Simple FBI Sml

8/3/2019 Simple FBI Sml

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Fig. 14.1 SilUrian rocks showmg effects of the Caledonian Groger y, Shap ell. Cumbna

eoThe oldest rocks in Britain were formed over 2900million years ago; the youngest are still being de-posited today The long story of Britain's geologicaldevelopment has been worked out by the followi g. Studying the mineralogy, texture, structure andfossil content of rocks. This gives informationabout how and where they were formed and whathas happened to them since formation. Applying dating methods to rocks and the eventswhich have affected them. This ay i volvrelative dating (see pages 110 and 118) orabsolute (radiometric) dati 9 (see page 90). Correlating information from different areas. Thisgives a better understanding of past events by

showing the possible variation and extent of pastenvironments.With so much time and evidence to consider, Britain'sgeological history can appear to be a huge list ofseemingly separate events. However, you shouldremember that geological events are not completelyseparate but follow each other in a continuous suc-cession. To recognise the most important trends ofthis succession we shall study Britain's past in rela-tion to plate movement. This can provide possibleanswers to questions such as the following.1. Why has Britain had different climates during dif-ferent periods of time (as shown from the evidence

r,Ies

of sedimentary rock sequences and fossils)) Tbe explained quite logically if we think of hotectonics causes continents to move about tface of he planet. There is considerable evidsuggest that. since the Cambrian period, thepart of the uropean plate system has moved northwards from approximately 40 southequator to its present position of 55 nortmakes it easy to see why a sequence of dclimates and a sequence of different rockc urred. For example, it is thought that the of the Upper Carboniferous (which eventually boal depo. i ) grew particularly well at that ticause Britain was the close to the equator. SiBritain'S ermian an Triassic red sandstonevaporites developed because, by that time, had moved north from the equator into deserttions (that is, it was in the same latitudesSahara is now).2. Why have mountain building, igneous activregional metamorphism only occurred in Brcertain specific times) It seems logical to sthat these events happened when Britain wato an active plate margin.3. Why have major changes in sea level takenIt is likely that some of these resulted from cIn the rate of development of spreading ridgexplained on page 148.


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ij

by

01

~ ~ ~ ~ ~ ~ ~ ~ - - - - - - - - - - - - - - - - - - - - - - - -

(f )I7

40 C N

3;; Nzoe N t:az15N 115 S

41, 57

30 S?

Era Period Age (Ma)Qllilter 2.Cainozoic TcniJry

5Crcl,ICSl)cIS

Mesozoic 13.Juras.>ic"r l a s ~ i c 225

Permian280

Upper Carl)oniferousPill eOZQlc345

Devunian395

Siluria 43!JLower Orc!ovicli'lI\PalaeozoIc 500

Cambrian570

To explain Britain's geological history in term of platemovement we shall divide the story into five mainepisodes; each separated from the next by the

te

ostdramatic effect of plate motion: an oroge y. 00Fig. 14.2 which Illustrates these episo as dorogenies associated with them. Note that thesodes vary in length and the orogenie vary In eamount of deformation they caused. Rocks frol e chepisode also tend to be separated by unconformities'this is because the uplift of each oroge y eant lherosion generally took place before the next se es ofsediments was deposited.

74.2 The Jor episode' In nt olog'ca'l1lstory(a) t reight of each biock OW $ ag range nor hicK, essrock Involved: (b) blo ks r 'slep d' to 5 ow tha so aposed ( J wh Ie thers r main hi denbeneatn rocks from the iater episodes

Fig. 14.3 shows the solid (bed-rock) geology ofSri i Isles. Notice how rocks from the oldest s es tend to occur In the north and west wtho e from er isodes are mainly found insour a d ea 1. tice too that because the mapbee drawn to show the solid geology, materials epi ode 5 have had to be left out. These remat n I ave n t yet been lithified into solid rockfor a surfac covering over much of the lowlandpractically all of the sea floor.Before studying each episode more closely, sevother poi ts must be made. The f rther ba , in time we go, the less infotion w a e. Th story of the older episode

therefore Ie s re ise than that of more rene The res nt geographical shupe of Britain has xlste fo r about 10 000 years (and It is still slcha ging). When we talk of 'Britain in the pastreally mean 'Ihe area which eventually becBrit in'. In th ollowing pages the letters Ma are useshorth nd for 'million years ago'.

PRE-CAMBRIAN

Episode 4 Rocks r n91n9 inage from Permian to Tertiary.P r IV lolde i south eastEn!] and by Alpine Orogenybo II 25 1111 on years ago.

Episode 3 Rocks formp.d In theDavor la aHd 'rbof1lrerous p .noFolded ( .speoally in south westEngland) by Variscan Orogeny


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Fig. 14.3 Rock outcrops of rna'or pisode

Episode 1 Pr e ~ m b r i ~ n (over 1 0 0 0 M a ~ ' sev ra 0 r o ~ e r \ l : Episode rm "Devon! n dod C' rbol if",rous7;: ">':'. Hec;ttll lin SOlllh w ~ lyVariscan OrogenyI Ie Pr -Cambrian, 11mbn n.Episode 2 OrdnVl


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Episode l' he ea he t eVidence(see Fig. 14.4)Here we deal with events which took place over 1000million years ago. It is the largest but Ie st understood episode of Britain's geological his ory. A lookback to page 89 will remind au of the difffcul iesinvolved in the interpretation of such ancienl PreCambrian rocks. Nearly all the eVlde ce has by nowbeen lost, hidden or altered by erosion, unal anmetamorphismBritain's oldest rocks are the Lewisian Gneissesexposed in the Hebrides and alan Scotland's nor hwest coast. Studies show that their high-grade re-gional metamorphism was produced during twoseparate orogenies. However, th .re is simply no tenough evidence to suggest what late movem ntsor continental collisions could ha e been responsibl .The orogenies are known as the Seo rlan (abo2700 Ma) and the laxfordian (aboul 1800 a), buteven they do not give the real beginning to our story.Other, even older, rocks must have existed bef rethis to provide the eroded material which formed t esediments which became aitered to gn. iss dur ng theScourian Orogeny!Apart from the Lewisian Gneisses, the only otherrocks over 1000 Ma to be seen In Bril in are somesmall areas of metamorphics in the Chan el lsi ndsand south east Ireland. However, it is likely thal otherequally ancient rocks could be found beneath much ofthe country if boreholes were drilled right through hethick sequences of later rocks which lie above them.Fig. 74.4 Distribution of rocks over 1000 mil n e rs Id rtheLewisian Gneisses)

Epi'sode 2,. a clos DC ,an _ringr 1aln together

Although this 's tl"1I:3 sec nd episode, It is the firsf he tory th t can be interpreted in a meanw y . l l ov r events during the late Pre-Caman the L wer PalaeozoIc (that is between 1000400 illion years ago). Rocks of this age are wexpo ed in 'he hi lands of north and west B(see Fi s. 14,3 and 14.13) an all show the effecth Ca edonian Oroge.ny.Before s udyi 9 any details of this episodeshou d look at Fi 14.5a. On this map EuropeNorth America re shown without the Atlantic Oin etween. Although this may look strange, you to realise ha the A.tl ntic did not open and spunliithe Mesozoic era (see page 147). Withoutra ion by the Atlantic, some important links belea.Similar la e Pr -Cambrian and Lower Palae

rocks I Scandi avia, Greenland and Canadawith those In Britain to for, a continuous beltthe same general direction ef folding. Geolocall this the Caledonia Fold Bel1t, The fold belt 'IS sandwiched between two are.ven 01 er rocks.This pattern suggests that Caledonian foldingproduced by the collision of two ancient continHowever, - s you would expect with events cov600 millior years, the story is open to several d

ent i terpretations. fhe following pages outlineposs' Ie stages, but as you read them keepfollowing. points in mind.Fi . 14.5 Th@ Caledonian Fold Selt

?& ' ~ l " : . ; : l Cal clonia Fuld . IN'!f.:'l!f; ItO b btt VII n OOQ. 400


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\ ::'. - " ~ " ..r ; ' . - " . ~ : ,")':-.....,,\-'"- := - - , /< ; ; .0 ' - " ' , A. .. ,?\ "'\ -." .., - < i ~ + -

1. The continents involved were basically thoseshown on either side of the fold belt (see Fig. 14.5b).We shall call them:(a) the Laurentian Continent (the remains of this

now form the ancient Pre-Cambrian regions ofCanada and Greenland);(b) the Baltic Continent (the remains of this nowform the ancient Pre-Cambrian regions of Northern Europe).2. During the late Pre-Cambrian and the LowerPalaeozoic, these continents were separated by anocean known as Iapetus (remember this was not theAtlantic but a different. much earlier, ocean). Iapetus

also separated Britain into two parts because,time, the area which was eventually to bScotland and northern Ireland formed partLaurentian Continent while England, Walesouthern Ireland were part of the Baltic CoBritain was 'brought together' when these concollided during the late Silurian. It is an intethought that the present day border between Eand Scotland runs approximately along the junwhat were once parts of separate continents I3. Palaeomagnetic evidence suggests that and its surrounding continents were in the shemisphere.

,.: : : - ~ ? ? LScotland and northern Ireland. 17 Shallow-water sediments:.; ~ . , : . ~ . ; ! 1 _ Torridoniar in late Pre~ ; 1= Cambrian, sandstones ndJt...;jf,.------, I Ilmestoncs In Cambr, 11

.. : i ~ ) ~ ' / L Offshore sediments::. '. Moine followed by Dalradl n: (- '. ,. " ~ ~ , : ~ : : __ i ~ ~ 1 ~ ~ ~ V O j . _ - ~ : : ; ? - _ 'J

.. } i - p ; ~ ~ - . 0':' '- ' --=- A - - " .. !Jj,'h ' f ~ ' ; ; " ; , ,/-- - _-=-- \ ,0 _-...=:::- southern Ireland::,?...., L_.;;: \.'?-\ ~ ~ '/ -=- _ - ~ - G o


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While the prolonged deposition of offshore sediin the trough, a much thinnerof shallow-water se Iments was f rminge. During the late Predeltas produced Torridanian Sandstonesin sea level meant that some of theseeroded before limestones and orthoquartzitesdeposited in the Cambrian.

on the edge of the Baltic Continent andand subsiding on this continelltal shelf. About

of this chain can be seen in Preand south east Ireland.Since there are so few outcrops of Pre-Cambrianin England it is difficult to tell what was hap

in Shropshire)sea covered the area and thathad been folded,

By the beginning of the Cambrian period, subof the Baltic Continent seemsen ed, At about this trme sea level rose (indian increased rate of spreading by Iapetus?) andfold elt of Pre-Cambrian rocks then stood

as an offshore ridge. Thi f a ure {known as theSea Ridge: ee Fig 14.6} supplied se ;menl tsea 0 e" her side. To i s norlh. shales were dsited on e edge 0 h conti nUl helf hese rnow lie b de ea h rdovicia material In splaces as the I ie of Ma an th. Lake Dis n .t}the south side of le Iri h ea Ridge, Ca brian sments ac u ulated in a region of dee ning wcalled the WeFsh Basin. ock from thi envirm mare now seen I nonn west Wales whe heyclude som excellen roofing sla es (see Fig. 14.8note that these rocks we e not metamorp ased the original shale until Iapetus closed in the Silurian),Across the south ast f th elsh asin, closthe shore of 'lle Baltic Co tinent, was a r giomuch shallower sea. During the Cambrian perrod beds of sandstones and limest n s were deposhere to lie unconformably 0 eroded lat Pre-C mbsediments and volcanicA note on Cambrian fossilsThe Cambrian is the first period when fossilsfound frequently enough to be use 'or correlaTrilobites are most common an , in ad irian 0value as zone fossils, they add f rther e i neeIapetus eXisted. Two dlrrerent set of Cambspecies are found: one occurs in the rocks of Scotand Canada wi le e other is reserved In 1he roof England, Wales and southern Scandinavi , separation of species waul seem suggest thatwo regions were then separ te continent WIwide enough ocean belwee hem to prevent migration of sea-froor cr atures su h s rilobites.

156


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Iapetus in the Ordovician Sea land and no hem Ire/andvents here we e dommate by mountain bUil(500 435 M : i 14 9 e nor h r d e of I- petus began to ubdAlthough ntaln rem'ln I wo S6 arate par oea h e Laurentian ontlnen. oine a d Daduring t e rdovician, Iapetus was, oy t en, a closing sediments (which ad been lying in heocean. hi pen d IS easter to correlate than earlie tr ugh) w re lote Iy fa ed an me a orphotime : gr polites r Vide good one ossi or fine lh wh Ie area 0 he helf and slop asgrained offshore sediments whil rHobit nd pressed and uplifte Com es Ion w a 0racillopo are used for deposits rom shallower 1hat ~ o m e Moine rocks were pushed manw I r', me res ow' r h north we alan t e Thrus {see Fig 4. 0) The subductlo and oddlng I lude Ie m erup Ions, the Inof gr nit b hohths nd h orma 10 a m Josuch as th Highland Boundary and are(e F!g. 10.19 on page 9).I Scotian and northern Irolan/., - MOlml emu DOli dUll

.::.:. tTll l l i lrl lurplws cl ill1U LJl=1llf. cl/- - os 'Alldelln Ly }' , IIluU lail'l>

LAL.R TIACONTIN[, T

KEYS a o .. W I ..sedimenlD Du pi!1 ...twas ofc mill !!. l...tl LIII II

~ o c e j J n l c c r u s t

'.

.~ ,.......7..... f ',"

,, ..,

7?7

--I \ / \A MoulI1am:;fi> OIl"flnOp"

"7 I Itile inrorml'llloll. lor I 1$ Clr it ? .. r-..-:=. ,'" v.'"'"-r

-./-

Section acroSs ocean

Even as th mountainsrrslng, new sedIments wereera e from them. Thl, mw s transpo ad on 0 theremain t e on ineotaan Into he rench above thdue io 70ne beyond. Deepshale and greywac e sandfrat lese "nvi 0 m nt cab see as part of he Ordsequence in he ou hem Uof Sea ,- n .In to ai, a dovician even th Laurentian sid 0 Iapetbe Ii ene to the more receue ion f Ih _ aze Pia e South Am 'ca n he forof he Annes Mountains.EnglandlW; leslsouthern IrSedi en tlon h r con inuImrlar pa ern t ambrian WI h h Ie in offshor basih lIow w ter deposi neahe shore of th Baltic ConHowever. during the Ordowides d volca ic a tivr ylarge amounts of ava, tua glomerate to t e seqThese vol ani rocks now

ut as the highest most eshill' of e Lake District, SoIrel nd and W Ie (or xSnowdonial.As Ig. 14,9 hows, hani is 0 ght to av cause by s bd c 10 b t 'horne arg ment abou wheactu' lIy 00 place Possibearlier su uctio zone (which

57


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nally formed he Irish Sea Ridgeduring the re-Ca n n} b _ ameactive again. or (mar lik Iy) n wzone dev I e to e north ofthis. Notice th I the vol oesformed islands in the ea 0 thecan line al edge and did not risedirectly from a .eanic rus: for thisreason they should not be houghtof as an island are of he typedescribed on a e 14

Fig. 14.11 Resi an rdol/lc!an volccmi_rock at t e summt ot Iyder. Snowdonia

The closure of Iapetus inthe Silurian(435 to 39 M : Fig. 14.12)By the ea Iy ilurian. s d tionhad r duced Iapetus 0 a narrowoc an, an the rest o the periodmarks the final s ages of closure.Even before otal collision tookplace, a series of parallel landridges develo e s pressurebuckled and uplifted parts of bothontinen I edges. idges uch asSolw yl nd nd Coe burnfandstood betwe n the ainlan sand

use he a ern 0 de osition tobecom mor omplex y eachsupplying sediment to he r adjacent floode basin. The horehneof the main Baltic ontinent asalso eha ging as a rise i ea evelallowed hallow-water sedi entsto spread acros w .. t had previously been Ian area in theMidlands and South Eas ngland.By he la e Silurian. Iapetus nolonger existed. he d es or hLaur n ia and Sal ic ommo t(in luding. of cou e. e twohalve 0 ri ainl had erged to-

Fig. 14.10 Section of MOine Thrust area

..

Geological l . G . f d ' I P . bhistory 1'tlllii&1 ()WISllIn nelss orme In em y re-Carn nan1orridonifln Sandstone laid unconformably2 . on le isran II111llePre--Carnbrian

3 ea br i 11. hallow- ntOl s dirnents- I d unconformably a T rridonianMoil'll; Schists (originiilly dep(ISlled as dp.ep-waler offshore s dllnen4 la e reCarnbrian bu t OffCI:1Cd by orog ny III Ordovicinn li,nes and Ifrom ~ o u t h uast to rid up OVEr yOll er rocks)

Fig. 14.12 Iapetus in the Silurian periodKEY

D Deeper paof eontine[ashelfOceanic c/ \I { \ Mountain? 7 little infor

fo r this are

I+_ - - - -Ca l l J don i an Mountains---- ... I New nrea ofSection showingfinal closing inlate Silurian

sea to southof mountainI

SOLllillH>l Uplands Lak I01 Scotland l),slncl


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gether and raised the mountains ofthe Caledonian Fold Belt. However, these final phases of collisionand orogeny were not as powerfulas might have been expected.Lower Palaeozoic rocks of England,Wales, southern Scotland andmost of Ireland show only lowgrade regional metamorphism (forexample, Welsh slate) and although folding is obvious it doesnot usually involve overfolding orthrusting. Certainly the forces ofthis Silurian deformation were lessintense than those that had occurred earlier when the ScottishMoine and Dalradian groups wereaffected during the Ordovician.Perhaps the final stages of theorogeny were relatively 'weak'because, even as collision tookplace, the pattern of continentswas changing. A new ocean calledthe Rheic was by then beginningto open to the south of Britain.The total effect of the CaledonianOrogeny was to produce a range ofmountains whose features are stillevident today. (Note: by using theword 'total' we are including thecomplete series of events whichlead up to collision as well as theeffect of the collision itself.) Fig14.5 shows how fragments of theCaledonian Fold Belt can be seenin highland areas of Canada. Greenland and Scandinavia. Fig. 14.13shows the main Caledonian features in Britain. Notice that thetrend of the fold axes is north eastto south west; this shows that theoriginal pressure of the orogenymust have acted from the northwest and south east.

Fig. 14.13 Distribution of late Pre-Cambrian,Cambrian. Ordovician and SilUrian rocks, andthe effects of the Caledonian Orogeny

I, . ./" ~ : , _ : :' . . ' . , ,Faults /:. ,: : -1 Mmn Irust /::::." .. :.:.:.:.:.:

I. . . . . . .2 Gre I -1l1n T ar F ~ u l t / . ' " .' .' .'*3 Hi hland 80 ndary Faull /:.:.:. '. . , . , ' , .

4 OLlt I ' f ll UplJnd F ult 1:-:-:/ -------_/: : : : : : , : : : :Ar south of thiS line deformed - - .... :by I, I r VCJriS(;clll r04eny (Caledonian ~ slructLJrP-S now unlr ' (;8i-lble)

Rocks from these periods (now seenilurlan in Southern Uplands of Scotland. LakeDistrict. Wales and Southern I land)were folded but only slightlyJ ~ ~ ; " j Ordovician metamorphosed (some slates) durng. ",'. the final closure of Iapetus in theCalmaiCambnan Rocks from these periods (now seen

.................., ,fate Silurian/early Devonian. tren-__________________ in Scotllsh Highlflnds and Northern {NEIreland) w re folde ,intruded und-=::: Lale Pre-Cambrian I eglonally metamorphosed (schists).' - (1000 to 570 Ma) by bducliOn In the lower OrdOVician................. - .r - : - : - ~ l Arf! thoug to be underlain byL....... ~ ' J rAIn-Hns 01 Caledonian Mountains *Note that faults 3, 4 andhave moved considerabv Volc"nlc rocks their original fornt


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/ \/ \

R he ' c

/\ 15 S

/ \ / \1\

c \)~ O l \

1\/\/ \ / \

/ \ / \

of the

With he closing 0 lapetu , the Bal ic and Laurentianregi n became a singl la dmas known as theCale-onian Continent. T the outh of this, a newocean all d the R e c e n developing with part ofIts shor line extend in across southern Britain.

This episode traces the rory lhrough evonian andCarboniferous time un iI the eventual C osing of theRherc cear caused t e Var can Or09 ny by bringin S u hem urope into contact with the CaledonianContinent Th ougho 1e e ages climate had ani po ant effect on geology becau e, by that time,the 0 hward drift had carri Britain into equatoriallatitudes.Devonian events (395 0345 Ma: see Fig. 14.14)

or thi perio It is c nvenient to divide Britain by an-wes line rom Sri I to London. This marksth approximate sh reI ne of t e Rheic Ocean.

Northern re (land)This area was dominated by the newly formed mountains. he last phases of th Cale onian Orogenywere SLlll ta ing place w en he period began but. astime passe, rosion ecame the most importantprocess. Ouri 9 the v nia. , thou ands of metreso ower Pala OZOIC an late Pre- ambrian rockswere w m away.

ot all the erode sedim ,t reached the sea. Someof 't was deposited on la d to produce a series ofrocks nown as the Old Red Sandstones. A largeamount of I Red S ndstone was laid down by'vers lowing into lower Iyina area between themain peaks, Such areas ar calle termontainebasins. As 'g. 14.14, shows, one f th basins I yin he region f the Or ney n Shetland Islands.e e the e u nee includes mudstones which weree ited I lake (fo lis 0 resh-water fish haveeen ound. Another I te ontai e basin lay between th Hig land Boundary nd Southern Uplandsaults. As s n s one we e deposited here, thewhol area subsided to produce a !'itt valley stretching from central Scotland to nor e Ireland. Numerus volcanic eruptIons accompanied the subsidence.her deposi sold ed Sandstone were formedto he south of the mountains, 0 flood plains andde tas close 0 the Rhei sh reo These sediments,now found in p s of South Wales and southernIreland. s ow fe res sue as easonal depositionand dessication crack. hi Iden e supports theidea hat ri ain was then situated about 15-20south of th e uator an ad y Ii ,ited seasonalaInfall.

Fi . 74.14 Britain in the Devoni n period

Ighland ..: . , owland(areas of erosion) '" :1,'.: (ClreCls of deposition)A;(" Mountains ? ? Little info

Britain's position in th e Caledonian Continent

f? VolcaniC area

Equato

o\ C

r1 P-

160


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Fig. 14.15 Devonian sandstone at Duncansby Head near Johno'Groats, north east ScotlandSouthern area (sea)This was an area of continental shelf beneath theRheic Ocean but the exact shoreline must havevaried. At times the sea spread northwards to leavemarine sediments interbedded with the Old RedSandstones of South Wales. At other stages it musthave retreated south because delta deposits areknown from parts of Devon.The sea floor also seems to have varied in depth;deeper subsiding areas (for example, west Cornwall)received thousands of metres of muds and greywackes, while other regions (for example, northDevon) acquired shallow-water limestones with fossilcorals and brachiopods.Carboniferous events(345 to 280 Ma: See Fig. 14.17)Carboniferous rocks and fossils show that northwarddrift had moved Britain (as part of the Caledoniancontinent) close to the equator. It is convenient todiscuss this period in two parts: Lower Carboniferous. a time of warm seas andlimestones; Upper Carboniferous. a time of deltaic conditionswith gritstones and coal deposits.Lower CarboniferousAlthough the main Rheic Ocean still lay across thesouth of Britain, a rise in sea level meant that itswaters now spread slowly north over the remains ofthe Caledonian Mountains. This produced a majorunconformity with Carboniferous sediments beingdeposited on folded eroded Lower Palaeozoic rocks(see Fig. 10.27 on page 102).The warm tropical environment of the LowerCarboniferous sea favoured limestone deposition butthe pattern of sedimentation also depended on theshape of the drowned landscape beneath. Some high

areas remained as is and t rougho t and dreceive sediment Hor example, orgeThe deeper w t flo ad valleys and tended to accumul te beds of muddy limestonshale. Only in the really hallow and w r e s t reof the sea was U e grey/w ita limestone ormLower Carboniferou (sh How-water) lime can now be seen in he 0 hem Pennines.District, Mendips an much of central Ireland.provide atl ire, mg landscape WI h cave poand undergr und streams (Fig. 6.25 on p gethe rocks are well known or the fossils ( or exabrachiopods. rin ids and goniatites) lhey coThe presence of colonial corals nd algal ra fs sthat tropical condi ion x. te du -ng IS Ime.lim tone sea a f r lthough th preadScottish Rift Valle , thl P ieul area a'so resediment rom maining aledonran Mouto the north. As ime assed. the depo 'I Ion ofand muds pro uce a enes of dis alonshore.

Vo canic activity als 0 curre urin the Carboniferou . B sal war; rupted in cen ralland (for example, Arthur's ea In Edlnbu 9eroded remains of a vole nic one) and lavatuffs have been found In the Scottish or eDerbyshire.Fig. 74.16 Carbonif r u hmlSSl ne a dscape. no em Pe

161


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/ \. ~ : ; ~ ' "

1\

Sha I w-watwhite limesto

1\

La d

(now muc redu !!d in Si78 tlnd depth:soon to ci se at C'limax of Variscan Oroge y)R H E

KEYtboth lap ' ,,-:.:.'. l--'~

14.17 Britain In the Carboniferous period (both maps are very generalised a

E / C r' o----C:;, E _,--A ,-' N/ ._ , _ ~ r ~ >.-r ( / . ;-.:l ....

This area received of shore deep-water 5Ssediments throughout the p riod

o Volcanic orca/\;(' Mountains ? ? Little informa

R H t:'L IDellas ..Upper Carboniferous

sediments continued to be deposited ine Rheic Ocean across southern England but, overe rest of the country, conditions were changing. A

of uplifts (presumably early effects of thethe depth of water lying

of sediment to the northern deltas. As a result,deltas began to infill the shallow water andtowards St George's Land. The ad-'an interbedded sequence of

known asGrit. These rocks now give a distinctiveto the dark moorlands of the mid Pennines

Mid-way through the Upper Carboniferous, the

as St George's Land and the Caledonian Mounof northern Scotland: see Fig 14.17b) had bea vast swampy deltaic area of river channels,

was this environmentprovided Britain's most valuable geologicalCoal _easures.

Newly evolved species of land plants grew rain the hot wet equatorial climate, and the swaconditions allowed vegetation to sink and accumurather than rot away However, beds of coal (sewould not have been formed unless, every so ochanges in the pattern of deposition caused osediments to bury and preserve the layers ofmaterial.Fig. 14.18 Mill stone gflt landscape near Oldham, Lancashire

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- - - - - - - - - - - -

~ ~ ~ ~ _ ~

b ~ ~ ~ = = ~ - - - - - - - -

Shale

Shale -

Mudstone i 1 i i l ~ i a _I:: ~ o Coal seam

Sandstone

-----_._._.-

Fossils offrashwater Lakellagoonbivalves

To work out how the patterns of deposition werechanging we must study sequences of rock whichcontain coal seams. Although there are many slightvariations, Coal Measure sediments usually occur inthe order shown by Fig. 14.19. Look carefully at thisdiagram and note how each complete sequence ofrocks (called a cyclothem) can be explained in termsof the sediments produced by a delta building outacross a low-lying lake or lagoon. Notice also that thesame sequence of rocks is repeated again and againthroughout the Coal Measures; this suggests thatwhen deltas had developed to a stage where forestswere growing on their surface, they were eventuallyflooded over and the whole process of building outhad to begin again. Possible explanations for theperiodic 'drowning' of deltas include the following.

Fig. 14.19 Generalised sequence of Coal Measures sediments

1:: _Lake/lagoon~ ~ ~ ~ : ~ : : - - - _ - _ - _ l D _ e _ : _ . : _ : _ s w _ a n _ l _

Order of position

_Fossils ofplant f r i J ~ J m e l l t ! j

n l l c k n e ~ s varies trom2 to 30 metres

i l i l l i l ~ ~ ~ - ~ ~ ~ ~ ~ - ~ ] : ~ ~ : ~ ~ = ~ I,'toDelta

ubsidenoe. Mode n del as ten 0 ubsid and only remain b e wa er i enough sedi being supplied to k e bUilding them up. f heof sediment to part of he delta is re uceexample, by a storm altering the paUern of dtaries), then that ar is Ii ely to be 'drownebecome a lake or lagoRi e of sea level. In some yelot ems, marinesuch as brachiopod or fish have been found than the more usu I fre h-wa er ivalveslln hei mediately above t e oal sea . This evshows that some delta wer 'drowned' by silevel. Fluctuations sea level dunng he Carboniferous c uld have been caused by -Itperiods of freezin hawin in the ice which covered m ch of Go d a ala at ha(see page 1 7).

1equenp alabove

Ir nl;,equen


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Theariscan Orogeny: late Carboniferou learly Permian 290 to 280 M )This ra eny was caused ' - 'e cl sure of the RheicOcean brough Sou hem Europe into contact wi h t eCaledonian ontin nt The most powe ul d form lionoccurre in central France and southern Germany utits e f cls can als be learly een in ou h westBritai . (Note: par Iy ec e It has een studied nwr,itt n a U 111 several Europ a coun nes. heVariscan has two al erna ive names: the Armoricanor Hercynian.)As Fig. 14.20 ~ h o w s . B "tain1s mai V nsca foldsar , in Devon, Cornwall an he most ou them panso Wale and Irelan . Folds In th s . reas end to betight, vertical or e en overturn (se Rg. 10,10 onage 9 land haY Iheir axes running in a east-w stdirection. This evidence shows thar intense orces

must haY been acting north OUlh as Lhe orcom ressed Devonian and arboniferous against the edge 0 the Caledoni n Contmegional e orphism, he L'z8,rd Thrust f uthe south wes r nite b tholi provide urthed nee 0 Variscan nou'l In Dllidm In this areaNorth he main fol h It the defo Hanless intens nd the east-west fold rend lesvi lJS. Here. Variscan r rees ended t causemovements on older buried atedonian st ucwhich in tum produced Folding in the overlyingments. For exampl movemenl on l1ePeBloc Fa It s pllfted the centr of he area tduce a bra nicllne structure a d en weK:Jence o. e Scottis RKt Valley formed a syn

KFY

, itlle aero fTlatiort;y me s ti n almOSI Ilorizontal.-....fI---1!!-Synchne 9xis

/Mldland V i . l l l ~ " , Syncline . . ... Tm 5lJ I II__ ~ . . . . . . (In 'ift Villi V'

_----"n'rie Carboniferous/ ror. s 501lrt:e o./ ' GQmtl orth e.fl f.I as

InlrllSlOll of south w e ~ t 1:1


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Episo, .. itain a- p of rglandmass with ucto tlng ea levThe closure of the Rheic was part of s ne of platemovements which (by early in tl e Permian period)had brought all the world's cont'inents together as thhuge landmass called Pangaea (see page 147). Although Pangaea became separated into m lIer c n tinents during the Mesozoic era. it was not untilTertiary times (when the North A I 'ic began opening) that Britain was once again lace ear a properocean or an active plate margin.

Irom the Permian to th T rti ry Brit i geologjcal development was greatly influenced by i OSItion within a large continental unit. Although sea didperiodically flood the area, they were continental seasrather than real spreading and subduction oceans likethe earlier Iapetus or Rheic. (Note: a continental seais one which covers only continental crust, like, forexample, the present-day North Sea)Fig. 74.21

,l),,I...__ _"r ... "

-.\,.,I-'

,\I,IIr/

~ _ ...:.}, " , - - ,)

?ITTTm M 111 , FJ6 ilOV' ilil byImnw Z ch&l 111 Se c111f Ir1'1 Pnrnlli:l11+-- Wind IJir chon Ikflown If(.)rll

pall r ofcr05S IrlnlUlilti n)77 i rl Inforrn


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Cheshire and north east Yorkshire became sabkhaenvironments were halite was precipitated.Although a large amount of sediment was depo-sited in the lowlands during the Permian and Triassic,there was also a great deal of erosion during theseperiods. Both the Variscan Mountains of southernBritain and the remaining older Caledonian Mountainsof the north were greatly reduced in height.Jurassic events (195 to 135 Ma : Fig. 14.22)Conditions changed considerably with the start of thisperiod. Britain had now moved north from desertlatitudes to lie about 35 0 north of the equator (thelatitude of the present Mediterranean). and anothercontinental sea was beginning to advance acrossEurope.Fig. 74.22 Britvin In the Mid Jurassic. Sea level changes meantthat the coastline varied considerably during thiS period see textfor details

NORTHSEABASIN

o Volcanic area 7 7 Little InformalionA

By the early Jurassic this sea had reached souBritain and, as the waters deepened and snorth, fine muds were deposited. This sedproduced the Lias Clays which are famous forfossil ammonites, belemnites and marine resuch as Ichthyosaurs and Plesiosaurs. Good expoof these rocks can now be seen in the soft uncliffs of Whitby (Yorkshire) and Lyme Regis (Dsee Fig. 14.23) Sedimentary ironstones, suthose now found in Lincolnshire and Northamshire, were deposited in some shallower parts Lower Jurassic sea and a large island callelondon Platform occupied much of southEngland at this time.As sea level dropped during the Mid Jurasswaters across southern and central England beshallow and warm. These conditions produceoolitic limestones (page 70 and Fig. 7.15) can now be seen, for example, in the Cotswoldthe north, deltas spread into the shallow sea froremaining areas of highland. These deltaic sediare now found in North Yorkshire and parts of west and north east Scotland. They contain stones, shales and the occasional thin seam of

166


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By the start of the Upper Jurassic, sea level wasrising again. This time the sea reached i maXllTlUmextent for the period and deposited the Oxfo,rd aaysand the Kimmeridge Clays. Both have proved economically important: Oxford Clays are used for brickmaking (see page 69) and Kimm ridge Clays arethought to be the sour,ce rock from which muchNorth Sea oil has corne (see pa.ge 182).

Towards the end of the Jura sic another loweringof sea level took place. This allowed the London Fig. 74.24 Close-up of Portland LimestonePlatform to link with the northern highlands andcreate a continuous barrier of land. On one side of this was a sea where sandstones and mudstones C,etaceous ev:ents (135 to 65 Ma: Fig. 14.2were forming (this area was approximately in theposition of the present North Sea). On the other side The period began (as the Jurassic had endeof the land barrier a warmer shallower sea left lime two main areas of deposition Sands and dastones (for example, the Portland Limestone: Fig tinued in the North Sea Basin but by now, 14.24) across southern England. water lake had replaced the limestone sea sthe land barrier. This was the Wealden Lahe interpretation of Britain's Jurassic geology hasbeen greatly helped by the presence of ammonite atOund it a mixed sequence of river sands anzone fossils. (An example of Jurassic correlation is formed in a swampy environment. These segiven in Fig. 12.8 on page 118). contain types of plant and dinosaur fossilssuggest that Britain still had a very warm(latitude was then approximately 40 N).Near the end of the Cretaceous the land barriflooded over when sea levFig. 74.25 Britain in the CretClceo


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"

Cretaceous

.. WcBldc 11Cds--------_.

--'- ~ - - . Jura j .TertTriassicPennia

~ 00.0./0

Frg 74 7 Permian to Cretaceous sea leval c anges

Teniary events(65t02 a: ig14.28)Thi period began with yet another major lowerinsea I vel, and most of ritai emer ed as J dmore. However. the Nort Sea Basi emaine

rocess they event ally produced up to 1000 mof chalk. About 30 yea s was needed 0 give eachillimAtre.Calk i now exposed in the Chll ems, the Northand S Ih Downs and the 'whi e Ii fs' of south stngland. It i q" med for cemenl m ki 9 and arsopr vides supplies of undergroun at r (see page18 ). The rock" well nown for fossil echinoid nbivalve plu the oule and layers of flint r co

ins. orne flint proba Iy formed from th remainsea- loor span e w ich built theIr skeletons 0 silicSea level changeAs you will have realise, B I in 'ment ro the rmla to the Crec n rolled by the many c a ge 0 s a Ilace. Fig. "14.27 su mari es he e c anges In agraph and remln S yOl! 0 he mai rocks that wereforme. ossible xpJa a io f events Illay befo nd in the pi te movements w ich were occur ngat the Ime.

Pa e 48 sugge ed that world sea level is ffec tedby the n mbar of preading oceanic ridge and therat t Which they are d velo In ak"ng his i eain 0 account, i eerns probable at Me ozo c changesIn 'ea Ie el could hav been cause by s ages in thgrow h of new oceans h t were e sep ati 9Pangae into smaller co tinent . or co rse, h 0 r-ail pattern may have been omp icated y 0 her actor For xa pie. some contine al areas y havesubsi ed a d alowed he sea [0 100 over themwitho any c ange in i sown leve at all.

the waves, and here up to 3 km of sedimentsdeposited durin the period. Subsid nce and rthe older ocks some of which were ri h in orgremains) beneat this m terial is thought tocaused the formation of some North Sea oilpage 182). he exact coastline of the North Sea Bvaried during the Tertiary and a times marine ditions pread into sou h east ngland to derocks such as the Lo don Clay.

Oellas

M' rinesedime

Marinesedime

0 0 I .rll Ions r vole nie. , . . . . Will, contmental break up

~ II ) dr il af! r lerj by ILal rill rhe T rtiAfV~ , IhcAlplf '0 oglmy It I 1 lh t time show

168


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While sedimentary rocks werebeing deposited in the North Sea.the other side of Britain was affected by a constructive platemargin. As part of the break-up ofPangaea, the Atlantic Ocean hadbegun opening between SouthernEurope and America during theJurassic. By early in the Tertiary (60Ma), the spreading ridge had developed far enough north to beginsplitting Northern Europe fromGreenland. As this took place.magmas from the ridge rose intothe separating continental edgesSimilar igneous rocks of Tertiaryage can now be seen on eitherside of the North Atlantic. Thebasalt flows of Northern Ireland (forexample. Giant's Causeway) andthe volcanic rocks and intrusions ofScottish islands such as Skye andMull are of the same general ageand type as the igneous rocks ofeast Greenland.The opening of the North Atlanticended a 350 million year old geological link between Europe andNorth America. However. the newbreak was not exactly along theline of the Caledonian collisionwhich had originally brought themtogether (as the Baltic and Laurentian continents) in late Siluriantimes. In effect. Scotland andNorthern Ireland were 'left behind'on the European side.

Another series of Tertiary platemovements left its impression onsouth east England when, between25 and 30 Ma, the Alpine Orogenytook place. Britain was very muchon the fringe of this event so majorfeatures such as regional metamorphism or large granite batholithsdid not occur here. However, foldssuch as the Wealden Anticlinewere produced, and many of theolder fault lines moved again inresponse to the pressure, As Itsname suggests. the Alpine Orogenywas centred in Southern Europe; itwas brought about by the northward movement of Africa and oc-curred at about the same time asthe Himalayan Orogeny broughtIndia into contact with Asia.

Fig. 14.30 Alpine folding at t i 01, Dar el

KEY

,.

Wealdel1(AlpIn fo

and ert . ro

F"l1nlan, Tri S ~ J rr aSSiC, Cr I-BC


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--

Clim tic c ange and the Ice Agepisode 5: the final shaping of theBritish IslesThe opening of the North Atlantic and the AlpineOrogeny were the last major structural events toaffect Britain. By Mid Tertiary times the basic patternof solid (bedrock) geology had been f'irmly estab-lished. Since then the 'final shaping' has taken severalforms.Tertiary tilting and erosionThe combined effect of continental rifting on one sideand a subsiding North Sea Basin on the other pro-duced a general tilting of Britain towards the southeast. The rising western and northern areas suffereda great deal of erosion during the Upper Tertiary andtheir covering of Mesozoic sediments, such as thewidespread Cretaceous Chalk, was removed to re-expose older PalaeozoIc rocks below.r--,IL __~

M -

M

Centres of IceL. ......... .1 (highland)Maximum extent of iGeduring the most severeperiod of glCiciation(approx 400000yei'u'51lg0)

. . . . . . - - Main direction ofIce movement

.I MaXimum extent of Ice dUringJ last period of glaciation

By the end of the Tertiary, Britain had moved norabout its present lall u e. At this time a gelowering of world temperatures took place andIce Age began. It was a not single simple evduring the tw o illion years of OuatRrnary timesheets repeatedly advanced over large areas of \America and Northern Europe. Between glaci

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