Palaeozoic History of the UK - Part 1: Cambrian to Silurian

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Imperial College London

Introduction

During the early Palaeozoic Scotland and England were still on separate continents, Laurentia and Avalonia respectively. Cambrian, Ordovician and Silurian rocks are found mainly in Scotland and Wales. This article covers the time period 550Ma to around 410Ma.

Cambrian

The Cambrian in Scotland is located in the north-west (sedimentary succession) and the central regions (metamorphic rocks of the Dalradian). The general sedimentary stratigraphy is shown in Figure 1. The sedimentary succession shows a progression from tidal sands to a limestone dominated marine shelf. The most famous rock is the pipe rock, which shows abundant vertical fossil burrows, Skolithos and Monocraterion. The reduction of tidal strength is shown by successively thinning beds. The fucoid beds are a mix of carbonate and clastic storm-dominated sediments, populated by Olenellid trilobites. The salterella grit or serpulite bed comprises of cross-bedded and bioturbated sandstone, probably deposited as offshore sandbanks. The final facies is the Durness limestone, which at its base shows stromatalites, oolites and psuedomorphs of gypsum, indicating an arid environment. The upper part of the sequence has been dated as lower Ordovician from fossil evidence.

Cambrian straigraphy of Scotland

Figure 1: Cambrian sequence stratigraphy of north-west Scotland

There are some Cambrian rocks in Wales, which mainly show fluctuations between deep marine and inter-tidal. The sucession in Wales is capped by Ordovician arc volcanics, indicating the start of subduction.

Ordovician and Silurian

The main Ordovician exposures are located in Wales. They show a subduction zone to the north, bordering a deep basin. This basin shallows up where magma is raising, creating a shallow basin (Figure 2). The sucession here is mainly offshore marine mudstones, rich in graptolites. These rapidly evolving creatures provide an excellent zone fossil for the whole of the Ordovician and Silurian sucessions.

Ordovician palaeogeography Silurian palaeogeography

Figure 2a: Ordovician palaeogeography

Figure 2b: Silurian palaeogeography

At end of the Ordovician a back-arc basin had formed in Wales, causing huge amount of extension. This was caused in part by the ceasing of subduction, due to the collision of Avalonia with the Iapetus spreading ridge. The end of the Ordovician also saw a major ice age. This was mainly due to Gondwana (the southern supercontinent) straddling the South Pole. Land ice reached up to 70-50° south. This ice age resulted in a major drop in sea level, resulting in a marked regression in the Welsh basin. This caused the sediment to swing from deep marine to shallow sandstone, conglomerates and the occasional limestone.

The beginning of the Silurian started with a major transgression as the ice age ended. The Iapetus closure continued with subduction under Laurentia, until the end Ludlow, when Laurentia finally collided with Avalonia along the Iapetus suture, which is located throught the Solway Firth on the west to around Holy Island on the east coast. This collision resulted in the Caledonian Orogeny, although orogeny is best thought of as several separate events.

Marine deposition continued to the end of the Silurian, when there was a steady regression, leading to terrestrial deposition in the Devonian.

Conclusion

The lower palaeozoic of Britain commences with a shallow marine setting over the most of what is now Scotland. There are few un-metamorphosed rocks of Cambrian age which can tell us about the sedimentary environemts at the time. The Silurian and Ordovician sawh the closure of the Iapetus ocean. This joined the landmass that is now Scotland with England and Wales. The Ordovician ended with an ice age. The Silurian ended in a large-scale regression.

References

G. Y. Craig Geology of Scotland. Buy It

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