DescriptionIgnimbrite stratigraphy and reworking : analogue modelling of dense, polymict granular flow deposition and Kelvin-Helmholtz (K-H) instabilities
Sectioning and 3D visualisation of laboratory flume deposits from dense, polymict granular flows reveals intricate structures typical of pyroclastic density current (PDC) deposits. Particle mixtures containing analogies to juvenile pumice and lithic clasts generate structures analogous to PDC features including ‘rafting’ and inverse grading of pumice, and normal grading of lithics by simple gravitational granular sorting. In addition, sequential charges generate complex shear-derived reworking and interaction between the active flow and underlying deposit. Recumbent flame structures occur alongside features produced by larger scale Kelvin-Helmholtz (K-H) instabilities that result from perturbations in the shear-layer during flow and deposition. Mathematical modelling indicates that similar K-H instabilities should also form in natural PDCs. Hence there would be strong mixing at the boundaries between successive flows and this would have significant implications for unravelling eruption histories, temperature proxy data using in-situ charcoals, and dating using phenocrysts (40Ar/39Ar) or charcoals (14C). K-H instabilities provide a syn-depositional mechanism for recumbent flame structure formation in laminar shearing systems.
Assuming laminar shear is prevalent in the depositional flow-boundary region of dense PDCs, the lack of recorded K-H instabilities in field deposits is assessed. Possible explanations include: 1) masking due to the “uniformity” in colour, constituents and composition of successive PDC deposits, and 2) rapid vertical migration of the shear zone during deposition precluding full K-H growth and preservation. In situations where reworking has obscured boundaries between separate multiple flow units, inferences based upon a single unit interpretation of eruption volumes and periodicities, flow volumes, and flow thicknesses would be incorrect. The ability of thin dense granular currents to remobilise significant volumes from underlying loose material suggests an important role for reworking in the stratigraphies of sequential density-stratified pulses and flows. Furthermore, the degree of particle sorting over short distances during flow demonstrated in the flume experiments suggests the classic ignimbrite inverse-graded to massive division stratigraphy can be reproduced by remobilisation of pre-existing PDC deposits.
|Period||13 Dec 2009 → 20 Dec 2009|
|Location||San Francisco, United States|