TheHebridean terrane is one of theterranes that form part of theCaledonian orogenic belt in northwestScotland. Its boundary with the neighbouringNorthern Highland terrane is formed by theMoine Thrust Belt. The basement is formed byArchaean andPaleoproterozoicgneisses of theLewisian complex, unconformably overlain by theNeoproterozoicTorridonian sediments, which in turn are unconformably overlain by a sequence ofCambro–Ordovician sediments.[1] It formed part of theLaurentian foreland during the Caledoniancontinental collision.
The Hebridean terrane forms the westernmost strip of mainland Scotland, most of theInner Hebrides and all of theOuter Hebrides. Similar rocks are also thought to be present inShetland and have been proved west and north of the Outer Hebrides byBGS shallow boreholes andhydrocarbon explorationwells. The full extent of this terrane to the west is obscured by the effects ofMesozoicrifting.[1]
The Lewisian complex consists of mainlygranitic gneisses, subject to a series of metamorphic and tectonic events, interrupted by the intrusion of a majordyke swarm.[1]
Theprotoliths for the Scourian complex were mainlygranitic plutonic rocks intruded in the interval 3.0–2.7Ga. They were metamorphosed togranulite 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 toamphibolite facies in the Inverian event, which overlapped in time with the emplacement of the Scourie dykes.[1]
This swarm of mainlydoleritedykes, postdates the Scourian deformation and metamorphism and has therefore been used to identify the later Laxfordian event.[2] Many of the dykes were intruded into hotcountry rock.[1]
This event is associated with the development ofshear zones, in which the Scourie dykes occur as concordantamphibolite sheets. This deformation was accompanied by retrogression to amphibolite facies and subsequently locally togreenschist facies.[1]
The Torridonian is a sequence of Neoproterozoic sediments, mainly sandstones that restunconformably 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[example needed] suggest that this unconformity represents a major time break.[further explanation needed] These sediments are interpreted to have been deposited during a period ofrifting.[1]
The Stoer Group outcrops on the peninsula ofStoer, nearAssynt,Sutherland. A basalbreccia is present in many areas with large clasts derived from the underlying Lewisian. The breccia passes up into muddy sandstones, often with well-preserved desiccation structures, and into deposits of abraided river system,trough cross-bedded sandstones andconglomerates. A thin sequence of siltstones and fine sandstones alternate with muddy sandstones, suggesting deposition in alacustrine environment. The uppermost part of the sequence consists of trough cross-bedded sandstones thought to have been deposited by braided rivers.[1]
The Sleat Group, which outcrops on theSleat peninsula onSkye, underlies the Torridon Group conformably, but the relationship with the Stoer Group is nowhere exposed. It is presumed[according to whom?] 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.[1] The sequence consists of mainly coarse-grainedfeldspathic sandstones deposited in a fluvial environment with some less common greyshales, probably deposited in a lacustrine environment.
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 asalluvial fans 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 abajada environment.[1]
This thin strip of Lower Paleozoic strata lie unconformably on both the Lewisian complex and the Torridonian. The sequence is divided into the lowerCambrianArdvreck Group overlain by theDurness Group of Cambrian to lowerOrdovician age.[1]
The ca. 200 m thick lower Cambrian Ardvreck Group consists of two formations, the basalEriboll Formation and the overlyingAn-t-Sron Formation. The Eriboll Formation contains the Basal Quartzite member, with a basalconglomerate overlain by the heavilybioturbated Pipe Rock member with its distinctiveskolithostrace fossils.[3] The An-t-Sron Formation consists of calcareous siltstones of the Fucoid Beds[4] and overlying sandstone of the Salterella Grit.[5]
The Durness group lies conformably on the Ardvreck Group. It consists of several hundred metres of mainlydolomites with somelimestones andcherts, although the effects of later faulting make precise estimates of thickness difficult.[1] It is subdivided into seven formations.
The earliest event recorded in the terrane is the intrusion of theprotoliths 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 byextensional 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 marinetransgression and deposition of shallow marine sandstone and carbonates continuing into the Ordovician. In theSilurian the terrane became involved in the Caledonian continental collision, with the Highland Terrane beingthrust over this segment of the Laurentian foreland.[1]