The Stac Fada Member is a distinctive layer towards the top of the

Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic), ...

Bay of Stoer Formation, part of the

Stoer Group In geology, the term Torridonian is the informal name for the Torridonian Group, a series of Mesoproterozoic to Neoproterozoic arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. The s ...

(lowermost Torridonian Supergroup) in northwest

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...

. This rock unit is generally thick and is made of

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

that contains accretionary

lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range ...

and many dark green glassy fragments of

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks in ...

composition. Evidence for a

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...

impact in the area of

the Minch The Minch ( gd, An Cuan Sgitheanach, ', ', '), also called North Minch, is a strait in north-west Scotland, separating the north-west Highlands and the northern Inner Hebrides from Lewis and Harris in the Outer Hebrides. It was known as ("Sco ...

or near

Lairg Lairg ( gd, An Luirg, meaning "the shank/shin") is a village and parish in Sutherland, Scotland. It has a population of 891 and is at the south-eastern end of Loch Shin. Lairg is unusual in the northern Highlands in being a large settlement th ...

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 Assynt ( gd, Asainn or ) is a sparsely populated area in the south-west of Sutherland, lying north of Ullapool on the west coast of Scotland. Assynt is known for its landscape and its remarkable mountains, which have led to the area, along with ...

, Sutherland (at ).

Extent

The Stac Fada Member is exposed at a series of localities on or near the coast of

Wester Ross Wester Ross () is an area of the Northwest Highlands of Scotland in the council area of Highland. The area is loosely defined, and has never been used as a formal administrative region in its own right, but is generally regarded as lying to t ...

, extending for about from the western side of

Loch Ewe Loch Ewe ( gd, Loch Iùbh) is a sea loch in the region of Wester Ross in the Northwest Highlands of Scotland. The shores are inhabited by a traditionally Gàidhlig-speaking people living in or sustained by crofting villages, the most notab ...

in the south to the Stoer peninsula in the north.

Description

The Stac Fada Member is an

impactite Impactite is rock created or modified by one or more impacts of a meteorite. Impactites are considered metamorphic rock, because their source materials were modified by the heat and pressure of the impact. On Earth, impactites consist primarily o ...

consisting of a mixture of

suevite Suevite is a rock consisting partly of melted material, typically forming a breccia containing glass and crystal or lithic fragments, formed during an impact event. It forms part of a group of rock types and structures that are known as im ...

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 The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. These rocks are of Archaean and Pale ...

are locally common and samples have been taken to establish which

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its ow ...

within the Lewisian the samples came from based on their

geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing th ...

Interpretation

The unit was initially interpreted by the Geological Survey to be a

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** ...

with clasts derived from

dykes. Later interpretations invoked a volcanic origin for the unit, based on the presence of pieces of green devitrified glass, with

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

peperite A peperite is a type of volcaniclastic rock consisting of sedimentary rock that contains fragments of younger igneous material and is formed when magma comes into contact with wet sediments. The term was originally used to describe rocks from th ...

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

and

lahar A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley. Lahars are extr ...

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 An ejecta blanket is a generally symmetrical apron of ejecta that surrounds an impact crater; it is layered thickly at the crater's rim and thin to discontinuous at the blanket's outer edge. The impact cratering is one of the basic surface format ...

. Evidence for a meteorite impact close to

Ullapool Ullapool (; gd, Ulapul ) is a village and port located in Northern Scotland. Ullapool has a population of around 1,500 inhabitants. It is located around northwest of Inverness in Ross and Cromarty, Scottish Highlands. Despite its modest size, ...

was published by a combined team of scientists from the

University of Oxford , mottoeng = The Lord is my light , established = , endowment = £6.1 billion (including colleges) (2019) , budget = £2.145 billion (2019–20) , chancellor ...

and the

University of Aberdeen , mottoeng = The fear of the Lord is the beginning of wisdom , established = , type = Public research universityAncient university , endowment = £58.4 million (2021) , budget ...

, in March 2008. Additional evidence for an impact origin for the deposit comes from the identification of the mineral

reidite Reidite is a rare polymorph of ZrSiO4 created when zircon experiences high pressure and temperature. Reidite is denser than zircon and has the same crystal structure as scheelite. All natural occurrences of reidite are associated with meteorite ...

as lamellae in

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...

grains, indicating pressures of at least 30 GPa.

Authigenic Authigenesis is the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described as authigenic. Authigenic sedimentary minerals form during sedimentation by precipitation or rec ...

potassium feldspar Potassium feldspar refers to a number of minerals in the feldspar group, and containing potassium: *Orthoclase ( endmember formula K Al Si3 O8), an important tectosilicate mineral that forms igneous rock *Microcline, chemically the same as orthocla ...

, found in vesicular pipes associated with the degassing of the unit immediately after emplacement, have been dated using the

argon–argon dating Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer ...

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

, is currently presumed to either lie to the west under the

Minch The Minch ( gd, An Cuan Sgitheanach, ', ', '), also called North Minch, is a strait in north-west Scotland, separating the north-west Highlands and the northern Inner Hebrides from Lewis and Harris in the Outer Hebrides. It was known as ("S ...

, the waterway that separates the

Isle of Lewis The Isle of Lewis ( gd, Eilean Leòdhais) or simply Lewis ( gd, Leòdhas, ) is the northern part of Lewis and Harris, the largest island of the Western Isles or Outer Hebrides archipelago in Scotland. The two parts are frequently referred to a ...

in the

Outer Hebrides The Outer Hebrides () or Western Isles ( gd, Na h-Eileanan Siar or or ("islands of the strangers"); sco, Waster Isles), sometimes known as the Long Isle/Long Island ( gd, An t-Eilean Fada, links=no), is an island chain off the west coas ...

from the north-west Highlands of Scotland, or to be the cause of the

Lairg Gravity Low The Lairg gravity low is a possible impact crater in Scotland about 40 kilometres in diameter, with a centre near the town of Lairg in the Scottish Highlands. Its identity as an impact crater is suspected due to the impact deposits present in the ...

, beneath the Moine Thrust Belt to the east. It has been estimated that the crater is about across. The impact would have created a blast with the energy of 145,000

megatons TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. The is a unit of energy defined by that convention to be , which is the approximate energy released in the detonation of a ...

and the shock wave would have created winds of as far away as the site of modern

Aberdeen Aberdeen (; sco, Aiberdeen ; gd, Obar Dheathain ; la, Aberdonia) is a city in North East Scotland, and is the third most populous city in the country. Aberdeen is one of Scotland's 32 local government council areas (as Aberdeen City), a ...

. 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 fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s and folds and striae found locally at the base of the unit have been combined with measurements of the

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not ...

in the rocks using anisotropy of

magnetic susceptibility In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...

. 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.

Extent

Description

Interpretation

See also

References

Further reading