The Torridonian is the informal name given to a sequence of Mesoproterozoic to Neoproterozoic sedimentary rocks that outcrop in a strip along the northwestern coast of Scotland and some parts of the Inner Hebrides from the Isle of Mull in the southwest to Cape Wrath in the northeast. They lie unconformably on the Archaean to Paleoproterozoic basement rocks of the Lewisian complex and unconformably beneath the Cambrian to Lower Ordovician rocks of the Ardvreck Group.
The sequence was first mapped as a unit by John MacCulloch and was initially assumed to be part of the Old Red Sandstone. [1] The first name used specifically for this sequence was "Torridon Sandstone" introduced in 1866 by James Nicol. [2] By 1892 the term was shortened to "Torridonian" by the Geological Survey. [3] 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). [4] At about the same time Lower Cambrian fossils were found in the sequence above the unconformity, suggesting that the Torridonian was of Precambrian age. [1]
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. [1]
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 concretions in the Diabaig Formation (994±48 Ma and 951±120 Ma respectively). [5] [6]
Variations in thickness and lithology 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 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. [5] More recent work has suggested that although the Stoer and Sleat groups were probably deposited in a rift setting, the scale and continuity of the Torridon Group, particularly the Applecross and Aultbea Formations, is more consistent with a molasse type foreland basin setting possibly related to the Grenville Orogeny. [7]
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. [8]
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. [8]
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 190 kilometres (120 mi) south-west to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of thrust faults that branch off the Moine Thrust itself. Topographically, the belt marks a change from rugged, terraced mountains with steep sides sculptured from weathered igneous, sedimentary and metamorphic rocks in the west to an extensive landscape of rolling hills over a metamorphic rock base to the east. Mountains within the belt display complexly folded and faulted layers and the width of the main part of the zone varies up to 10 kilometres (6.2 mi), although it is significantly wider on Skye.
The Torridon Group is a series of Tonian arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. These strata of the are particularly well exposed in the district of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks by James Nicol. Stratigraphically, they lie unconformably on gneisses of the Lewisian complex and sandstones of the lithologically similar Mesoproterozoic Stoer Group and their outcrop extent is restricted to the Hebridean Terrane.
The Stac Fada Member is a distinctive layer towards the top of the Mesoproterozoic Bay of Stoer Formation, part of the Stoer Group in northwest Scotland. This rock unit is generally 10 to 15 metres thick and is made of sandstone that contains accretionary lapilli and many dark green glassy fragments of mafic composition.
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 Paleoproterozoic age, ranging from 3.0–1.7 billion years (Ga). They form the basement on which the Stoer Group, Wester Ross Supergroup and probably the Loch Ness Supergroup sediments were deposited. The Lewisian consists mainly of granitic gneisses with a minor amount of supracrustal rocks. Rocks of the Lewisian complex were caught up in the Caledonian orogeny, appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event.
The Hebridean Terrane is one of the terranes that form part of the Caledonian orogenic belt in northwest Scotland. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt. The basement is formed by Archaean and Paleoproterozoic gneisses of the Lewisian complex, unconformably overlain by the Neoproterozoic Torridonian sediments, which in turn are unconformably overlain by a sequence of Cambro–Ordovician sediments. It formed part of the Laurentian foreland during the Caledonian continental collision.
The Colonsay Group is an estimated 5,000 m thick sequence of mildly metamorphosed Neoproterozoic sedimentary rocks that outcrop on the islands of Colonsay, Islay and Oronsay and the surrounding seabed. They have been correlated with the Grampian Group, the oldest part of the Dalradian Supergroup.
The geology of the Isle of Skye in Scotland is highly varied and the island's landscape reflects changes in the underlying nature of the rocks. A wide range of rock types are exposed on the island, sedimentary, metamorphic and igneous, ranging in age from the Archaean through to the Quaternary.
The Aravalli Mountain Range is a northeast-southwest trending orogenic belt in the northwest part of India and is part of the Indian Shield that was formed from a series of cratonic collisions. The Aravalli Mountains consist of the Aravalli and Delhi fold belts, and are collectively known as the Aravalli-Delhi orogenic belt. The whole mountain range is about 700 km long. Unlike the much younger Himalayan section nearby, the Aravalli Mountains are believed much older and can be traced back to the Proterozoic Eon. They are arguably the oldest geological feature on Earth. The collision between the Bundelkhand craton and the Marwar craton is believed to be the primary mechanism for the development of the mountain range.
The Badenoch Group is a sequence of metamorphosed Tonian age sedimentary rocks that outcrop across the Central Highlands of Scotland, east of the Great Glen. This rock sequence has formerly been referred to as the Central Highland Migmatite Complex and the Central Highland Division.
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 conglomerates developed near the base It is subdivided into three formations. It lies unconformably on the underlying Archaean to Paleoproterozoic age gneisses of the Lewisian complex and is in turn unconformably overlain by the Neoproterozoic Torridon Group.
The Sleat Group, which outcrops on the Sleat peninsula on Skye, underlies the Torridon Group conformably, but the relationship with the Stoer Group is nowhere exposed. It is presumed to have been deposited later than the Stoer Group, but possibly in a separate sub-basin. It is metamorphosed to greenschist facies and sits within the Kishorn Nappe, part of the Caledonian thrust belt, making its exact relationship to the other outcrops difficult to assess. The sequence consists of mainly coarse feldspathic sandstones deposited in a fluvial environment with some less common grey shales, probably deposited in a lacustrine environment.
The Mazatzal orogeny was an orogenic event in what is now the Southwestern United States from 1650 to 1600 Mya in the Statherian Period of the Paleoproterozoic. Preserved in the rocks of New Mexico and Arizona, it is interpreted as the collision of the 1700-1600 Mya age Mazatzal island arc terrane with the proto-North American continent. This was the second in a series of orogenies within a long-lived convergent boundary along southern Laurentia that ended with the ca. 1200–1000 Mya Grenville orogeny during the final assembly of the supercontinent Rodinia, which ended an 800-million-year episode of convergent boundary tectonism.
The Picuris orogeny was an orogenic event in what is now the Southwestern United States from 1.43 to 1.3 billion years ago in the Calymmian Period of the Mesoproterozoic. The event is named for the Picuris Mountains in northern New Mexico and interpreted either as the suturing of the Granite-Rhyolite crustal province to the southern margin of the proto-North American continent Laurentia or as the final suturing of the Mazatzal crustal province onto Laurentia. According to the former hypothesis, this was the second in a series of orogenies within a long-lived convergent boundary along southern Laurentia that ended with the ca. 1200–1000 Mya Grenville orogeny during the final assembly of the supercontinent Rodinia, which ended an 800-million-year episode of convergent boundary tectonism.
The Loch Ness Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks in the Scottish Highlands. It is found everywhere in tectonic contact above the older Wester Ross Supergroup. It is thought to be unconformably overlain by the Cryogenian to Cambrian Dalradian Supergroup.
The Wester Ross Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks in the Scottish Highlands. It lies unconformably on medium to high-grade metamorphic rocks and associated igneous rocks of the Archaean and Paleoproterozoic age Lewisian complex or locally over the Mesoproterozoic sedimentary rocks of the Stoer Group. The contact between the Wester Ross Supergroup and the next youngest of the Neoproterozoic sequences in the Scottish Highlands, the Loch Ness Supergroup, is everywhere a tectonic one.
The Knoydartian Orogeny is a Tonian tectonic and metamorphic event, or group of events, that is recognised in the rocks of the Wester Ross and Loch Ness supergroups of the Scottish Highlands. It is dated to about 820–725 Ma, predating the deposition of the Cryogenian to Cambrian Dalradian Supergroup. It is named after Knoydart, one of the localities where the event was first recognised.
The Renlandian Orogeny is a Tonian tectonic and metamorphic event that is found in East Greenland, on Svalbard, on Ellesmere Island and in Scotland. It takes its name from Renland in East Greenland, where the event was first recognised.
The Sgurr Beag Thrust is an important tectonic structure within the Neoproterozoic metasedimentary sequences of the Scottish Highlands. The thrust, or similar structures correlated with it, form the boundary between rocks of the Glennfinnan Group and the underlying Morar Group. It divides the Wester Ross Supergroup from the Loch Ness Supergroup. The history of this structure remains poorly understood although it is thought to be at least partly of Caledonian age.
The Morar Group is a sequence of Tonian sedimentary rocks that have been subjected to a series of tectonic and metamorphic events since their deposition. Originally interpreted to be lowest (oldest) part of a "Moine Supergroup", this sequence now forms part of the Wester Ross Supergroup. They lie unconformably on Archean to Paleoproterozoic basement of the Lewisian complex. The contact with the overlying Glenfinnan Group of the Loch Ness Supergroup is everywhere a tectonic one, formed by the Sgurr Beag Thrust or related structures.
The Moinian or just the Moine, formerly the Moine Supergroup, is a sequence of Neoproterozoic metasediments that outcrop in the Northwest Highlands between the Moine Thrust Belt to the northwest and the Great Glen Fault to the southeast and one part of the Grampian Highlands to the southeast of the fault. It takes its name from A' Mhòine, a peat bog in northern Sutherland.