The Stretton Group is a group of rocks associated with 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 ...

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 th ...

age, in

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 th ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

. The rocks are located within the tract between two elements 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 ...

, the Church Stretton Fault and the Pontesford-Linley Lineament.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed The Stretton Group is a predominantly sedimentary group with a range of

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 ...

attributable to that of a closing ocean.J. N. Carney, J. M. Horak, et al., ''Precambrian Rocks of England and Wales'', ''Joint Nature Conservation Committee.'' Geological Conservation Review Series 20 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. ...

overlies the Stretton Group of rocks and although the units are separate, together they show a good geological progression. At the base of the Stretton Group the rocks are of basinal oceanic facies and as time goes a coarsening occurs with increased terrigenous input from the continent.

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 were ...

s are observed and deltas form latterly with alluvial plains with occasional marine washovers. This creeps up into the Wentnor Group where alluvial plains latterly with

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

and

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

deposits noted in the uppermost (youngest) Bridges 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 IslesW. 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.J.C. Pauley, “A revision of the stratigraphy of the Longmyndian Supergroup, Welsh Borderland, and its relationship to the Uriconian Volcanic Complex”, 1991, Geological Journal. 26 167-183 The information below is present oldest to youngest. The progradational Longmyndian Sequence from oldest to youngest is: Ragleth Tuff Formation; Stretton Shale Formation; Burway Formation; Synalds Formation; Lightspout Formation; Portway Formation; Bayston-Oakswood Formation; Bridges Formation. The latter two units belong to the

. Below we carry on from the underlying Portway Formation (Stretton Group).

Ragleth Tuff Formation

This is a unit of coarse epiclastic sediments that may pass upward into the Helmeth Grit. Its position is stratigraphically tentative.

Stretton Shale Formation

A grey/green shale with normally graded siltstone which is interpreted as a distal turbidite with basin floor facies. The Helmeth Grit is located at the base of the unit (medium-grained sandstone). The Stretton Shale has been assigned a geochronological time of 566Ma+/-2.9.

Burway Formation

The base of this formation is defined as the base of the

rhyolitic 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 ...

tuffs of the Buxton Rock Member. It coarsens upwards from thin to thick bedded turbidite lobe facies. Above the turbidite facies are shallow marine mudstones and then deltaic sandstones. The uppermost unit, the Cardingmill Grit, at the top is interpreted as a fluvial deposit. '' Beltanelliformis minutae'' and ''Beltanelliformis brunsae'' occur in the succession.

Synalds Formation

A mudstone lithology with inter-bedded, laminated and cross stratified sandstones is interpreted as alluvial plain, fluvial and possibly deltaic deposits. Mudstones are green at the base of the unit and red at the top. ''Beltanelliformis miuntae'' and ''Beltanelliformis brunsae'' occur in the succession.

Lightspout Formation

Laminated mudstones that coarsen upwards with upward fining sandstones interpreted as alluvial plain, fluvial and possibly deltaic facies. ''Beltanelliformis miuntae'' occurs in the succession. The succession is geochronologically dated at 555.9Ma+/-3.5. The formation also contains matrix and clast supported conglomerate members with sub-rounded lithic clasts and sub-angular sedimentary clasts. These are interpreted as braided fluvial deposits. The unit is barren of fossils.

Portway Formation

This succession comprises red mudstones and siltstones interbedded with fine grained sandstones although it is sandier and coarser than the Synalds and Lightspout formations but also interpreted as alluvial plain and fluvial deposits. The base is marked by the Huckster Conglomerate which is thought to be deposited in a braided fluvial (palaeo)environment.

References

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External links

Shropshire Geology
Geology of Shropshire Precambrian Europe