Stac Fada Member

The Stac Fada Member is a distinctive layer towards the top of the Mesoproterozoic Bay of Stoer Formation, part of the Stoer Group (lowermost Torridonian Supergroup) in northwest Scotland. This rock unit is generally 10 to 15 m thick and is made of sandstone that contains accretionary lapilli and many dark green glassy fragments of mafic composition.

Evidence for a meteorite impact in the area of the Minch or near Lairg has been published and refined in a series of studies from 2008 to 2019. The unit dates to approximately 1.2 billion years ago.

The name comes from a small promontory to the west of the village of Stoer, in Assynt, Sutherland (at 58.201°N, -5.349°W).

Extent
The Stac Fada Member is exposed at a series of localities on or near the coast of Wester Ross, extending for about 50 km from the western side of Loch Ewe in the south to the Stoer peninsula in the north.

Description
The Stac Fada Member is an impactite consisting of a mixture of suevite and clast poor impact melt rocks. At the Stoer peninsula, the basal part of the unit contains large clasts of sandstone within a matrix of melt rocks. At the same locality, lenses of accretionary lapilli are developed in the uppermost part of the unit, although these are better developed at Enard Bay. The thickness of the unit is quite variable, although it generally lies in the range 10–15 metres. At the southern end of Loch Ewe it is only 5–6 metres thick, while at Bac an Leth Choin, southwest of Loch Ewe across the Loch Maree Fault, it reaches more than 30 metres.

The unit contains clasts of melted material, although these are unevenly distributed through the sequence. Clasts derived from the Lewisian complex are locally common and samples have been taken to establish which terrane within the Lewisian the samples came from based on their geochemistry.

Interpretation
The unit was initially interpreted by the Geological Survey to be a conglomerate with clasts derived from mafic dykes. Later interpretations invoked a volcanic origin for the unit, based on the presence of pieces of green devitrified glass, with pyroclastic flow, peperite, tuff and lahar all being proposed. When shocked quartz, a higher-than-expected concentration of platinum-group metals and the presence of a non-terrestrial chromium isotope were all identified in the unit, it was reinterpreted as part of an impact ejecta blanket. Evidence for a meteorite impact close to Ullapool was published by a combined team of scientists from the University of Oxford and the University of Aberdeen, in March 2008. Additional evidence for an impact origin for the deposit comes from the identification of the mineral reidite as lamellae in zircon grains, indicating pressures of at least 30 GPa.

Authigenic potassium feldspar, found in vesicular pipes associated with the degassing of the unit immediately after emplacement, have been dated using the argon–argon dating method. Specimens from four localities give very similar results, indicating an age of 1177 ± 5 Ma (millions of years ago) for the formation of this deposit.

The crater, preserved under sedimentary layers of sandstone, is currently presumed to either lie to the west under the Minch, the waterway that separates the Isle of Lewis in the Outer Hebrides from the north-west Highlands of Scotland, or to be the cause of the Lairg Gravity Low, beneath the Moine Thrust Belt to the east. It has been estimated that the crater is about 10 km across. The impact would have created a blast with the energy of 145,000 megatons and the shock wave would have created winds of 420 km/h as far away as the site of modern Aberdeen.

In order to reduce the uncertainty in the location of the impact, the main exposures of the Stac Fada Member have been re-examined to look for evidence of the direction that the ejecta from the impact was moving as it was deposited. The orientation of small-scale thrust faults and folds and striae found locally at the base of the unit have been combined with measurements of the fabric in the rocks using anisotropy of magnetic susceptibility. These observations are most consistent with an impact location in the Minch, rather than at Lairg. The presence of large sandstone clasts in the outcrops on the Stoer peninsula suggests that this was the area most proximal to the impact. The Minch location is also consistent with the observed types of clasts of Lewisian gneiss found.