GCR block - Caledonian Structures of Wales (CAL-STR-WL)

Caledonian Structures of Wales

Block Description

UK map showing distribution of GCR sites of GCR block CAL-STR-WL
Distribution of GCR sites of GCR block Caledonian Structures of Wales

The GCR sites selected for this GCR Block represent a major episode of Earth history that occurred as a result of the ‘Caledonian Orogeny’, lasting from about 500 million years ago (Ma) to around 360 Ma, which was a period of mountain building and continental collision that began in the Ordovician Period (which lasted from 495 to 440 Ma) and continued through Silurian (440–417 Ma) and Devonian(417–354 Ma) time, as represented by geological features present in Wales; geological structures south of the Southern Upland Fault produced elsewhere by the same events are covered by other Blocks: the Caledonian Structures of the Southern Uplands and Caledonian Structures of Lake District, for both geographical and geological reasons. See the GCR Blocks CAL-STR-SU and CAL-STR-LD.Igneous rocks that formed as a result of the Caledonian Orogeny are encompassed by the CAL-IGN GCR Block

There is at present no agreed definition of the term ‘Caledonian’, but is taken here to include all of the convergent tectonic and magmatic events arising from the closure of the ‘proto-Atlantic’ Iapetus Ocean in which many of the rocks of Late Proterozoic and early Palaeozoic age had been deposited. It therefore encompasses subduction beneath the continental margins; the accretion or obduction of oceanic crust and island-arc material onto these margins; and ultimate collision of the continents, uplift and development of extensional molasse basins. Within this broad orogenic framework many separate ‘events’ are identified, such as the ‘Grampian Event’ and the ‘Acadian Event’.

For details of stratigraphy related to the Caledonian Orogeny see GCR Blocks for Ordovician, Silurian and Devonian Stratigraphy: ARE-LLV; ARE-TRE; CAR-ASH; LDO; LDY; LUD; MAR-DEV; NMAR-DEV).

Tectonic setting and evolution

The Iapetus Ocean was created in Late Proterozoic time by the rifting and pulling apart of a large supercontinent known as ‘Rodinia’. The opening started sometime around 650 million years (Ma) ago and by the beginning of Ordovician time, at 510 Ma, the ocean was at its widest development of possibly up to 5000 km across. On one side of the ocean lay the supercontinent of Laurentia, which is represented today largely by the Precambrian basement rocks of North America, Greenland, the north of Ireland and the Scottish Highlands. On the opposite side lay the supercontinent of Gondwana, consisting of the basements of South America, Africa, India, Australia, East Antarctica and Western Europe (including south Ireland, England and Wales). A separate continent, ‘Baltica’ (the basement of Scandinavia and Russia), was separated from Gondwana by an arm of the Iapetus Ocean, known as the ‘Tornquist Sea’. The wide separation is supported by palaeontological data, which shows distinctly different faunal assemblages in the Lower Palaeozoic rocks of each continent and by palaeomagnetic interpretations.

The continental plates of Laurentia, Gondwana and Baltica started to converge during the early part of the Ordovician Period, initiating new tectonic and magmatic processes which marked the start of the Caledonian Orogeny. The Iapetus oceanic crust was consumed in subduction zones beneath oceanic island arcs and beneath the continental margins. Magma was being created by the melting of mantle and oceanic crustal material within and above the subduction zones and by melting within the thickened continental crust.

The generally accepted sequence and timing of events as the three plates converged is as follows.

• Closure of the Tornquist Sea between Eastern Avalonia and Baltica, followed by strike-slip movement along the Tornquist Suture.

• Anticlockwise rotation of Baltica, followed by convergence with Laurentia, with subduction beneath the ‘Scottish’ sector of the Laurentian margin and closure during early to mid-Silurian times.

• Oblique convergence of Eastern Avalonia with Laurentia, with subduction beneath the Laurentian margin, resulting in closure by the early Silurian in the ‘Irish’ sector and later, mid-Silurian closure in the ‘Scottish’ sector. The junction between the two fused plates passes through the Solway Firth in Britain and is known as the Iapetus Suture.

• Protracted continent–continent collision between Laurentia and Eastern Avalonia plus Baltica, with underthrusting beneath part of the Laurentian margin (?mid-Silurian to Mid-Devonian).

• Separation of a further microcontinent, Armorica, from the margin of Gondwana, which then collided with Eastern Avalonia during Early Devonian times (the Acadian Event).

• Sinistral re-alignment of terrane boundaries (?mid-Silurian to Mid-Devonian).

The Caledonian Orogenic Belt

The Caledonian–Appalachian Orogen can be traced (pre-Atlantic drift), for some 7500 km south-west to north-east, from south-eastern USA through the British Isles to Scandinavia, Greenland, and Ny Friesland. It is generally accepted that sedimentation and igneous activity took place at, or near, the margins of an ocean (the Iapetus) that separated the Laurentian and Gondwanaland plates, over a period from the Precambrian through the early Palaeozoic. From studies of fauna, sedimentary history, igneous activity, structural and metamorphic evolution, and palaeomagnetism on its two sides, it is considered that deformation of sediments and volcanic rocks, resulting from the episodic closure of the Iapetus Ocean, took place through the early Palaeozoic to culminate in continental collision during the early Devonian.

The British Caledonides–Wales

The Welsh Basin is now interpreted as a back-arc extensional basin within continental crust. Its original relationship to the Lake District (and the Irish equivalents) is not clear. Between the two, lies the small, isolated area of the Precambrian rocks of Anglesey. The boundary of the Anglesey terrane with the Lake District terrane is not exposed. Its south-eastern boundary, with the Welsh terrane, which has previously been interpreted as a subduction zone active in early Palaeozoic times, is now seen to be a fault boundary marking Late Precambrian strike-slip docking of the small terranes that now make up Anglesey. The significant deformation related to folds and cleavage in Wales represents, as in the Lake District, essentially an early Devonian event. There are, however, many variations on a simple pattern, attributed, variously, to soft-sediment, tectonic, and volcanic activity.


Volume Introduction

Caledonian Structures of Wales


Site List

Your selection found 19 GCR sites. Sites are sorted alphabetically by country, local authority and then by site name -
CodeNameCountryLocal AuthorityGrid RefGCR Site Account
2723Cae'r Hafod QuarryWalesCONWY AND DENBIGHSHIRESJ215476Not available
2893Cwm IdwalWalesCONWY AND DENBIGHSHIRESH645585Not available
2559Dyffryd Mymbyr (Capel Curig Volcanics)WalesCONWY AND DENBIGHSHIRESH696573Not available
2722Alexandra QuarryWalesGWYNEDDSH517560Not available
2561Tan-y-GrisiauWalesGWYNEDDSH683453Not available
2313Traeth PenbrynWalesGWYNEDDSN289521Not available
2560Trum y DdysglWalesGWYNEDDSH544518Not available
2720Carmel HeadWalesISLE OF ANGLESEYSH308930Not available
2557Lligwy Bay (Traeth Lligwy)WalesISLE OF ANGLESEYSH494878Not available
2721Ogof GynforWalesISLE OF ANGLESEYSH378948Not available
2558RhosneigrWalesISLE OF ANGLESEYSH317734Not available
2556Dolyhir Quarries, Old RadnorWalesPOWYSSO245581Not available
2572Llanelwedd Bay QuarryWalesPOWYSSO051522Not available
2318Allt-WenWalesSOUTH WEST WALESSN575793Not available
2317Craig y Fulfran (Cormorant Rock)WalesSOUTH WEST WALESSN583830Not available
2314Cwm Rheidol Track SectionWalesSOUTH WEST WALESSN700795Not available
2315Cwm TuduWalesSOUTH WEST WALESSN359579Not available
2316North ClarachWalesSOUTH WEST WALESSN585843Not available
2312Ponterwyd QuarryWalesSOUTH WEST WALESSN740808Not available
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