Hebridean Terrane
Encyclopedia
The Hebridean Terrane is one of the terrane
s 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
gneiss
es 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 Laurentia
n foreland during the Caledonian continental collision
.
and all of the Outer Hebrides
. Similar rocks are also thought to be present on Shetland and have been proved west and north of the Outer Hebrides by BGS
shallow boreholes and hydrocarbon exploration wells
. The full extent of this terrane to the west is obscured by the effects of Mesozoic
rift
ing.
gneisses, subject to a series of metamorphic and tectonic events, interrupted by the intrusion of a major dyke swarm.
s for the Scourian complex were mainly granitic plutonic rocks intruded in the interval 3.0–2.7 Ga. They were metamorphosed to granulite facies
and deformed towards the end of the Archaean. At the beginning of the Proterozoic, the Scourian gneisses were locally affected by deformation and retrogression to amphibolite facies in the Inverian event, which overlapped in time with the emplacement of the Scourie dykes.
, postdates the Scourian deformation and metamorphism and has therefore been used to identify the later Laxfordian event. Many of the dykes were intruded into hot country rock
.
sheets. This deformation was accompanied by retrogression to amphibolite facies and subsequently locally to greenschist facies.
The Torridonian is a sequence of Neoproterozoic sediments, mainly sandstones that rest unconformably
on an old land surface, with up to 300 m of relief locally. The Torridonian is divided into the older Stoer Group and the younger Sleat and Torridon Groups separated by an angular unconformity. Paleomagnetic data
suggest that this unconformity represents a major time break. These sediments are interpreted to have been deposited during a period of rift
ing.
, near Assynt
, Sutherland
. A basal breccia
is present in many areas with large clasts derived form the underlying Lewisian. The breccia passes up into muddy sandstones, often with well-preserved desiccation structures, and into deposits of a braided river
system, trough cross-bedded sandstones and conglomerates
. A thin sequence of siltstones and fine sandstones alternate with muddy sandstones, suggesting deposition in a lacustrine
environment. The uppermost part of the sequence consists of trough cross-bedded sandstones thought to have been deposited by braided rivers.
peninsula on Skye
, underlies the Torridon Group conformably, but the relationship with the Stoer Group is nowhere exposed. It is presumed to have been in deposited later that 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 shale
s, probably deposited in a lacustrine environment.
s building out into ephemeral lakes. The upper parts of the group consist of an overall fining-upward sequence of sandstones, interpreted to have been deposited in a bajada
environment.
Eriboll Group overlain by the Durness Group of Cambrian to lower Ordovician
age.
overlain by the heavily bioturbated
Pipe Rock member with its distinctive skolithos
trace fossil
s. The An-t-Sron Formation consists of calcareous siltstones of the Fucoid Beds and overlying sandstone of the Salterella Grit.
The Durness group lies conformably on the Eriboll Group. It consists of several hundred metres of mainly dolostone
s with some limestone
s and chert
s, although the effects of later faulting make precise estimates of thickness difficult. It is subdivided into seven formations.
s to the Scourian gneisses at about 3.0–2.7 Ga. Blocks within the Lewisian complex were then juxtaposed by Inverian and Laxfordian deformation from about 2.4–1.7 Ga. At about 1200 Ma, the terrane was affected by extensional tectonics
causing rifting and the deposition of the thick coarse clastic sequence of the Torridonian. This was interrupted by a tilting event of unknown origin that caused a hiatus of about 200 Ma, followed by further deposition. At the start of the Cambrian, there was a marine transgression
and deposition of shallow marine sandstone and carbonates continuing into the Ordovician. In the Silurian
the terrane became involved in the Caledonian continental collision, with the Highland Terrane being thrust
over this segment of the Laurentian foreland.
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
s that form part of the Caledonian orogenic belt
Caledonian orogeny
The Caledonian orogeny is a mountain building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly...
in northwest Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt
Moine Thrust Belt
The Moine Thrust Belt is a linear geological feature in the Scottish Highlands which runs from Loch Eriboll on the north coast 190 km south-west to the Sleat peninsula on the Isle of Skye...
. The basement is formed by Archaean and Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es of the Lewisian complex
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. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 Ga. They form the basement on which the...
, unconformably overlain by the Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
Torridonian
Torridonian
In geology, Torridonian describes a series of Neoproterozoic arenaceous sedimentary rocks, extensively developed in the Northwest Highlands of Scotland, and particularly in the district of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks...
sediments, which in turn are unconformably overlain by a sequence of Cambro
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
–Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
sediments. It formed part of the Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
n foreland during the Caledonian continental collision
Continental collision
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together...
.
Extent
The Hebridean Terrane forms the westernmost strip of mainland Scotland, most of the Inner HebridesInner Hebrides
The Inner Hebrides is an archipelago off the west coast of Scotland, to the south east of the Outer Hebrides. Together these two island chains form the Hebrides, which enjoy a mild oceanic climate. There are 36 inhabited islands and a further 43 uninhabited Inner Hebrides with an area greater than...
and all of the Outer Hebrides
Outer Hebrides
The Outer Hebrides also known as the Western Isles and the Long Island, is an island chain off the west coast of Scotland. The islands are geographically contiguous with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland...
. Similar rocks are also thought to be present on Shetland and have been proved west and north of the Outer Hebrides by BGS
British Geological Survey
The British Geological Survey is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS headquarters are in Keyworth, Nottinghamshire, but other centres...
shallow boreholes and hydrocarbon exploration wells
Oil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
. The full extent of this terrane to the west is obscured by the effects of Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
ing.
Lewisian complex
The Lewisian complex consists of mainly graniticGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
gneisses, subject to a series of metamorphic and tectonic events, interrupted by the intrusion of a major dyke swarm.
Scourian complex
The protolithProtolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
s for the Scourian complex were mainly granitic plutonic rocks intruded in the interval 3.0–2.7 Ga. They were metamorphosed to granulite facies
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...
and deformed towards the end of the Archaean. At the beginning of the Proterozoic, the Scourian gneisses were locally affected by deformation and retrogression to amphibolite facies in the Inverian event, which overlapped in time with the emplacement of the Scourie dykes.
Scourie dykes
This swarm of mainly dolerite dykesDike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
, postdates the Scourian deformation and metamorphism and has therefore been used to identify the later Laxfordian event. Many of the dykes were intruded into hot country rock
Country rock (geology)
Country rock is a geological term meaning the rock native to an area. It is similar and in many cases interchangeable with the terms basement and wall rocks....
.
Laxfordian event
This event is associated with the development of shear zones, in which the Scourie dykes occur as concordant amphiboliteAmphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
sheets. This deformation was accompanied by retrogression to amphibolite facies and subsequently locally to greenschist facies.
Torridonian sediments
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
on an old land surface, with up to 300 m of relief locally. The Torridonian is divided into the older Stoer Group and the younger Sleat and Torridon Groups separated by an angular unconformity. Paleomagnetic data
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
suggest that this unconformity represents a major time break. These sediments are interpreted to have been deposited during a period of rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
ing.
Stoer Group
The Stoer Group outcrops on the peninsula of StoerStoer
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....
, near Assynt
Assynt
Assynt is a civil parish in west Sutherland, Highland, Scotland – north of Ullapool.It is famous for its landscape and its remarkable mountains...
, Sutherland
Sutherland
Sutherland is a registration county, lieutenancy area and historic administrative county of Scotland. It is now within the Highland local government area. In Gaelic the area is referred to according to its traditional areas: Dùthaich 'IcAoidh , Asainte , and Cataibh...
. A basal breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
is present in many areas with large clasts derived form the underlying Lewisian. The breccia passes up into muddy sandstones, often with well-preserved desiccation structures, and into deposits of a braided river
Braided river
A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with high slope and/or large sediment load...
system, trough cross-bedded sandstones and conglomerates
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
. A thin sequence of siltstones and fine sandstones alternate with muddy sandstones, suggesting deposition in a lacustrine
Lacustrine
Lacustrine means "of a lake" or "relating to a lake".Specifically, it may refer to:*Lacustrine plain*Lacustrine delta-Fish:*Lacustrine goby , a type of small fish found in Philippine waters belonging to the Gobiidae family, known in Tagalog as dulong-See also:*Fluvial - of or relating a...
environment. The uppermost part of the sequence consists of trough cross-bedded sandstones thought to have been deposited by braided rivers.
Sleat Group
The Sleat Group, which outcrops on the SleatSleat
Sleat is a peninsula on the island of Skye in the Highland council area of Scotland, known as "the garden of Skye". It is the home of the clan MacDonald of Sleat...
peninsula on Skye
Skye
Skye or the Isle of Skye is the largest and most northerly island in the Inner Hebrides of Scotland. The island's peninsulas radiate out from a mountainous centre dominated by the Cuillin hills...
, underlies the Torridon Group conformably, but the relationship with the Stoer Group is nowhere exposed. It is presumed to have been in deposited later that 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
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
sandstones deposited in a fluvial environment with some less common grey shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s, probably deposited in a lacustrine environment.
Torridon Group
The Torridon Group infills an irregular land surface with up to 600 m of topography locally, cutting down through the previously deposited Stoer group sediments, resting in many areas directly on the Lewisian. The lowest part of this group consists of a basal breccia passing up into sandstones and subordinate shales deposited as alluvial fanAlluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
s building out into ephemeral lakes. The upper parts of the group consist of an overall fining-upward sequence of sandstones, interpreted to have been deposited in a bajada
Bajada
Bajada may refer to:*Bajada a festival common to the Canary islands* Bajada, a compound Alluvial fan* La Bajada, an escarpment of the Caja del Rio, New Mexico, USA* Clint Bajada , Maltese TV and radio personality...
environment.
Cambro-Ordovician sediments
This thin strip of Lower Paleozoic strata lie unconformably on both the Lewisian complex and the Torridonian. The sequence is divided into the lower CambrianCambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Eriboll Group overlain by the Durness Group of Cambrian to lower Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
age.
Eriboll Group
The ca. 200 m thick Eriboll Group consists of two formations, the basal Eriboll Formation and the overlying An-t-Sron Formation. The Eriboll Formation contains the Basal Quartzite member, with a basal conglomerateConglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
overlain by the heavily bioturbated
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
Pipe Rock member with its distinctive skolithos
Skolithos
Skolithos is a common trace fossil ichnogenus whose original form consisted of approximately vertical cylinders. One well-known occurrence of Cambrian trace fossils is the famous 'Pipe Rock' of northwest Scotland...
trace fossil
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
s. The An-t-Sron Formation consists of calcareous siltstones of the Fucoid Beds and overlying sandstone of the Salterella Grit.
Durness Group
Dolostone
Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. In old U.S.G.S. publications it was referred to as magnesian limestone. Most dolostone formed as a magnesium replacement of limestone or lime mud prior to lithification. It is...
s with some limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s and chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
s, although the effects of later faulting make precise estimates of thickness difficult. It is subdivided into seven formations.
History
The earliest event recorded in the terrane is the intrusion of the protolithProtolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
s to the Scourian gneisses at about 3.0–2.7 Ga. Blocks within the Lewisian complex were then juxtaposed by Inverian and Laxfordian deformation from about 2.4–1.7 Ga. At about 1200 Ma, the terrane was affected by extensional tectonics
Extensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
causing rifting and the deposition of the thick coarse clastic sequence of the Torridonian. This was interrupted by a tilting event of unknown origin that caused a hiatus of about 200 Ma, followed by further deposition. At the start of the Cambrian, there was a marine transgression
Transgression (geology)
A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water...
and deposition of shallow marine sandstone and carbonates continuing into the Ordovician. In the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
the terrane became involved in the Caledonian continental collision, with the Highland Terrane being thrust
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
over this segment of the Laurentian foreland.