Abstract
AbstractPyroclastic 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. Pyroclastic currents 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. This article presents 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 (a few centimetres) to the largest dune bedform scale (several metres in 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 centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. 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 (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler 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 is made via comparison with wind tunnel experiments. It is estimated 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 to 40 m.s−1.
Highlights
Deposits containing fine-scale laminasets forming dune bedforms have been interpreted as resulting from the low-concentration, turbulence-dominated, end-member of pyroclastic currents
Dune bedforms emplaced by pyroclastic currents are notorious for their stoss-aggrading nature producing a variety of backset laminations
As already suggested for turbidites (Ponce and Carmona 2011) and pyroclastic currents (Douillet et al 2013b), we interpret that the stoss-depositional crests result from a process of differential draping, whereby falloutdominated deposition is enhanced on stoss-faces, due to the simple combination of the bed topography and trajectory of particles (Fig 18b)
Summary
Pyroclastic currents are ground-hugging gas-pyroclast mixtures whose flow is generally driven by gravity and triggered by volcanic eruptions (Dufek 2016, Sulpizio et al 2014, Palladino 2017). Le Roux 2005-, Douillet et al 2013b) Such "dilute pyroclastic currents" may result from decreased density resulting from thermal expansion of entrained air (Andrews 2014), flow stripping at cliffs (Douillet et al 2013a), or may be related to the initial eruptive dynamics, in particular for highly explosive maar volcanoes A smooth continuum between the granular and turbulent end-member flow behaviors is often modeled and expected in nature (Breard et al 2016, Burgisser and Bergantz 2002)
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