Bouma division boundaries, but the sandy zone usually consists of A, B, and occasionally C divisions, the silty zone is composed of one or more of the B, C, and D divisions, and the muddy zone correlates with division E. Pyroclastic density current deposits likewise display a tripartite zonation with prominent grain-size breaks. Divergence of particle-laden gravity currents into a lower dense body and an upper dilute turbulent wake can explain the formation of the lower grain-size break. Fine particles are elutriated into the wake region by ambient entrainment through the current head, while coarse grains are transported in the body. Particle size and concentration stratification is sustained by the turbulence and velocity structure of gravity currents, with a velocity maximum and turbulence minimum also dividing currents into two regions. Two stages of deposition result. The upper grain-size break may be produced by buoyant lofting, with fine particles convected into the water column or atmosphere as the flow wanes.

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