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 .

Abstract. Luminescence dating has become a pillar of the understanding of Pleistocene glacial advances in the northern foreland of the Swiss Alps. However, both quartz and feldspar from the region are equally challenging as dosimeters with anomalous fading and partial bleaching being some of the obstacles to overcome for the establishment of decisive chronologies. In this study, luminescence properties of coarse- and fine-grained quartz, feldspar, and polymineral fractions of eight samples from a palaeovalley, Rinikerfeld in northern Switzerland, are systematically assessed. Standard performance tests are conducted on all four fractions. Deconvolution of luminescence signals of the quartz fractions is implemented and shows the dominance of stable fast components. Reader-specific low preheat temperatures are investigated on the infrared stimulated luminescence (IRSL) signal of feldspar. Thermal stability of this signal is found for low preheats, and thermal quenching could be excluded for higher preheats. However, anomalous fading is observed in the feldspar and polymineral IRSL signals and two correction approaches are applied. For one approach, fading corrected coarse-grained feldspar ages are consistent with those derived from quartz. In general, coarse-grained quartz and feldspar, as well as the fine-grained polymineral fraction of one sample, are in chrono-stratigraphic agreement and present negligible evidence for partial bleaching. However, ages derived from fine-grained quartz are found to underestimate those of the coarse-grained quartz fractions. Hence, the impact of alpha efficiency and water content on the dose rate and thus the ages are assessed. A finite explanation for the observed discrepancies remains lacking, but this systematic investigation of different luminescence signals allows for the establishment of a chronology for the palaeovalley fill dating back to at least Marine Isotope Stage 6 (MIS 6).

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