Article 63

Knowledge Of Coral Reefs, Graptolites And A Closing Ocean – The Silurian Period.

In his Chapter 4, Peter Toghill reviews our knowledge of the coral reefs and graptolites associated with the final episodes of marine sedimentation on either side of the Iapetus Ocean before it finally closed at the end of the period. Although subduction took place on both sides, much of the closure may have been caused by lateral shearing movements. Volcanic rocks are rare in the Silurian sequences, only occurring in South Wales (Pembrokeshire) and the Mendip Hills. However, bentonite volcanic ashes occur throughout the sequences on both sides of the Ocean, and in Llandovery Epoch, rocks of the Moffat area and make up 10% of the black shale sequences. On both sides of the closing Ocean, early Silurian (Llandovery) transgressions occurred on the shelf areas to be followed by shallow-water deposits (with corral reefs in the English Midlands) and these passed laterally into deep water turbidite environments with great thicknesses of greywakes and thin condensed sequences of anoxic black shales full of graptolites. The final infilling of the marine areas was followed by a continental collision as the newly formed continent of Avalonia and Baltica collided with Laurentia at around 20 degrees South (Chapter 5). The resulting Caledonian fold mountains covered most of Britain and heralded a continental phase of deposition, the Old Red Sandstone of the Devonian Period.

His Chapter 5: The Himalayas of North-West Britain – The Caledonian Orogeny, reviews the geological conditions after the Iapetus Ocean had closed during the late Silurian Epoch, by which time, the Caledonian Mountains formed along the suture line between Laurentia, Baltica and Avalonia, and rose to heights comparable to the Himalayas prior to being eroded to the current heights of the Scottish uplands, while the Rheic Ocean began to open to become what is now the Atlantic Ocean. This Chapter also describes collage collisions and terranes which arise because ocean floors are not usually simple structures comprising oceanic crust plus sediment. They may contain oceanic islands, sea mounts and plateaux, small continental fragments called microcontinents, and volcanic island arcs, all of which are called terranes, and many fold-mountains are complex structures including various terranes welded onto the edge of continental margins. This type of mountain building is often called collage tectonics because of the wide variety of terranes involved. This Chapter goes on to describe island arcs and accretionary prisms. As an oceanic plate is subducted, sediment may be taken down into the subduction zone, or it may be sliced off by the over-riding plate and added to it as a series of fault slices. The build-up of these slices with each one under-thrusting those formed earlier will build up a mass of faulted sedimentary rocks called an accretion prism. The prism may form in a fore-arc basin between the trench and the arc margin and eventually the whole island arc and accretion prism will be welded onto the continental margin as part of the fold structure as the ocean closes and the continents collide. The Southern Uplands of Scotland are the site of an ancient accretionary prism.

This Chapter also describes the Caledonian orogeny or epoch of mountain building, named after Scotland, where evidence for such mountain-building is well exposed. This episode includes a number of orogenic events which span the period from the early Ordovician to the early Devonian, an interval of 100 million years. But, though the whole orogeny took this long, the main activities were early Ordovician and late Silurian, a timescale compatible with the formation of the Himalayas which started to form 30 million years ago. The various movements were all caused by subduction zones on either side of the Iapetus Ocean which started in the early Ordovician and ended in the late Silurian, during which time southern Britain moved from a position near to the Antarctic Circle at 60 degrees South to around 20 degrees South, a distance of around 4500 km in 100 million years. Thus all of the Iapetus Ocean crust was subducted and destroyed in 100 million years at the rate of 4.5 cm per year, a similar rate to that measured in the Pacific Ocean at present, but the rate was probably more rapid, perhaps 7.5 cm per year from the late Ordovician to the Wenlock Epoch of the Silurian. During the Caledonian orogeny fold mountains of Himalayan proportions, as evidenced by the deep rock structure and fold amplitudes now exposed, were formed over Scotland and north-west Britain. The number of rocks thus metamorphosed to the highest grades, subsequently became the sediment source for later systems. In general terms, the intensity of earth movements decreases as one moves south over the British area. Areas such as the Lake District and North Wales do not show the intense folding seen in the Scottish Highlands.

In this Chapter 5, Peter Toghill then reviews our knowledge of the main episodes of the Caledonian orogeny under the headings, Grampian orogeny, igneous intrusions, chemical composition, Dalradian rocks, sediments, metamorphism, southern Scotland, the Lake District and other regions of England and of Wales from the Early Ordovician to the Late Silurian, during which time the British area was part of a huge continent straddling the equator called the Old Red Sandstone Continent. The rapid erosion of the Caledonian mountains provided the sediment for the Old Red Sandstone of the succeeding Devonian period and the final suturing of the continental masses which came together to form this new continent took place during the early Devonian, and Caledonian Granites of northern Britain are of this age and mark this event. This is my summary of Peter Toghill’s Chapter 5. He didn’t provide one.