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 The Stoer Group is a sequence of Mesoproterozoic sedimentary rocks that outcrops on the peninsula of Stoer, near Assynt, Sutherland. The dominant lithology is sandstone with breccias and conglomerate (geology), conglomerates developed near the bas ...

(lowermost Torridonian Supergroup) in northwest

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

. This rock unit is generally thick and is made of

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

that contains accretionary

lapilli Lapilli (: lapillus) 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'' is Latin for "little stones". By definition lapilli range from in dia ...

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 include ...

composition. The member records evidence for an

impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...

in the near vicinity (likely in

the Minch The Minch () is a strait in north-west Scotland that separates the mainland from Lewis and Harris in the Outer Hebrides. It was known as ("Scotland's firth") in Old Norse. The Minch's southern extension, which separates Skye from the midd ...

or near

Lairg Lairg (, 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 that is not on ...

), dating to around 1 billion years ago. The name comes from a small promontory to the west of the village of

Stoer Stoer () is a crofting township in the parish of Assynt, Sutherland, in the Highlands of Scotland and in the council area of Highland. It is located about five miles north of the village of Lochinver. Norman McLeod, a presbyterian ministe ...

, in

Assynt Assynt ( 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 neighbour ...

, 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 th ...

, extending for about from the western side of

Loch Ewe Loch Ewe () 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 notable of which, si ...

in the south to the

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 ...

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 impact ...

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 Enard Bay is a large remote tidal coastal embayment, located 10.5 miles northwest of Ullapool, in northwestern Ross and Cromarty, Scottish Highlands in the west coast of Scotland. The mouth of the bay is about 4.5 miles across running from the ...

. 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 ''Loch'' ( ) is a word meaning "lake" or " sea inlet" in Scottish and Irish Gaelic, subsequently borrowed into English. In Irish contexts, it often appears in the anglicized form "lough". A small loch is sometimes called a lochan. Lochs which ...

, 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, North Atlantic Craton. These rocks are of ...

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 d ...

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 the e ...

. Isotopic analysis suggests that the impactor was

chondritic A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form primiti ...

in nature, and that the surface rock that was hit during the impact included

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

material.

Interpretation

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

and

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

all being proposed. When

shocked quartz Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), the crystalline structure of quartz is deformed along planes inside the crystal. These pla ...

, a higher-than-expected concentration of

platinum-group metals The platinum-group metals (PGMs) are six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block (groups 8, 9, and 10, periods 5 and 6). The six platinum-group m ...

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 (; ) is a village and port located in the civil parish of Lochbroom in the county of Ross and Cromarty, Scottish Highlands. It is located around northwest of Inverness. According to the Scottish Government in 2016, the village had a ...

was published by a combined team of scientists from the

University of Oxford The University of Oxford is a collegiate university, collegiate research university in Oxford, England. There is evidence of teaching as early as 1096, making it the oldest university in the English-speaking world and the List of oldest un ...

and the

University of Aberdeen The University of Aberdeen (abbreviated ''Aberd.'' in List of post-nominal letters (United Kingdom), post-nominals; ) is a public university, public research university in Aberdeen, Scotland. It was founded in 1495 when William Elphinstone, Bis ...

, 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 th ...

grains, indicating pressures of at least 30

GPa Grading in education is the application of standardized measurements to evaluate different levels of student achievement in a course. Grades can be expressed as letters (usually A to F), as a range (for example, 1 to 6), percentages, or as num ...

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 Mineral, minerals form during or after sedimentation by Precipita ...

potassium feldspar Potassium feldspar refers to a number of minerals in the feldspar group that contain large amounts of potassium in the crystal lattice. *Orthoclase (endmember formula K Al Si3 O8), an important tectosilicate mineral that forms igneous rock *Microcl ...

, 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 Potassium-argon dating, potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measur ...

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. However, a later analysis of shocked zircon crystals using

uranium–lead dating Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routi ...

found the age to be somewhat younger, around 990 ± 22 million years ago. The crater, preserved under sedimentary layers of

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

Minch The Minch () is a strait in north-west Scotland that separates the mainland from Lewis and Harris in the Outer Hebrides. It was known as ("Scotland's firth") in Old Norse. The Minch's southern extension, which separates Skye from the middle ...

, the waterway that separates the

Isle of Lewis The Isle of Lewis () or simply Lewis () 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 as if they were separate islands. The t ...

in the

Outer Hebrides The Outer Hebrides ( ) or Western Isles ( , or ), sometimes known as the Long Isle or Long Island (), is an Archipelago, island chain off the west coast of mainland Scotland. It is the longest archipelago in the British Isles. The islan ...

from the north-west Highlands of Scotland, or to be the cause of the Lairg Gravity Low, beneath the

Moine Thrust Belt The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast southwest to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of thr ...

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. A ton of TNT equivalent is a unit of energy defined by convention to be (). It is the approximate energy released in the det ...

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

Aberdeen Aberdeen ( ; ; ) is a port city in North East Scotland, and is the List of towns and cities in Scotland by population, third most populous Cities of Scotland, Scottish city. Historically, Aberdeen was within the historic county of Aberdeensh ...

. 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. I ...

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, and different types of fabric. At first, the word "textiles" only referred to woven fabrics. However, weaving is no ...

in the rocks using anisotropy of

magnetic susceptibility In electromagnetism, the magnetic susceptibility (; denoted , chi) 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 applied magnet ...

. 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