Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador).

Abstract

Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. They occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute, low concentration, and fluid-turbulence supported end member of the pyroclastic currents. Here, we present a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss-face deposition. From the ripple scale (few cm) to the largest dune bedform scale (several m length), similar patterns of erosive-based backset beds are evidenced. Recurrent trains of sub-vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse-grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimeters, with very local truncations both in flow-perpendicular and flow-parallel direction. We infer that the bedforms sedimentary patterns result from four formation mechanisms: differential draping , slope-influenced saltation , truncative bursts , and granular-based events . Whereas most of the literature makes a straightforward link between backset bedding and Froude-supercritical flows, we reconsider this interpretation. Indeed, features that would be diagnostic of subcritical dunes, antidunes, and chute and pools can be found on the same horizon and in a single bedform, only laterally separated by short distances (10s of cm). Our data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Gortler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre-existing morphology. Quantification of near-bed flow velocities are made via comparison with wind tunnel experiments. We estimate that shear velocities of ca. 0.30 m.s-1 (equivalent to pure wind velocity of 6 to 8 m.s-1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30-40 m.s-1.