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. 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 subvertical 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 1531 role of coherent flow structures such as G€ ortler 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 0Á30 m.s À1 (equivalent to pure wind velocity of 6 to 8 m.s À1 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 À1 .

Revisiting the lacquer peels method with pyroclastic deposits: sediment plates, a precise, fine scale imaging method and powerful outreach tool

“…d Accretionary pellets possibly linked with the encounter of pyroclastic currents with the Chambo river (see Kueppers et al 2016) field investigations (Douillet et al 2013b). In particular, centimeter scale shear features forming "shark fins" (Douillet et al 2015(Douillet et al , 2018b, overturned lamination underlining erosion planes (Douillet et al 2017), or "backset ripples" (Douillet et al 2018a) which have been recognized for the first time here in pyroclastic deposits. Even outcrops that appeared completely massive in the field actually exhibit faint lineations highlighted through grain fabrics and subtle stratification that is readily visible in the sediment plates.…”

“…The Tungurahua deposits represent a unique chance to date to investigate in detail well-preserved pyroclastic bedforms with surface exposures in 3D. Adding greatly to the research context of these deposits is the fact that a thorough documentation of outcrops through a survey combining ground penetrating radar (GPR, Dujardin 2014), terrestrial laser scanner (TLS), and the impregnation method has also been accomplished (Douillet et al 2018a(Douillet et al , 2018b.…”

“…Over 50 m 2 of sediment outcrops were impregnated in order to preserve each of the four types of dune bedforms previously recognized (Douillet et al 2013b(Douillet et al , 2018a. For each outcrop, a set consisting of 6 neighboring plates was produced aligned with the inferred flow direction (Fig.…”

“…Thus, when assembling the plates, they form sets, each of which reflects a 3 m wide outcrop, which can in some cases cover the entire thickness of a Tungurahua 2006 deposit. For some bedforms, several sets parallel to each other as well as perpendicular cuts between transects were impregnated, in order to document lateral variations within a single bedform (Douillet et al 2018a). One 6 m long set was created from the stoss side of one dune bedform to the crest of the next one, in order to document the relationship between successive structures.…”

Hazardous Base Surges of Taal’s 2020 Eruption

“…The dune field in the southeast sector offer a rare, largely undisturbed, and nearly complete picture of a base surge deposit field, which we describe concisely in this article. Often, these deposits are studied as discontinuous outcrops years after an eruptive event, when erosion, deposition, and vegetation have obliterated or concealed important features that hinder better understanding of flow emplacement and dynamics [10][11][12] . Taal Volcano's 1965 eruption was well-documented by J.G.…”