A Brief Phanerozoic Geological History of Britain
Index
Cambrian Ordovician Silurian Devonian Carboniferous Permian and Triassic Jurassic Cretaceous Tertiary Quaternary
During this period the area that is now occupied by Britain was flooded by lapetus. This ocean, similar to the modern Atlantic, contained hard-shelled animals and so, for the first time the marine sediments contained fossils. Initially beach and shallow marine sediments were deposited on both sides of the ocean and these can mainly be found in Scotland and North Wales, but as the sea deepened these sediments were replaced by muds and calcareous ooze and can only be found in NW Scotland. The rest of the Lower Palaeozoic tended to be dominated by these muddy sediments. During this period, Northern Ireland and Scotland were on the opposite side of lapetus to Wales and Southern Ireland and hence were separated by a vast expanse of deep water. Therefore the faunas of the shallow seas on the margins of lapetus evolved separately because they could not mingle and hence they are completely different to each other. These shallow shelf seas represent the flooding of the continental margins possibly caused by the eustatic rise in sea-level due to the development of an oceanic ridge.
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Lapetus was now beginning to shut as subduction became faster than the spreading rate. In Scotland and Northern Ireland, sedimentation ceased and the compression accompanying the closure led to the first phase of the Caledonian Orogeney known as the Grampian Orogeney. This orogenic phase occurred in the Lower Ordovician (535Ma - 465Ma) and it affected the rocks on the NW margin of lapetus. In early Ordovician times, NW Britain was a shallow shelf sea in which carbonates were slowly accumulating while to the SE the sea was deepened into a turbidite filled basin. The Grampian Orogoney uplifted this basin and shelf sea into a series of mountain chains, evidence of which can still be seen today. Lower Ordovician rocks are the youngest to be affected by deformation and metamorphism whilst it is late Silurian rocks which are the oldest above the associated unconformity in Ireland and Devonian in Scotland. In the SE side of lapetus the sediments continued to be deposited and were interspersed with volcanic rocks ranging in composition from andesites to rhylites. This can be seen in Snowdonia and the Lake District. Towards the end of the Ordovician, lapetus had closed sufficiently for the shelf faunas to mingle, resulting in a more unified fauna.
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The final closure of lapetus not only occurred during this period but also not before the vast thicknesses of muddy sediments were deposited across Wales, N England and Scotland where sands probably derived from Cockburnland replaced the more typical L Palaeozoic muds around the middle of the period. The later phases of the Caledonian orogeney uplifted, folded and faulted these rocks.
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Thick marine sediments were deposited in a shallow shelf sea in what is now SW England while further north lay a newly created continent formed from the closure of lapetus during the Caledonian orogeney. A collective name for the sediments deposited in subsiding basins on this new continental surface is the Old Red Sandstone. It was probably deposited by streams and rivers flowing off the newly formed mountains and the large outcrops of it in S Wales and the Welsh Borderlands is thought to represent a large delta of one of the larger rivers.
The Old Red Sandstone facies can be seen in a transitional stage in the Welsh Boarderlans and Ireland. In the Orcadian Basin a classic Old Red Sanstone facies can be seen and it contains some fossils of fish. Here the facies is up to 10km. thick and is belived to have been deposited ina great lake that covered the NE of Scotland and was linked to the Baltic. The conglomerates found at it's base are derived from the surrounding uplands.
The Old Red Sanstone was deposited over most of the British Isles with the exception of the marine sediments of SW England which were formed in a marine basin. These sediments are a classitype with limestones and corals, turbidites with fluvial and deltaic sediments also present. There is an almost comlete sequence of strata covering almost 50 million years which is dateable by goniatite fossils and somne radiometric dating of volcanics
Some late Caledonian volcanaicity occurred more notably in Scotland.
During this period there was a move of the plants and animals from a marine environment to a terrstrial one. Globally, there was a eustatic sealevel high but this had dropped since the end of the Silurian. The climates were warm and semi-tropical over most of the Earth.
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The Old Red Sandstone continent had begun to subside at the beginning of this period to a sufficient level where it became flooded by a shallow shelf sea in which muddy goniatitic sediments were deposited. The rate of subsidence varied and areas such as the Pennine Block subsided more slowly and were eventually subsided by the Dinantian. Deposits contain corals and brachiopods in large numbers, which were teeming in the clear tropical water.
As the period progressed, river deltas bounding the continent to the north spread over large areas of Britain from the Midland Valley Southwards giving rise to the Millstone Grit sequence.
There were periodic marine transgressions over a predominantly peneplain landsurface but they receded later in the period and the deltas became forested. The remnants of which can be found in the coal seams most of which have been worked in recent times. The cyclicity of the Upper Carboniferous gave rise to the repetition of the coal seems at different depths. This is generally caused by repeated advance and retreat of the sea and this could possibly be associated with a variation in the spreading rate of a distant palaeo-ocean.
Globally, the early Carboniferous was in equitorial zones, alloiwing for vast thicknesses of limestones to accumulate in North America and Europe and the humid forests with gave rise to the coal deposits. It was also the begining of a period of glaciation in the south.
Towards the end of the Carboniferous a continental collision occurred known as Hercynian Orogeney, to the South of Britain. South Wales and Southern Ireland were the areas most intensely affected by this collision and intense deformation occurred with the same low grade metamorphism and thrusting occurring. It was during this time that SW England botholith was implaced. Northwards, much more gentle faulting and folding occurred resulting in the preservation of the coal seams in the down faulted/folded regions.During the Dinantian Britain was divided into three parts; a SW province, a central province and a Northern province. The SW province was called St. George's Land and it extended over much of Wales, E Ireland and the Midlands and there are marine sediments from Bristol to W Ireland recording the marine transgression that occured during this time. The central province covers huges parts of Ireland and the English Midlands. It has more mudstone sediments but there is good evidence of cyclicity and there are six recognisable cycles. There are major developments of lime mudbanks and these can be seen in Ireland and Yorkshire. The Northern province contains thinner limestone and sandstone sequences showing a similar cyclicity as those in the central province. In the Northumberland basin, rivers flowed from the north and east, eroding debris from the Southern Uplands block. There is also evidence of active faulting and vulcanism along the margins of the province.
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Britain essentially became a land area on the northern fringe of a new continent after the Hercynian Orogeney. The climate was arid and so a characteristic new deposit was laid down in desert environment. This was called the New Red Sandstone. This was produced by rapid erosion from the uplands producing this coarse sandstone. There is also evidence of some fluvial sediment which would have transported this coarse sandsone.
During the Permian, a crustal warping of part of NE England caused the land surface to be invaded by the Zechstein Sea. This invasion is possible due to the deglaciation which then caused a eustatic rise in sea level. It is possible to recognise up to five major cycles of transgression by studing the salt deposits whichnare precipitated out in a set sequence. the base of the transgressionis a Marl Slate which is abundant in fish and plant fossils and this is then followed by the Magnesian Limestone which is then overlain by typical evaporite deposits.This occured as the coninents continued to fuse together after the Hercynian Orogeny. There was also some continued vulcanism in SW England, the Midland Valley and the North Sea. In general there was an increase in land area as many areas were fused together.
Most Triassic deposits are typically terrestrial. In the Lower-Mid Triassic there are three types of sandstone facies; coarse grained breccias,conglomerates and sandstones; red sandstones and siltsones; and evaporites.Triassic salt deposits can also be found, especially in Cheshire and they were believed to have been formed in a smaller sea or large lake on the opposite side of the Pennine Ridge. In the Mid-Upper Triassic there are again another three types of facies; the Mudstone group ( reprenseting great lakes or inland seas); the Penarth group (representing a transition from shalow to deep marine sediments) contain the classic Rhaetic bonebeds; and a set of arid deposits towards the end of the Triassic.
One of the most famous episodes in the Permo-Triassic is the mass extinction that occured. It involved the extinction of most of the major groups of organisms that had dominated the Palaeozoic seas in a time of less than 15 million years. Many of the organisms that wre not made extinct were rapidly reduced in numbers. Despite these extictions there were also some organisms that were unaffected and this causes some curiosity as some were morphologically similar to some of those that had died out, E.g. ammonoids died out where as nautiloids survived. As a result of all these changes, the early Triassic seas conained an unusually narrow range of organisms.
Towards the end of the Triassic the super continent of Pangea began to break up, rifting between N Africa, Europe amd N America.
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The land had now reached an essentially peneplain and so a eustatic sea level rise probably caused the birth of the Atlantic Ridge in the New Atlantic Ocean. This flooded much of Britain thus most British Jurassic sediments are typically marine, with oolites being amongst the most important, with their greater development being seen in the Cotswolds Clays accumulated in the deeper parts of the marines basis, eg Oxford Clays.
Due to the recent mass extinction during the last period, dinosaurs evolved and dominated both the seas and the land. Their life span though would soon prove to be relatively short due to the immenent mass extinction at the Cretaceous-Tertiary boundary. Again this was not a suuden or catastrophic event but occured over a considerable time period of a few million years. it has been calculated that 70% of all living species of all forms of life died out at this time. The difference between this mass extinction and the Permo-Triassic is the dead of the dinosaurs which were then replaced by mammals as the dominent terrestrial animals.
One of the more favoured theories for this mass extinction is a huge asteroid impact in the sea off Mexico but this is by no means the definitive answer and it is still open for questioning.
Towards the end of the Jurassic the North Atlantic had opened splitting up the land masses even more.
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The sea had retreated from most of Britain by the end of the Jurassic but the highest Jurassic deposits represent a coastal legend area in SE England and the earliest Cretaceous sediments, called the Wealdon Series are a continuation of these. Subsequent Lower Greensand deposits represent an important marine transgression as they are overlain by Gault Clay. The former of the two being a shallow marine deposit whereas the later is deeper marine facies. A brief regression resulted in the deposition of the Upper Greensand before a further transgression produced the Chalk beds.
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The opening of the North Atlantic seems to be responsible for the igneous activity found in NW Scotland and Northern Ireland as well as a certain amount of crustal flexing which led to the flooding of the Irish Sea. Alpine Orogeney was too distant to have any effect except for some buckling of S England, particularly the Isle of Wight monocline and the Wealden Anticline. In SE England, in an extension of the North Sea predominantly muddy sediment was deposited, notably, the London Clay. The mid period Wealdon Anticline has since separated these early Tertiary deposits into the London and Hampshire basins.
This period began warm and arid and then began cooling in the Pliocene. It was dominated by mammals and birds which radiated through the terrestrial surface.
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This relatively short period is essentially dominated by the climatic changes, especially the Great Ice Age in which the whole of Britain was covered in ice to a boundary line approximately running from Avon to Cambridge. This moulded the topography and scenery giving us the landscape that we see today. During the Ice Age Britain was still attached to the continental mass of Europe but due to a combination of the retreating ice and a eustatic sea level rise and isotatic subsidence the continental shelf was flooded resulting in the formation of the English Channel and thus resulting in the current nature of the British Isles.
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Last updated 21 april 1997 ©1997
Sources:
Britain before Man
Mr D. N. Knight
Background courtesy of the Cyberspace Inn.
An alternative Earth History guide
