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 and the overlying Wentnor Group. 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 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 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 Wentnor Group is made up of the fluvial 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 Wentnor Group.

Stretton Group

The topmost of the Stretton Group, the Portway Formation, consists of braided fluvial 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 wer ...

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-m ...

and lapilli

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 ...

horizons that allow for zircon U-Pb dating and noted within the Batch Volcanics of the Synalds Formation, toward the base of the Stretton Group, a date is returned of . The Lightspout Formation toward the top of the Stretton Group returned a date of .

References

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External links

Shropshire Geology
Geology of Shropshire Precambrian Europe Geology of Wales