Sedimentary records of changes between granular-fluid based and fully dilute pyroclastic density currents.Michael J. Branney (University of Leicester) and Peter Kokelaar (Liverpool University).Pyroclastic density currents have been modelled variously as grainflows, laminar semi-fluidised dispersions, and low-concentration turbulent suspensions. However, they are inherently inhomogeneous, polydisperse, spatially and temporally variable, and exhibit a wide range of particle concentrations. Simultaneously operating clast-support mechanisms (aerodynamic lift, traction, saltation, hindered-sedimentation, fluidization, granular interactions, clast buoyancy and interlocking) each have associated particle segregation effects. The relative importance of each support and segregation mechanism varies with clast type, clast-concentration, shear rate, and deposition rate (e.g. effects of granular temperature may decrease with height though a density stratified current).A major challenge is to use deposit characteristics (ignimbrite lithofacies) to infer flow conditions, so that vertical successions of lithofacies in the field can be used to constrain temporal changes in currents, and lateral variations can be used to constrain how flow properties vary spatially within a current. Deposit lithofacies are created within a flow-boundary zone, which spans the basal part of the current and the upper part of the deposit. Downcurrent and lateral variations in deposit character, e.g. between stratified and massive lithofacies, indicate that the nature of the current flow-boundary zone varies with location. We explore what may control such variations in sustained currents.