The Torridonian is the informal name given to a sequence of

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 ...

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s that outcrop in a strip along the northwestern coast of Scotland and some parts of the

Inner Hebrides The Inner Hebrides ( ; ) is an archipelago off the west coast of mainland Scotland, to the south east of the Outer Hebrides. Together these two island chains form the Hebrides, which experience a mild oceanic climate. The Inner Hebrides compri ...

from the

Isle of Mull The Isle of Mull or simply Mull ( ) is the second-largest island of the Inner Hebrides (after Skye) and lies off the west coast of Scotland in the Council areas of Scotland, council area of Argyll and Bute. Covering , Mull is the fourth-lar ...

in the southwest to

Cape Wrath Cape Wrath (, known as ' in Lewis) is a cape in the Durness parish of the county of Sutherland in the Highlands of Scotland. It is the most north-westerly point in Great Britain. The cape is separated from the rest of the mainland by the Ky ...

in the northeast. They lie unconformably on the Archaean to

Paleoproterozoic The Paleoproterozoic Era (also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the Proterozoic eon, and also the longest era of the Earth's geological history, spanning from (2.5–1.6 Ga). It is further sub ...

basement A basement is any Storey, floor of a building that is not above the grade plane. Especially in residential buildings, it often is used as a utility space for a building, where such items as the Furnace (house heating), furnace, water heating, ...

rocks of 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 ...

and unconformably beneath the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to Lower Ordovician rocks of the Ardvreck Group.

History of research

The sequence was first mapped as a unit by John MacCulloch and was initially assumed to be part of the

Old Red Sandstone Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It ...

. The first name used specifically for this sequence was "Torridon Sandstone" introduced in 1866 by James Nicol. By 1892 the term was shortened to "Torridonian" by the Geological Survey. In 1893 the survey had subdivided the Torridonian into four units, the Diabaig, Applecross, Aultbea and Cailleach Head groups (which are now the names of formations within the Torridon Group). At about the same time Lower Cambrian fossils were found in the sequence above the unconformity, suggesting that the Torridonian was of Precambrian age. Little further work was carried out on the Torridonian until the 1950s when Edward Irving and Keith Runcorn sampled the sequence and determined paleomagnetic pole directions, observing a major change between samples from part of the Diabaig Group (as then understood, now known to be part of the Stoer Group) and the overlying Torridon Group. In 1969 Sandy Stewart subdivided the Torridonian into the groups that are in current use. He had already recognised the existence of a major angular unconformity between the Stoer Group and the Torridon Group. The age of the main part of the Torridonian and of the older Stoer Group is constrained by the youngest ages from the Lewisian complex (~1100 Ma) and the age of the oldest fossils in the Ardvreck Group (~544 Ma). Direct dating of the Torridonian is restricted to: Pb-Pb dating of a limestone in the Stoer Group (1199±70 Ma) and Ar-Ar dating of the Stac Fada Member ejecta blanket deposit at a slightly lower stratigraphic level (1177±5 Ma); Rb-Sr and Pb-Pb dating of

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

concretion A concretion is a hard and compact mass formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular shapes a ...

s in the Diabaig Formation (994±48 Ma and 951±120 Ma respectively). Variations in thickness and

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

were interpreted to mean that both the Stoer and Sleat/Torridon Groups were deposited in a rift setting. Evidence from seismic reflection data 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 middle ...

suggested that the Minch Fault was active throughout the deposition of the Torridon Group. This is consistent with the generally westerly derived pebbly material throughout the thickness of the Applecross Formation, suggesting a constantly rejuvenated sediment source in that direction. More recent work has suggested that although the Stoer and Sleat groups were probably deposited in a

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben ...

setting, the scale and continuity of the Torridon Group, particularly the Applecross and Aultbea Formations, is more consistent with a

molasse __NOTOC__ In geology, "molasse" () are sandstones, shales and conglomerates that form as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse deposits accumulate in a foreland basin, especially on top of flys ...

type

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

setting possibly related to the

Grenville Orogeny The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, ...

Revised stratigraphy

Going back to the late 19th century, geologists had speculated that at least the lower part of the Moine sequence might be a lateral equivalent of the Torridonian, based on similarities in lithology, sedimentary structures and thickness. The application of detrital zircon geochronology to the Torridonian and Moine sequences has allowed this proposed correlation to be tested. The Morar Group] of the Moine has a very similar spectrum of detrital zircon ages to the Torridon Group. Both have a population of youngest zircons that give ages that match the Grenville Orogeny. In contrast, the Glenfinnan Group has a youngest zircon population that matches the Renlandian Orogeny, an event that is now known to have affected the Morar Group. The detrital zircon and other geochronological data show that there is major break in the Moine stratigraphy at the base of the Glenfinnan Group and that it can, therefore, no longer be regarded as a supergroup. In the revised stratigraphic framework for the Proterozoic sedimentary rocks of the Highlands: the Stoer Group is part of a separate sequence whose deposition predated the Grenville Orogeny and has no equivalents in Scotland; the Torridon Group (together with the Sleat Group, the Iona and Tarskavaig groups and three groups on Shetland) are correlated with the Morar Group forming the Wester Ross Supergroup; after the Renlandian Orogeny, the Glenfinnan, Loch Eil and Badenoch groups were deposited and together form the younger Loch Ness Supergroup.

References

{{Reflist Geology of Scotland