GROWTH OF PELLETS AND ACCRETIONARY LAPILLI DURING THE EMPLACEMENT OF VOLCANIC AND METEORITE IMPACT EJECTA BLANKETS BY CATASTROPHIC DENSITY CURRENTS

Aggregation of airborne dust particles is an important mechanism by which the atmosphere cleanses itself after catastrophic dust-forming events. Evidence for how and where this happens is preserved in deposits of both volcanic and meteorite impact ejecta. We distinguish three aggregate-bearing facies (ABF1-3) in the geological record and propose distinct mechanisms of formation within catastrophic atmospheric density currents and associated dust plumes. The key to their origin lies on Tenerife where the origin of individual deposit layers can be deduced from exceptionally-exposed field relations. ABF1 (laterally consistent layers of clast-supported accretionary lapilli) forms by direct fallout from vigorous volcanic plumes and is not documented in impact ejecta blankets. ABF2 (laterally impersistent layers of matrix-supported whole and broken accretionary lapilli with well-developed concentric lamination) was emplaced by ground-hugging density currents, and is characteristically overlain by ABF3 (thin laterally consistent layers of framework-supported pellets) that records direct fallout from an associated atmospheric dust plume. An extensive, well-exposed meteorite impact ejecta blanket, the Stac Fada of Scotland, displays a remarkably similar sequence, with a single ABF2-ABF3 couplet and is similarly inferred to record emplacement predominantly as a vast, ground-hugging density current with a basal granular-fluid. The impact-generated density current was depletive, and deposited progressively as it waxed and waned, generated a phoenix plume, then dissipated and was followed by pellet fallout. Distinction of aggregate-bearing facies in other volcanic and meteorite impact ejecta deposits should help reconstruct emplacement: it can inform hazard assessment at explosive volcanoes and dynamic models of meteorite impact ejecta emplacement.