Uriconian rocks are

volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...

s found in parts of

Shropshire Shropshire (; alternatively Salop; abbreviated in print only as Shrops; demonym Salopian ) is a landlocked historic county in the West Midlands region of England. It is bordered by Wales to the west and the English counties of Cheshire to ...

, United Kingdom. The name relates to '' Uriconio'', the Latin name for an Iron Age hillfort on the summit of

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 ...

, a hill formed of Uriconian rock. The Uriconian rocks of

(

Wrekin Terrane The The Wrekin, Wrekin 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 in the region are the Charnwood Terrane, Fenland Terrane, Cymru T ...

) are thought to be potentially related to the

Longmyndian Supergroup 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 geol ...

of the Stretton Hills, Shropshire, United Kingdom. Current geological profiling of the terranes suggests that the Uriconian rocks are of

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 ...

age (

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 prec ...

Phases 2 and 3).P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed The Uriconian Rocks outcrop to the southeast of the

Long Mynd , photo = , photo_alt = , photo_caption = View down Townbrook Valley toward Burway Hill , country_type = , country = England , subdivision1_type = County , subdivision1 = Shropshire , border ...

area of 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 beyond the Church Stretton Fault which trends northeast-southwest across the area.W. Compston, A. E. Wright, P. Toghill, ''Dating the Late Precambrian volcanicity of England and Wales, Journal of the Geological Society 2002; v.159; pp 323–339 The Stretton Hills are composed primarily of

arenaceous Arenite ( Latin: ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00246 in) and 2 mm (0.08 in) and contain less than 15% matrix. The related adjective is ''arenaceous''. The equivalen ...

(quartz rich sand) beds assigned to the Longmyndian Supergroup. The Longmyndian rocks are a c.6 km thick group of

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments ( clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

and bentonitic

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

horizons. The Uriconian have long thought to be older than the Longmyndian and generally

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...

to the southeast of the latter within northeast-southwest trending lineaments suggesting basement influence for the regional structure. The Uriconian rocks outcrop in areas from Wellington,

to Primrose Hill on the southwest side of

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 ...

, east of Caer Caradoc and in the Craven Arms

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 obser ...

.Pauley, J.C., ''A revision of the stratigraphy of the Longmyndian Supergroup, Welsh Borderland, and its relationship to the Uriconian Volcanic Complex'', 1991 Geological Journal; v.26; pp167-183 Primarily the strata exist as fault-bounded slices within splays of, and to the southeast of, the main

Church Stretton Church Stretton is a market town in Shropshire, England, south of Shrewsbury and north of Ludlow. The population in 2011 was 4,671.

fault system. The Uriconian rocks comprise both intermediate to acidic and basic (

bimodal In statistics, a multimodal distribution is a probability distribution with more than one mode. These appear as distinct peaks (local maxima) in the probability density function, as shown in Figures 1 and 2. Categorical, continuous, and d ...

) volcanic suites that reflect largely intraplate origins for the complex although some

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, ...

signatures have been identified. Further work has led to suggest that the locality of this kind of volcanism is related to marginal basin volcanicity (behind the main arc) influenced by trans-tension brought about by oblique

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

sediments were deposited on

Avalonia Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, southern Ireland, and the eastern coast of North America. It is the source of many of the older rocks of West ...

in various strike-slip faulted basins and they comprise predominantly volcaniclastic and siliciclastic sediments.

Overview

Britain during this period (late

/early Cambrian) lay about 70 degrees south of the equator at the margin of a large ocean (to the north) and with a large

continental plate Continental may refer to: Places * Continent, the major landmasses of Earth * Continental, Arizona, a small community in Pima County, Arizona, US * Continental, Ohio, a small town in Putnam County, US Arts and entertainment * ''Continental'' (a ...

to the south, the ocean is accepting sediment from the continental block. The

oceanic plate Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic c ...

is subducting southwards underneath the continental block which is being buoyed by the rock which is subsequently being melted (

partial melting Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...

) by the high temperatures and pressures associated with the process. As with all subduction zones an island arc is formed above the subducting plate (due to rising rock melt) with a back-arc basin existing between the island arc and the continental edge. A large continental block existed with oceanic crust subducting underneath it. In front of the block and above the subducting oceanic plate are volcanoes emitting a range of varied deposits that give a spectrum of signatures (basaltic eruption, intermediate eruptions and acidic eruptions, as the material melting and rising changes over the period of the subduction). These volcanic deposits get mixed up with the sediment from the continent (now in the form of a

back arc basin A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...

). These are the volcaniclastic deposits that we can see today at places such as

. During the formation of the volcanoes and their associated deposits of

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 ...

and rhyolite there are melts that never reach the surface and these shallow level intrusions are notable in the forms of their non-extrusive equivalents such as

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 ...

and rhyolite. Granophyre is quite evident in the Ercall Quarries. The sediments and volcaniclastic sediments are of Precambrian Uriconian age and were deposited in rifts and basins that would have been sub-parallel to the back arc basin as subduction continued. Deep basins will have formed and sediments will have infilled the open space. Significant temperature and pressures are applied to these ''infill sediments'' and as such they were transformed to schist and gneiss (Primrose Hill Gneiss and Rushton Schist are examples). The Uriconian Mountains were formed during this process to the northwest of the continental margin. The significant

sediments were deposited.

Geological relationships

There is much uncertainty about the exact age of the Uriconian rocks primarily due to the complex structure of the folded and faulted rocks. All contacts are discordant, hence no direct relationships with the Longmyndian Supergroup can de deduced, and a relative age has not been established. Relationships to other potential lateral equivalents (from the eastern part of the

palaeo micro-continent known as the Monian Terrane) are still conjectural albeit to varying degrees. The following relationships are all interpreted from figures presented in Brenchley (2006). The relationship to the Longmyndian Supergroup is generally accepted as coeval for the Shropshire/South Wales area. Other Neoproterozoic Phase II (NP2) rocks of similar age are the Warren House Formation of 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 ...

(566 +/-2 Ma), South Charnwood

Diorites 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-silic ...

(of the Charnwood Terrane – these stretch back into NP1 though), Fachwen & Minfordd Formations (Arfon/Llyn area), and the

blueschist Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (), approximately corresponding to a depth of . The blue ...

metamorphosed Aethwy Terrane Blueschists.

Other notes

Given the steep dip of the Uriconian rocks relative to the shallow inclination of the Cambrian, the Uriconian the

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

events can be classed as pre-

Atdabanian Cambrian Stage 3 is the still unnamed third stage of the Cambrian. It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4, although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approx ...

(Stage 2, Lower Cambrian).Tucker, R.D. & Pharaoh, T.C., ''U-Pb Zircon ages for Late Precambrian – early Palaeozoic plate tectonics evolution of England and Wales'', Journal of the Geological Society of London 1991; v.148; pp435-443 Zircon dating has given similar ages of 566.6+/-2.9Ma from the Longmyndian Supergroup and 566+/-2Ma for the Uriconian Volcanics. This leads to suggest that the area was affected by (Uriconian)

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

and associated events between 570-550 Ma including the intrusion of the Ercall

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 ...

. This potentially created a thermal extension of the area allowing

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...

to occur within the basin.

References

External links

Shropshire Geology
{{coord, 52.682, -2.529, region:GB_type:landmark, display=title Geology of Shropshire Precambrian Geology of Wales