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

Abstract

Pyroclastic sedimentary successions record an eruptive history modulated by transport and depositional phases.Here, a field technique of outcrop impregnation was used to document pyroclastic sediments at the scale of individual laminae. The details so obtained have the potential to reveal new insights into the dynamics of transport and deposition of pyroclastic currents and fallout. Sediment plates (a type of lacquer peels) represent a sampling method whereby a thin plate of undisturbed sediments is obtained directly from the outcrop. A low-viscosity, hardening epoxy resin is applied to a freshly exposed cross-section of an unconsolidated deposit and impregnates a surface layer of the cross-section via capillary forces before solidifying. Upon hardening, a solid plate (0.5-5 cm thick and up to 2 m in length) of the sedimentary formation can be recovered and transported with full preservation of the initial organization of the particles. Sediment plates are capable of recording and highlighting details of stratification to a very fine scale and high degree of precision. This method represents a valuable tool for research, education, and exhibition purposes. A dataset of 50 m 2 of sediment plates was created from the primary sedimentary structures emplaced during 1) the August 2006 eruption of Tungurahua volcano (Ecuador), 2) the Laacher See eruption (Germany) and 3) Astroni surge deposits (Campi Flegrei, Italy). This dataset has been put to use in several contexts: 1) analysis of sedimentation processes at the boundary-layer scale, 2) data comparison with remote measurements (e.g., ground penetrating radar), 3) archiving and conservation, 4) as teaching material and 5) for dissemination exercises during exhibition in museums.

Synthesizing Field and Experimental Techniques to Investigate the Enigmatic Processes Occurring at the Base of Pyroclastic Currents

“…In addition, the mixing structures produced in our experiments resemble recumbent flame structures found in the deposits of turbidity currents (Dasgupta, 1998), pyroclastic currents (Rowley et al, 2011;Douillet et al, 2018;Pollock et al, 2019), and tsunamis (Matsumoto et al, 2008). Following previous studies, we suggest that the structures form as the result of granular shear instabilities formed at the flow-bed interface (Rowley et al, 2011;Farin et al, 2014;Pollock et al, 2019).…”

“…Structures similar to those produced at the flow-bed interface in our experiments occur in the deposits of pyroclastic currents (Rowley 2010;Rowley et al 2011;Douillet et al 2018;Pollock et al, 2019), tsunamis (Matsumoto et al 2008), and turbidity currents (Dasgupta 1998). These structures are termed many things in the literature including vortical reworking features (Rowley et al 2011), sheared wavelike structure (Roche et al 2013), "shark fin" structures (Douillet et al 2018), and sheared, recumbent, or truncated flame structures (e.g.…”

“…These experimental studies propose that the erosion and mixing occurs because of the formation of granular shear instabilities at the flow-bed interface (Rowley et al 2011;Farin et al 2014). Additionally, field studies on the deposits of pyroclastic currents reveal sedimentary structures that suggest syn-depositional mixing between the flow and its bed (Rowley 2010;Douillet et al 2018;Pollock et al 2019).…”

The supercritical question for pyroclastic dune bedforms: An overview

“…• Common co-existence of peculiar finer-scale structures (Fig. 3; Douillet et al, 2015Douillet et al, , 2018Douillet et al, , 2019, and references therein) such as:…”

“…Pen for scale is 14 cm long. (B) Tungurahua, backset ripples in sediment plates/lacquer peels (see Douillet et al, 2018Douillet et al, , 2019. For all images, flow orientation roughly from left to right; photograph (A) has been mirrored for consistency.…”