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.The Mesoproterozoic Stac Fada Member (SFM) of the Stoer Group of NW Scotland was first described as a basal conglomerate containing clasts of mafic dyke material (Peach et al., 1907). The abundant mafic material was subsequently recognized as comprising dark green, vesicular, devitrified glass fragments, and a volcaniclastic origin for the deposit was proposed: as an ash or pyroclastic flow (Lawson, 1972); as a volcanic peperite formed by a phreatomagmatic eruption of basic magma in contact with groundwater or shallow lakes (Sanders & Johnson, 1989); as an airfall tuff (Young, 1999(Young, , 2002, or; as volcanic mudflow (Stewart, 2002). However, Stewart (2002) also recognized that none of the proposed origins adequately explain all the observations. The identification of quartz grains with multiple sets of planar deformation features, platinum group elements in higher than normal crustal abundances and a non-terrestrial chromium isotope anomaly led to the re-interpretation of the Stac Fada Member as a proximal impact ejecta blanket beyond the margins of an as yet undiscovered meteorite impact crater (Amor et al., 2008). Consequently many of the depositional, petrographic, and geochemical features of this unit and adjacent strata took on a new significance. Although no source impact crater (or related crustal structure) has yet been identified on the Scottish mainland, the possibility remains that it has been obscured by thrust sheets of the Moine thrust belt (~430 Ma) that formed during the Caledonian Orogeny in the course of the closure of the Iapetus Ocean. Recently, Simms (2015) suggested that the

A bigger splat: The catastrophic geology of a 1.2-b.y.-old terrestrial megaclast, northwest Scotland

“…Alternatively, it may have occurred due to freeze-thaw processes if the earliest period of Stoer Group deposition was influenced by glaciation, as suggested by Davison and Hambrey (1996). Since the CM demonstrably formed during early deposition of the Clachtoll Formation, it is clearly unrelated to the Stac Fada impactite event (e.g., Simms, 2015;Amor et al, 2019), which lies 300 m higher in the stratigraphic succession.…”

The Mesoproterozoic Stac Fada Member, NW Scotland: an impact origin confirmed but refined

“…These conclusions were based on the presence of shocked quartz grains (and biotite with kink bands, which are not, however, diagnostic of shock; French & Koeberl, 2010), chromium isotopes and elevated platinum group metal and siderophile element abundances. More recently, these authors referred to the Stac Fada Member as an impact melt rock (Amor et al 2019). The discovery of reidite (ZrSiO4) provided further evidence of an impact origin (Reddy et al 2015).…”

“…The location of the source crater remains unresolved: one suggestion is The Minch (Fig. 1), northwest of the outcrop belt (Amor et al 2019), and the other is the Lairg Gravity Low (Fig. 1) (Simms 2015;Simms & Ernstson 2019), which, if correct, has significant implications for the deep crustal structure of northern Scotland (Butler & Alsop 2019;Simms 2019;Simms & Ernstson 2019).…”

Discussion on ‘A reassessment of the proposed ‘Lairg impact structure’ and its potential implications for the deep structure of northern Scotland’ Journal of the Geological Society, London, 176, 817-829

“…Indeed, other studies argue that the Stac Fada rocks were derived from a broadly NW direction (e.g. Amor et al 2019). So what is the geological explanation of the negative Bouguer gravity anomaly in central Sutherland?…”

“…If these requirements are accepted, then Lewisian rocks preserved in thrust sheets in the Moine Thrust Belt of the Assynt district have been derived directly from the proposed impact site. As Amor et al (2019) note, they contain no such evidence. Simms & Ernstson (2019) suggest that structural interpretation in the Moine Thrust Belt is unimportant for their deliberations.…”