Erosion and re-deposition by pyroclastic flows: implications of experimental evidence for reworking of loose substrates and primary deposits

Pyroclastic deposits can remain unconsolidated for long periods after deposition (e.g. Taupo ignimbrite, New Zealand c.1.8ka). Subsequent erosion, entrainment and remobilisation of these loose deposits by younger flows can (a) reset depositional thickness, (b) reshape palaeotopographies, (c) redistribute charcoals and phenocrysts used for absolute dating and (d) mix juveniles used in understanding provenance. An experimental approach was used to assess the interaction between dry granular charges and static loose substrate materials. Flume experiments used a lock-gate release of coloured dry granular bead charges, deposited onto a variety of loose granular substrates. The results revealed the scale of erosion from granular temperature conduction from the “younger” flow into the “older” substrate. Flow thicknesses of approximately 10 mm removed an equivalent thickness of substrate in highly unsteady flow conditions. Furthermore, experiments using polymict charges assessing particle size and density reproduce a range of internal stratigraphies observed in field deposits, and grant insight into the depositional mechanisms active in these non-uniform unsteady dry granular currents. Analogue experiments demonstrate that reworking by dry granular flow can reproduce “ignimbrite stratigraphy” identical to the primary deposits formed by the fountaining of eruptive columns, including inverse graded “pumices”/ low density particles and normal grading “lithics”/high density particles