The

Wrekin The Wrekin is a hill in east Shropshire, England. It is located some five miles (8 km) west of Telford, on the border between the unitary authorities of Shropshire and Telford and Wrekin. Rising above the Shropshire Plain to a height of 4 ...

Terrane is one of five inferred fault bounded

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its ow ...

s that make up the basement rocks of the southern United Kingdom. The other notable geological terranes in the region are the

Charnwood Terrane Charnwood may refer to: County of Leicestershire, United Kingdom * Borough of Charnwood, a local government district in the county of Leicestershire, England ** Charnwood Borough Council elections * Charnwood (ward), an electoral ward and admini ...

Fenland Terrane Fenland may mean: * Fenland, or the Fens, an area of low-lying land in eastern England ** Fenland District, a local authority district in Isle of Ely, Cambridgeshire, England, forming part of the Fens ** Fenland Airfield, an airfield near Spalding, ...

Cymru Terrane The Cymru Terrane is one of five inferred fault bounded terranes that make up the basement rocks of the southern United Kingdom. The other notable geological terranes are the Charnwood Terrane, Fenland Terrane, Wrekin Terrane and the Monian Comp ...

and the Monian Composite Terrane.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed The Wrekin Terrane is bounded to the west by the

Welsh Borderland Fault System The Welsh Borderland Fault System is a zone of faulting and associated folding which runs northeastwards through Wales from Pembrokeshire through Carmarthenshire and Powys into Shropshire in England. It comprises the Tywi Lineament, Pontesford ...

and to the east by the

Malvern Lineament The Malvern Line or Malvern Lineament is the name applied to a north-south aligned lineament which runs through the Malvern Hills of western England and extends southwards towards Bristol and northwards past Stourport. It consists of a series of ...

. The geological terrane to the west is the Cymru Terrane and to the east is Charnwood Terrane. The majority of rocks in the area are associated with the outcrops that are evident at the faulted boundaries.J. N. Carney, J. M. Horak, et al., ''Precambrian Rocks of England and Wales'', ''Joint Nature Conservation Committee.'' Geological Conservation Review Series 20

Intrusive geology

The older rocks of the terrane rest in the

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of th ...

with younger

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...

rocks resting unconformably upon them. The Precambrian rocks are noted to be calc-alkaline plutonic intrusions dated to approximately 700-600 Ma (Phase 1 to Phase 2

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

). These have associated (younger) volcanics that dominate the overlying sedimentary rocks that date between 570 and 560 Ma. The signatures of the rocks are interpreted as having an intra-plate geochemical signature that may be attributed to arc-rifting. The diachronous formations of the bedded

Uriconian Uriconian rocks are volcanic rocks found in parts of Shropshire, United Kingdom. The name relates to '' Uriconio'', the Latin name for an Iron Age hillfort on the summit of the Wrekin, a hill formed of Uriconian rock. The Uriconian rocks of Sh ...

and Coomb Volcanic Formation.Pharaoh, T.C., Gibbons, W., ''Precambrian Rocks in England and Wales south of the Menai Strait Fault System'', 1987 A Revised Correlation of the Precambrian Rocks in the British Isles

Metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, cau ...

rock is thought to underlie the Wrekin Terrane and is inferred from the outcrops associated with the

Malvern Hills The Malvern Hills are in the English counties of Worcestershire, Herefordshire and a small area of northern Gloucestershire, dominating the surrounding countryside and the towns and villages of the district of Malvern. The highest summit aff ...

where outcrops of metasedimentary

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...

and

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

are noted at

Gullet Quarry The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

(southern Malvern Hills). There are also outcrops of gneiss at Primrose Hill on the Welsh Borderland Fault System.

Stanner Hanter

The Stanner-Hanter Complex outcrops in the terrane, the complex is a magmatic intrusion (non-preserved) with a four-stage cross cutting history where acidic and basic rocks have been intermingled and hybridised with an interpreted age of 702 Ma +/-8 Ma (Stanner Hill

Granophyre Granophyre ( ; from ''granite'' and ''porphyry'') is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image. The texture is called granophyric. The texture can b ...

).R. S. Thorpe, ''Possible subduction zone origin for two Precambrian calc-alkaline plutonic complexes from southern Britain.'', 2000, Bulletin of the Geological Society of America, Vol. 83, 3663–3668

Malvern Complex

On the other side of the terrane the

Malvern Complex Malvern or Malverne may refer to: Places Australia * Malvern, South Australia, a suburb of Adelaide * Malvern, Victoria, a suburb of Melbourne * City of Malvern, a former local government area near Melbourne * Electoral district of Malvern, an el ...

is composed of

diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sil ...

and

tonalite Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the tota ...

plutons that have been altered by Proterozoic metamorphism (modifying the original igneous textures) by still exhibit calc-alkaline, magmatic arc geochemical signatures. The date of emplacement is interpreted to be 677 Ma +/- 2 Ma with upper

greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite ...

-lower

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flak ...

metamorphic cooling ages of c.650 Ma although crustal recycling is inferred from the presence of inherited

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

of 1598 Ma +/- 32 Ma. The later phases of intrusion are noted within the Wrekin Range where the Uriconian

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained ( aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The min ...

is cross cut with the

Ercall Granophyre.

Volcano-sedimentary geology

The sedimentary record in the area varies between volcaniclastic and siliciclastic deposition, the latter being present in the Longmnyd area and the former being representative of the Stanner-Hanter and Malvern Complex. Depositional facies include the

and the

Longmyndian The Longmyndian Supergroup is a sequence of Late Precambrian rocks that outcrop between the Pontesford–Linley Fault System and the Church Stretton Fault System in the Welsh Borderland Fault System. The supergroup consists of two major geolo ...

sequences. The relationship between the two is complex with some suggestion that the two are coeval and faulted as well as two unconformable strata. The former (coeval but different depositional facies) is supported by U-Pb Zircon dating from the Longmyndian bentonites. However, this is not definitive.R. A. Strachan, R. D. Nance, R. D. Dallmeyer, R. S. D'Lemos, J. B. Murphy, G. R. Watt, ''Late Precambrian techtonothermal evolution of the Malvern Complex'', 1996, Journal of the Geological Society, London, 144, 753–764

Uriconian

The

sedimentary geology is mainly composed of bimodal (acidic, intermediate and basic) intra-plate volcanics with only minimal influence from

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...

. The interpreted overview of the depositional environment is that of a trans-tensional arc caused by oblique subduction. The sediments of the Uriconian are indicated to have been deposited rapidly with such volcanism being dated (U-Pb Zircon) at 566+/-2 Ma (Note the Uriconian is cross-cut by the Late

Ercall Granophyre dated 560+/-1 Ma.) The sequence is dominated by

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

deposits,

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of d ...

s and welded tuffs which is suggestive of a former volcanic centre being in close proximity and deposition in a

subaerial In natural science, subaerial (literally "under the air"), has been used since 1833,Subaerial
in the Merriam ...

environment. Uriconian Volcanics

Coomb Volcanic Formation

The Coomb Volcanic Formation occupies much of the south-west of the Wrekin Terrane and its associated

inlier An inlier is an area of older rocks surrounded by younger rocks. Inliers are typically formed by the erosion of overlying younger rocks to reveal a limited exposure of the older underlying rocks. Faulting or folding may also contribute to the obs ...

s. The sequence is the most recent suite of Neoproterozoic rocks identified in southern Britain. The formation consists of over 1000 m of sediments deposited in a subaqueous environment (in contrast to the subaerial Uriconian). Explosive ash-flow tuffs mark the initial phase of deposition followed by rhyolite flows and domes. Bimodal deposition followed with lavas, minor intrusions, ash-flows and volcaniclastic rocks with some ash-flows exhibiting

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and t ...

body fossils.J. C. W. Cope, ''An Ediacara type fauna from South Wales'' 1977, Nature, London 264,624 The geochemistry notes that fractional crystallisation process within the magma chamber links the lavas with the tuffs and gives intra-plate as well as

geochemical signatures.

Warren House Formation

The

represent another depositional environment for late

sedimentation with the

Warren House Formation A warren is a network of wild rodent or lagomorph, typically rabbit burrows. Domestic warrens are artificial, enclosed establishment of animal husbandry dedicated to the raising of rabbits for meat and fur. The term evolved from the medieval Angl ...

being in thrust contact with the Malvern Complex. The Warren House Formation is composed of

pillow basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

s, intermediate

lava Lava is molten or partially molten rock ( magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...

s, altered

rhyolites Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

and

welded Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as braz ...

and non-welded acid

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

s with evidence to suggest that they are linked via fractional crystallisation. U-Pb zircon dating gives an age of 566 Ma+/-2 Ma. The geochemical signature from this sequence is contrasting to that of the rest of the Wrekin Terrane being of a

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc ...

formed on

oceanic Oceanic may refer to: *Of or relating to the ocean *Of or relating to Oceania **Oceanic climate **Oceanic languages **Oceanic person or people, also called "Pacific Islander(s)" Places *Oceanic, British Columbia Oceanic is an unincorporated set ...

or on thin, immature

continental crust Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial' ...

Longmyndian Supergroup

The Supergroup consists of c.6000 m of sedimentary rocks with varying

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

from basinal and distal

turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites w ...

to deltaic and fluviatile successions which exhibits a generally regressive sequence. The

is separated into two main depositionally distinguishable units: the

Stretton Group The Stretton Group is a group of rocks associated with the Longmyndian Supergroup of Ediacaran age, in Shropshire, England. The rocks are located within the tract between two elements of the Welsh Borderland Fault System, the Church Stretton Fault ...

and the

Wentnor Group The Wentnor Group is a group of rocks associated with the Longmyndian Supergroup of Precambrian age in present-day Wales, U.K. The rocks are located within the confines between the Church Stretton Fault and the Pontesford-Lindley Lineament.P. ...

. The succession is geographically positioned between the Pontesford-Linley Fault System and the

Church Stretton Fault System Church may refer to: Religion * Church (building), a building for Christian religious activities * Church (congregation), a local congregation of a Christian denomination * Church service, a formalized period of Christian communal worship * Chris ...

which has been folded, possibly during the late

, and is inferred to gently plunge to the south. The succession is predominantly siliciclastic but with minor intrusions noted. A basal age (from a

bentonite Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-m ...

horizon) of the Stretton Group is given as 555.9 Ma+/-3.5 Ma.W. Compston, A. E. Wright, P. Toghill, ''Dating the Late Precambrian volcanicity of England and Wales.'' 2002, Journal of the Geological Society of London. 159 323–339.

Palaeontology

The Wrekin Terrane contains abundant evidence of Ediacara

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''biota''. Zoo ...

which include the body fossil

taxa In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...

''Beltanelliformis brunsae'' and ''B. minutae''. The body fossil Arumberia is also noted to be present in the Lightspout Formation of the Stretton Group.D. McIlroy, T. P. Crimes, J. C. Pauley ''Fossils and matgrounds from the Neoproterozoic Longmyndian Supergroup, Shropshire, UK.'' 2005. Geological Magazine 42,441–455

References

External links

Shropshire Geology
{{coord, 52.669, -2.552, type:landmark_region:GB, display=title Terranes Precambrian Europe Geology of Shropshire Geology of Wales