Impactoclastic density current emplacement of terrestrial meteorite-impact ejecta and the formation of dust pellets and accretionary lapilli: Evidence from Stac Fada, Scotland

Abstract

We present an impactoclastic density current model for terrestrial meteorite impact ejecta emplacement that is based on a hitherto-unreported threefold sequence of lithofacies in the recently identified Stac Fada impact ejecta outflow blanket in Scotland. Massive suevite (division A) grades seamlessly up into massive suevite containing whole and broken accretionary lapilli (division B), and this is overlain by a thin layer of clast-supported dust pellets (division C). The sequence closely resembles sequences in accretionary lapilli-bearing ignimbrites emplaced by pyroclastic density currents at volcanoes, and we deduce similar emplacement mechanisms. We propose that ≥99% of the thickness of the Stac Fada ejecta blanket as seen at outcrop was deposited from a single depletive (spatially decelerating), granular fluid–based density current that rapidly waxed and then waned. A previously unreported thin layer of pellets (division C) records direct fallout from a residual atmospheric dust plume. We propose that the impactoclastic density current was density stratified and comprised a ground-hugging component of granular fluid passing up into less concentrated, more turbulent levels and a buoyant, dilute upper component. Larger rock fragments rolled and saltated within the basal granular fluid. In the buoyant plume, dust agglomerated into pellets, and these dropped into lower, more vigorous levels of the density current, where successive concentric layers of fine dust accreted to them as they circulated and hardened. The resultant accretionary lapilli sedimented through the granular fluid into lower levels of the current and were deposited from its base. Dust pellets continued to form in the residual atmospheric dust plume after passage of the density current and fell out to form the thin layer (division C) of framework-supported pellets at the top of the ejecta blanket. The impactoclastic density current model may have applications to finer-grained parts of some continuous impact ejecta blankets elsewhere on Earth as well as on Mars and Venus, where features indicating flow processes are evident.