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 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 supergroup consists of two major geological groups, 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 Faul ...

and the overlying

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 rocks are a generally regressive sequence from basinal facies to clastic

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 ...

. The rocks are thought to be derived from

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 ...

mountains that were formed during the southward

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 ...

of an oceanic plate beneath a continental block (ocean closure). The rocks have since been folded due to fault movements and plunge gently to the south. The Longmyndian rocks were deposited in northeast–southwest trending faulted rift basins. These were deposited on top of the

volcaniclastic deposits. Subsequent ocean closure squeezed these rocks together to provide sub-vertical bedding in the synclinal sequence we see in exposed rocks today. As noted below, the deposited rocks of the Longmyndian show a variety of depositional

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 ...

that tell a story of ocean closure. The Longmyndian deposits rest upon Uriconian rocks but different sections are exposed exclusively between the Church Stretton Fault and the Pontesford-Linley Lineament.

Wentnor Group

The

is made up of the

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 ...

deposits of the Bridges Formation and is the topmost of the supergroup. This is underlain by the braided deposits of the Bayston-Oakwood Formation which completes the

Stretton Group

The topmost of the

, the Portway Formation, consists of braided

deposits with marine incursions. The underlying Lightspout Formation is interpreted as a fluvio-deltaic environment, this is succeeded by a similar facies in the Synalds Formation.

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 ...

facies are prevalent in the Burway Formation and the deep marine basinal environments continue toward the base of the supergroup within the Stretton Shale Formation. The Ragleth Tuff Formation completes the Stretton Group and the supergroup sequence.

Dating

Within the strata are

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-mon ...

and

lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range f ...

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 cont ...

horizons that allow for zircon U-Pb dating and noted within the

Batch Volcanics Batch may refer to: Food and drink * Batch (alcohol), an alcoholic fruit beverage * Batch loaf, a type of bread popular in Ireland * A dialect term for a bread roll used in North Warwickshire, Nuneaton and Coventry, as well as on the Wirral ...

of the Synalds Formation, toward the base of the

, a date is returned of . The Lightspout Formation toward the top of the

returned a date of .

References

{{Reflist

External links

Shropshire Geology
Geology of Shropshire Precambrian Europe Geology of Wales