Precambrian History of England and Wales

About the author


This is the companion tutorial to the Precambrian History of the Scotland.

This tutorial looks at the Precambrian of England and Wales which by the end of the Precambrian were on a completely different continent and separated from what is now Scotland by the Iapetus Ocean.

Introduction

During the Late Precambrian (c 600Ma) what is now England and Wales (and southern Ireland) was at a latitude of around 80°S, close to what is now North West Africa on the ocean margin of the Vendian Supercontinent. The geological setting was much like the Andes today with subduction of oceanic lithosphere beneath the continental coast with igneous activity in a volcanic arc. Meanwhile, the present-day Scotland was mid-continent along side modern day Peru. By the end of the Precambrian this supercontinent had split into Laurentia (North America/Greenland/Scotland), Baltica, Siberia and Gondwana (everything else including England and Wales).

During the Ordovician a sliver of this Gondwanan continental margin broke away as the Avalon microplate, the Iapetus Ocean closed and by the end of the Silurian Avalonia collided with the Laurentia, welding itself to modern day Scotland along the Iapetus Suture. The Precambrian of England and Wales forms the core of the Eastern Avalonian microcontinent. This 'Midlands Microcraton' or Avalon Terrane is an approximately equilateral triangular block with its northern apex approximately at Stoke-on-Trent in the north Midlands, a southwest corner off Pembrokeshire and a southeast vertex under the Thames Estuary. It is divided into two halves (western and eastern) along the Malvern Line. The margins and the central divide are zones of weakness in the crust and have been reused in many subsequent deformation events which have brought slivers of Precambrian rocks up to the surface in the west (Pembrokeshire & Welsh Borders), east (Charnwood) and central (Malvern) areas.

A second block of Precambrian Eastern Avalonian basement, the Monian-Rosslare Terrane, occurs further west. The boundary between this and the Avalon Terrane is along the Menai Straits Line, another major crustal weakness which has been reactivated bringing Monian-Rosslare and Avalon Terrane Precambrian rocks to the surface in Anglesey and the Lleyn Peninsula.

The Avalon Terrane

There are a few, poorly exposed metasediments representing oldest material outcropping in Shropshire and the Malvern Hills but the majority of the rocks are arc related volcanics associated with the subduction of oceanic lithosphere beneath the continental margin. These are a mixture of granites/granodiorites through diorites to dolerite/gabbros intusives and rhyolitic, andesitic and bassltic volcanics. They occur throughout the Avalon Terrane, the most important being the Uriconian Volcanics of Shropshire.

Precambrian sediments are present in several areas. At Llangynog in south Wales siltstones and sandstones contain Ediacaran fauna. In Shropshire the Longmyndian Supergroup records a marine regression from marine basinal facies to terrestrial alluvial plain and contain fragments of earlier metamorphic basement and volcanics. In Leicestershire the sediments contain far more volcaniclastics and form the Charnian Supergroup. Some of the fine grained volcaniclastics contain Ediacaran fauna including Charniadiscus. Again the sequence is regressive from marine turbidites to fluvial deposits.

These sequences are deformed before the deposition of the Cambrian by strike-slip tectonics, sinistral transpression in the west and dextral in the east.

The Monian-Rosslare Terrane

The geology of this terrane is more vague. Rocks include high grade gneiss and granite together with belt of blueschist grade metamorphic rocks and a zone of low grade metamorphic sediments (Monian Supergroup) on Anglesey. The contacts between these zones are all faulted so it is difficult to establish relationships between them. The blueschists are significant as they are associated with the metamorphism that occurs in subduction zones. The Monian Supergroup which continues into the Cambrian comprises about 6km of turbidites, the upper part (Gwna Melange) being an olistostrome produced by massive submarine debris flows.

These rocks are more deformed and metamorphosed than the Ordovician rocks that unconformably overlay them so must have been deformed in either the Late Precambrian or Cambrian.

Conclusion

The Precambrian of England and Wales comprises a minor amount of old, high grade metamorphic basement but mostly arc related Andean style volcanism and associated active continental margin sediments which contain an Ediacaran fauna. This sequence was deformed by strike-slip tectonics in the Late Precambrian, probably related to oblique subduction.

Reference

Woodcock N. & Strachan R. 2000. Geological History of Britain and Ireland.