The Lairg gravity low is a possible

impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

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 ...

about 40 kilometres in diameter, with a centre near the town of

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 ...

in the

Scottish Highlands The Highlands (; , ) is a historical region of Scotland. Culturally, the Highlands and the Scottish Lowlands, Lowlands diverged from the Late Middle Ages into the modern period, when Scots language, Lowland Scots language replaced Scottish Gae ...

. Its identity as an impact crater is suspected due to the impact deposits present in the

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 thick and is made of sands ...

50 km to the west. However, this has been disputed, with other studies suggesting that an impact 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 ...

is more likely.

Description

Relative to the residual gravity field, at the centre it is 23 mGal lower than the surrounding terrain. The

gravity anomaly The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity meas ...

is approximately 40 km across, and somewhat irregularly shaped towards the edges. It is quite similar to that of the Ries impact crater. A detailed analysis of the gravity data found that the anomaly has no central uplift as would be expected of a crater of this size, though it possessed anomalies that was suggested represented a

peak ring A peak ring crater is a type of complex crater, which is different from a multi-ringed basin or central-peak crater. A central peak is not seen; instead, a roughly circular ring or plateau, possibly discontinuous, surrounds the Impact crater, crat ...

. The authors proposed that the anomaly was an eroded remnant of a larger (~ 100 km diameter) crater, which would explain the anomalies. The authors also suggested that the impact originally took place further east, but had been moved westward by subsequent tectonic movements.

Correlation to the Stac Fada Member

It is suspected that the Lairg site is the source of the ejecta deposits in the Stac Fada Member for a number of reasons. The

palaeocurrent A paleocurrent or paleocurrent indicator is a geological feature (typically a sedimentary structure) that helps one determine the direction of flowing water in the geologic past. This is an invaluable tool in the reconstruction of ancient depositi ...

direction data across the N-S extent of the member changes and always points away from the site of the gravity low. The upper levels of target sediment were unconsolidated sand and basement felsic

gneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...

, according to the lithic fragments in the member. However a 2019 study disputed this based on the fact that the matrix of the Stac Fada Member had been derived from local

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 ...

sediments and the regional variation in the anisotropy of magnetic susceptibility, direction of compression and striations from the Stac Fada Member suggested that an impact somewhere in The Minch was more probable.

References

Geology of Scotland Mesoproterozoic Sutherland Possible impact craters on Earth {{scotland-stub