The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry

Abstract

The Stac Fada Member of the Mesoproterozoic Stoer Group (Torridon Supergroup) in NW Scotland is a proximal ejecta blanket surrounding an unidentified asteroid impact crater. A combination of field observations of the ejecta deposit and underlying strata, the geographical distribution of terrane-identified basement clasts found embedded in the impactite, and anisotropy of magnetic susceptibility of the impact melt rocks at different locations, can constrain the crater location to be about 15-20 km WNW of Enard Bay and thus buried under Mesozoic sediments in The Minch. Syn-compressional structures within the suevite at Stattic Point give a clear indication of a south-easterly direction of mass motion. The signatures of two different terranes within the Lewisian gneiss help identify the origin of clasts found in the impactite at three locations. These clasts are un-shocked and interpreted as having been swept up by the density current post-impact; their geographic distribution provides an important clue to ejecta emplacement pathways crossing the Assynt and Gruinard terranes. Anisotropy of magnetic susceptibility is used to measure flow direction in pyroclastic density current deposits and is applied here to derive a direction of motion for the impactoclastic density current. It provides good agreement with the other independent methods.