Bob Flashcards
The rules of stratigraphy
Superposition
• Younger rocks overlie older rocks
Inclusion
• Younger rocks include fragments of older rocks
Cross-cutting
• Younger features (e.g. faults) cut across older features
Lateral continuity
• Units can be matched across latter discontinuity
Extrusion
• Volcanic rocks post-date units below and pre-date units above
Intrusion
• Intrusive igneous rocks post-date rocks that they cut
Original Horizontality
• Bedded rocks were deposited horizontal
Deformation & Metamorphism
• Post-date the affected rocks
Geological Historical Record
- Lithostratigraphy (lithology)
- Biostratigraphy (fossils)
- Chronostratigraphy (radiometric dating)
- Sequence stratigraphy (unconformities & relative sea level changes)
Overall Geology of Britain & Ireland
• Strong contrast between Carboniferous & older rocks (NW Britain/Eire) vs Permian & younger rocks (SE Britain & offshore)
• Younger rocks are only weakly deformed (Not much metamorphism), whilst older rocks are affected by a number of orogenic episodes
• Concept of basement (older, more deformed, deeper) vs cover (younger, less deformed, shallower) – includes offshore surrounding UK
• Rock record becomes increasingly fragmentary back through time: why?
• Older rocks tend to get buried below younger rocks
• Geological processes – especially those at plate margins – tend to recycle & rework rock sequences during later events, obscuring or destroying earlier history
• Increasing uncertainty in extending modern plate tectonic model back through time
o Pre-cambrian + Proterozoic uncertainty + Archean there were probably many differences
Caledonian orogeny
• The most important orogenic unconformity formed during the Caledonian orogeny culminating in Silurian-Devonian times
o This event welded together microcontinent of Eastern Avalonia (England, Wales, SE Ireland) to the margin of the Laurentian continent (Scotland, NW Ireland)
o Prior to Silurian UK was in 2 parts
o Caledonian Orogeny brought them together
o Caledonian orogeny does not deform all pre-existing rocks, so regions of weakly deformed older strata occur in foreland regions (Welsh borders, NW Scotland)
• Before Silurian times, these two margins had very different histories being separated by the wide Iapetus Ocean – probably as wide as modern Atlantic
Variscan orogeny
• Devonian-Carboniferous cycle of rock accumulation ends with Variscan orogeny affecting S Britain & Ireland, with large weakly-deformed foreland region to the N
• By early Permian, British crust was ~ assembled in its present configuration
• Rock accumulation in Devonian and Carboniferous terminated by Variscan Orogeny
o Brought about modern assembly
o No longer plate boundary but intra plate
Post-Variscan cycle
• Post-Variscan cycle dominated by periods of crustal extension, sedimentation & magmatism related to marginal rifting & eventual opening of modern Atlantic Ocean
• Major magmatic event in Palaeogene in NW Britain related to Iceland plume, with associated underplating of magma leading to differential uplift & tilting: forms regional unconformity in NW Britain
o NW Britain majorly eroded, and sediment washed into North Sea
• Neogene folding & basin inversion in S Britain related to Alpine orogeny in Europe
• Thus post-Variscan history reflects intraplate setting & record of events is related to adjacent plate margins
Global climate & sea-level controls
Continents separated = much spreading = much CO2 = greenhouse
- Relative sea-level changes estimated locally using sequence stratigraphy
- If an event can be correlated over a broad region, it may be a global, eustatic event
- First order fluctuation on 100Ma scale related to volume of MOR & ocean capacity
Global sea-level change
• Highstands (early Palaeozoic, late Mesozoic marine sequences) vs lowstands (Late Carboniferous-Triassic marginal marine & non-marine sequences) – continents dispersed
• Second-order sea-level changes (<10sMa scale) contoversial: origins unclear
Climate Change
• Pre-Quaternary atmospheric composition uncertain: C & O stable isotopes provide constraints
• Calculated CO2 levels correlated to continental arrangement & magmatism curves: lower levels when C locked up in Lst & coals
• Global temps determined from O isotopes & distribution of climatically sensitive sediments: higher temps to times of higher CO2 levels
• Icehouse/greenhouse only weakly felt in British Isles – not near the equator
• Limestones + coals = biggest CO2 stores
Biological evolution
- Diversity of organisms fluctuates through time
- Divided into 4 faunal types: ‘Cambrian’, ‘Palaeozoic’, ‘Modern’ & ‘Microfossils’
- Increase in diversity spasmodic, with 5 key mass extinction events: various causes
- Emergence of specific organisms led to appearance of major biological rock types: limestone, coal, chalk
- Emergence of land-plants stabilized land surfaces for first time & had major effect on continent-ocean sediment fluxes
Geological Influence in Britain
- British geology influences the landscape, patterns of human settlement/development & the distribution of natural resources
- Marked contrasts across the lowland-upland divide known as the Tees-Exe line which approximately defines region of W British Isles uplifted during the Paleogene
- Natural parks & livestock farming in NW vs arable farming in SE
- Main industrial cities located close to coalfields: this is a key historical influence
- Vernacular building materials (walls, roofing)
- NW is mountainous, mainly due to uplifting during the Cenezoic
What are resources?
• Resources: commodities of use to mankind
• Geological resources: rocks, minerals, hydrocarbons, soils, subsurface (ground) water & geothermal heat
• Unsustainable (non-renewable) resources are those used up faster than natural processes can replenish them
• Sustainable (renewable) resources occur where the rate of extraction is less than rate at which natural processes can replenish or recondition them
• Geol. resources mainly unsustainable: why?
o Geological processes and therefore time are very slow and old, but consumption is rapid
Construction materials
• Generally common rock types: hard rocks (slate, Sst, igneous); monomineralic (Lst, gypsum); mudrocks (clay, shale); unconsolidated (sand, gravel)
• These may be cut, crushed, kilned/fired or mixed with other products
• Main end uses concrete; roadstone; agrregate; bricks; cement
• Volumetrically speaking direct use of rocks as dimension stones or roofing is minor
o Facing stones are a rare example of rocks being used as is, most are altered
• Distribution of construction materials in British Isles: the NW-SE divide
Industrial & metallic minerals
• Industrial minerals are mostly monomineralic rocks exploited for their own properties & not for their contained metals: examples & uses…importance of purity
o Not exploited for what they contain
• Metallic minerals exploited for contained metals, e.g. oxides, carbonates, sulphides or native minerals: examples & abundance
o Exploited for what they contain
• Minerals with a low crustal abundance are rarer and therefore valuable, e.g. gold
• Epigenetic deposits form later than host rocks, e.g. hydrothermal activity
• Syngenetic deposits form at same time as host rocks, e.g. evaporites
• Distribution in British Isles: NW-SE divide
o Epigenetic in NW, where deeper rocks are exposed
Petroleum: processes
• Fluid (gas, oil) & solid (asphalt) phases
• Economic accumulations need coincidence of:
o Lithologies: Carbon-rich source rock, porous/ permeable reservoir rock + impermeable seal
o Processes: organic material undergoes thermal maturation to form fluid hydrocarbons which then migrate & accumulate in reservoir rock
o Geometries: to form an economic accumulation, you need a trap (structural/stratigraphic)
UK CS= UK Continental Shelf = its borders in the North Sea
Petroleum: North Sea plays
• Oil/gasfield examples: Mesozoic/Cenozoic basins mainly in offshore regions
o This period is when most of the oil accumulations in the UK come from
• Hydrocarbon plays: common combinations of good source, reservoir & seal rocks, e.g.
• Sources: Kimmeridge Clay (oil) (Upper Jurassic); Carb coal (gas)
• Reservoir: Sst in Permian, Triassic, Jurassic & Paleogene… & fractured Chalk and fractured basement
• Seals: Permo-Trias evaporites (Zechstein); shales in Carb, Jurassic & Paleogene; Chalk (only reservoir when fractured)
• N Sea fields reflect distribution of source/ reservoir/seal rocks + geometries
• Northern N. Sea (mainly Viking, Central graben) has mainly oilfields, while Southern N. Sea has mainly gasfields – why?
o Distribution of source rocks
o South has lots of Carboniferous coal sources = gas
o North is mainly Kimmeridge clay = oil
• Middle of North Sea = Mid North Sea High – no hydrocarbons = no accumulation as rocks not buried far enough
Coal
• Coalfield location controlled almost entirely by Carboniferous palaeogeography & evolution – exposed vs concealed coalfields
• Note that coal is easily main fossil fuel reserve in the UK compared to oil/gas!
• Coal is key source rock for gas in S N.Sea
o And in North Sea in general, coal that outcrops in UK is also under the sea
• 95% of fossil fuels in the UK is coal
Geothermal energy
3 main ways of extracting geothermal energy:
• Hyperthermal schemes
• Geothermal aquifers
• Hot dry rock schemes
• Pump water into hot dry rocks = fractures = produces hot water and steam
Geothermal resources globally significant (3% of present energy consumption) but potential in Britain may be limited?
• No volcanism or plutonism in UK
• New tech does mean it’s becoming more viable – SW
Water: the NW-SE divide
- Annual rainfall highest in NW Britain whilst water demand highest in more heavily populated lowland areas in SE
- Main groundwater aquifers are in lowlands, e.g. Lst in Carb, Permian, Jurassic & Cretaceous + Sst in Permo-Trias & Carb
- ~50% of supply in SE England extracted from groundwater
- By contrast, few aquifers in older rocks of upland regions where >90% of water comes from surface water
- NW rich in water; SE increasingly depleted
- Rainfall higher in NW; higher topography
- Water demand higher in SE
Geological hazards
• Globally, most important are earthquakes, volcanoes, landslides & geomedical hazards
• In UK, earthquakes are relatively small, there are no volcanoes & landslides minor
• Coastal erosion is a problem along S & E coastal regions of England, whilst ground subsidence due to dissolution or mining activity is important in many areas
o Soft rock in SE + SE sinking due to deglaciation tilt
o Dissolution is mainly from limestones and gypsums
• Main geomedical hazard in UK is Radon gas from decay of natural U/Th in granites, mudstones & evaporites
o SW hotspot
o Is a carcinogen; increased risk of cancer
Geological legacy of British Isles
• Britain has a very diverse geology – both in terms of rock type and stratigraphic age – & is blessed with substantial & diverse geological resources
• These resources of Britain and Ireland are a key influence in the historical development of the UK, especially:
o Industrial/Scientific revolution & the development of the British Empire (coal)
o Continued economic prosperity (oil, gas)
• How will our geological resources contribute in the future?
Global Plate Tectonics: Back to the Jurassic
Plate tectonics in reverse
• Back to the Jurassic (ca. 200 Myr), we can use magnetic stripes in oceanic crust to determine past plate motions
• As molten rocks solidify at mid-ocean ridges, they acquire the contemporaneous polarity of Earth’s magnetic field
o Once cooled past the curie point the magnetic signature is preserved
• Movements in core periodically cause polarity flips of magnetic pole
• Generates magnetic stripes in ocean crust of normal & reverse polarity: use these to track plate motions
• Symmetrical strips across spreading ridges
Global Plate Tectonics: Back to the Jurassic
• We can use magnetic stripes & fixed hot spot reference frame to track plate movements back over the last 200 Myr
• Clear links between continental movements & global geological events
• Cannot go back further: older oceanic crust subducted
Pangea to present:
• Africa, India & Australia sequentially converge on Eurasia with time
• These events generate collisions forming the three most important modern mountain belts: Alps, Himalaya/Tibet & SE Asia/SW Pacific
• Note also that many oceans open approximately along the lines of old orogenic belts, e.g. N. Atlantic
• This tells us that:
o Continental interactions complex in space & time
o Continental templates have irregular shapes
o Rates of relative motion vary
o Collision/rifting events can be superimposed
• Pre-200Ma, no oceanic crust, so record of motion is only preserved in continents..and..
• From Jurassic-present, British Isles is intraplate: the complex history is earlier
• British Isles small, whilst continental interactions occur in broad complex areas
The tools of palaeogeographic reconstruction
• Inclination of palaeomagnetic field to paleohorizontal (e.g. bedding)
• Climatically sensitive lithofacies
Distribution of palaeo-flora & -fauna:
• Climatically controlled biofacies
o E.g. if flora-fauna suits cold/warm water
• Effects of continental separation
Terrane tools: • Essentially contact relationships o To determine age of boundary between 2 continental crusts • Provenance linkage • Overlap sequences, • Stitching plutons • Welding metamorphism
Global palaeogeographic history (pre-550Ma)
- Plate tectonics ca 3 Ga, with modern process by 750Ma
- 3-4 supercontinents recognised + 1 superterrane, likely more
- Nuna (1.9-1.2Ga)
- Rodinia (1100-720Ma)
- Pannotia (630-530Ma)
- Controversial
- Pangaea (320-195 Ma)
- Only phanerozoic supercontinent
• + Gondwana Superterrane (since ca 550Ma)
o Not supercontinent but long-lived cluster
British Isles: Palaeocontinental setting
• LAURENTIA o Paleo-North America o Includes Western B. Isles • BALTICA o Palaeo-Scandinavia o Includes North Sea • GONDWANA
• Rifted Gondwanan microcontinents o Avalonia Includes Eastern British Isles o Armorica o Iberia • Caledonian closure of Iapetus Ocean – surture line • Variscan closure of Rheic Ocean – suture line • Note that BI are close to triple point
Neoproterozoic supercontinents
1000-630Ma:
• 2 supercontinents based on palaeomag & matching parts of coeval orogenic belts
Rodinia (1100-720Ma):
• Amalgamated during the Grenvillian - Sveconorwegian orogenies.
• Laurentia ringed by E Gondwana, Siberia, Baltica & separated fragments of W Gondwana: surrounded by Mirovoi Ocean
• Breakup by ca 720Ma.
• British Isles on opposite sides of supercontinent
Pannotia (630-580Ma):
• Subduction of oceans leads to amalgamation of Gondwana, Laurentia & Baltica during Pan African-Baikalian-Brasiliano orogenies
o Continental amalgamation
Subduction, breakup & arc collision
600-540Ma:
• Andean-type convergent margins (‘peripheral orogens’) formed around Rodinia & Pannotia, e.g. Cadomian–Avalonian belt of S British Isles
• Subduction, arcs, arc-basins, strike-slip tectonics but no collisions
• Ca 600-580Ma: Pannotia breaks up, heralding opening of Iapetus ocean as Laurentia, Gondwana & Baltica break up
• Produced long lived subduction without collision
o Strike-slip motion
o Period of extension
Arc-continent collisions & Gondwana breakup
540-460Ma:
• Series of localised arc-continent collisions occur around periphery of Iapetus, e.g. Finnmarkian (520-500Ma), Taconic (495-450Ma), Grampian (470-460Ma) + marginal basin closure & associated ophiolite obduction
o Island arcs open around Iapatus
• Avalonia rifts from Gondwana ca 475Ma, opening Rheic ocean & starting Iapetus closure
o Rheic opens behind
• Baltica rotates ACW and starts to converge with Laurentia
Iapetus closure & Caledonian orogeny
460-400Ma:
• Avalonia proximal to Laurentia by 450Ma, with collisions & closure from 425-390Ma…Caledonian ‘orogeny’ polyphase & complex
• Colliding margins NOT the same as those that rifted apart during Pannotia breakup
• Strong Laurentia-Baltica oblique collision ca 425Ma (Scandian orogeny), affecting NW Scotland/Eire, followed by lateral dismemberment of Laurentian margin by sinistral strike-slip faults
• Baltica collision = oblique
Acadian & Variscan orogeny: formation of Pangaea
400-300Ma:
• Series of collisions of Gondwanan microcontinents with Laurussia, e.g.Armorica (Acadian ca 400-390Ma) & finally Gondwana (Variscan 370-290Ma in Europe, Alleghenian in USA)
o Rheic begins to close
• Thus Caledonian-Variscan cycle is in many ways all part of one ca. 300Ma-long continuum initiated with the breakup of the Pannotia supercontinent & culminating in the assembly of a new supercontinent Pangaea
Pangea and after
300-180Ma:
• Pangaea consolidated until ca 250Ma, Palaeotethys nearest seaway to UK
• ACW rotation of Pangaea caused Britain to move progressively N from equator (late Carb) to 40ºN by Jurassic: increasing rifting activity
180-80Ma:
• Central Atlantic opens in Jurassic along rifts created during Triassic extn
• South Atlantic opens in Cretaceous, with ACW rotation of Africa & W propagation + narrowing of Neotethys ocean as complex strike-slip rifts
Opening of N Atlantic & the Alpine Orogeny
80-0Ma:
• In Late Cretaceous, opening of N Atlantic
• Africa continues to rotate, driving it northwards into S Eurasian margin (Alpine Orogeny)
• Britain continues to drift northwards
PHANEROZOIC SUMMARY
• Phanerozoic broadly records progressive northward drift of British Isles
o Britain starts in southern hemisphere and at high latitude
o Britain now at most northern point ever
• Cambrian-Silurian sees divergence then convergence of Laurentian & Gondwanan parts of British Isles
• Post-collisional N drift recorded by regional sedimentary facies: e.g. arid Devonian & Permo-Trias bracketing equatorial humid facies of Carboniferous
o Devonian + Permo-Triassic = red beds due to the arid environment from being at the tropics
o Carboniferous = when Britain was at the equator
• Changing host continents for British isles: Laurentia (Gondwana, Avalonia, Armorica), Laurussia, Pangaea, Laurasia, Eurasia
Post-Permian
no major orogenic episodes - subsidence dominant - a uniform tectonic history INTRAPLATE location
Pre-Permian
multiple orogenies – 14 fault-bounded terranes – a complex disjointed history PLATE MARGIN location
- Terranes assembled from Cambrian-Carboniferous times Caledonian-Variscan cycle
- Formed due to multiple collisions of varying intensity + regionally significant strike-slip, especially in Silurian-Devonian
Foundation & basement
Precambrian & Palaeozoic important for 4 reasons:
• Straddle Caledonian orogenic belt & Iapetus suture separating Laurentia & E. Avalonia
• Lie close to bend in Laurentian margin & collisional triple point
• Carry evidence of pre-Caledonian orogenic events
• Preserve northern margin of Variscan orogenic belt & relicts of Rheic suture
• Triple point = known for complex history
• Rheic suture = closure of Rheic Ocean; final assembly of Pangea; seen in Lizard in Cornwall
Reconstructing an incomplete puzzle
3 factors make Caledonian & pre-Caledonian reconstructions difficult:
• Caledonian & older rocks either obscured by cover or reworked during later events; many are unfossiliferous (??ages)
• Hard to correlate with other rocks
• Complex, multiple, diachronous collisions
• Occurrence of one or more phase(s) of major orogen-parallel strike-slip movements which have sliced-up orogenic belt making reconstruction of the plate boundaries extremely difficult
• Suture parallel strike-slip; hard to re-construct with large-scale displacement
Terrane map of the British Isles
- Use major strike-slip or thrust faults to delimit crustal blocks with coherent internal histories (terranes)
- Bounding faults often reactivated with long histories of movement, but final large events are typically Silurian or Devonian, at least in area N of Iapetus suture
- Like fault-bounded tectono-stratigraphic terranes recognized in Mesozoic-Cenozoic W Cordillera of N America, but displacements not so large?
- Most boundaries are thrust faults or strike-slip faults
Two fundamental groups of terranes:
• Those N of the Solway Line (= Iapetus suture) which are thought to have Laurentian affinities
• Laurentian terranes = palaeo N. America
• Those S of the Solway Line (= Iapetus suture) which are thought to have Gondwanan affinities – Gondwana terranes
Pre-750Ma rocks of Laurentia
• In Hebridean, N. Highland & Highland terranes
• 4 main rock units (listed by age):
o Lewisian Complex: (ca. 3100-1500Ma) Mesoarchaean to Palaeoproterozoic gneisses
o Rhinns Complex: (ca.1700-1900Ma) Palaeo-proterozoic basement of C. Highlands Terrane
o Torridonian: (ca. 1200-1040Ma) At least two different sequences of Late Meso-to Neoproterozoic continental sedimentary rocks: little deformed, u/c overlie Lewisian
o Moine Supergroup: (ca. 900-1040Ma) Intensely deformed Neoproterozoic metasedimentary rocks that u/c overlie ‘Lewisianoid’ basement
• One other key event in this time period is the Grenvillian orogeny (ca. 1100Ma): = Rodinia assembly
o Record in British Isles shows this very poorly; reflects Britain is so small
o Lewisian, Moine & Rhinns affected by Precambrian orogenies of various ages; Torridonian is ‘post-tectonic’ cover
o Metamorphic rocks affected by orogeny’s
o Lewisian + Moine is a complex and fragmented record
o Complex histories of Lewisian & Moine ascribed to successive subduction, collision & rifting events
o Present structural configurations mainly due to Ordovician-Devonian events, e.g. Moine Thrust, Great Glen terrane-bounding faults
Lewisian Complex
o Oldest UK rocks: classic basement complex
o High grade (granulite-amphibolite facies), strongly deformed metamorphic rocks that originated as mostly acid-intermediate plutonic rocks, with some metasediments & metavolcanics
o Forms Laurentian basement as far SE as GGFZ trace? Lewisian ‘inliers’ in Moine
o Traditional model: single piece of Archaean crust heterogeneously reworked during later down-T orogenic & rifting events during later Archaean to Proterozoic
o Alternative model: Various regions of Lewisian are a collage of Archaean & Proterozoic terranes
o Final assembly of mainland blocks ca. 1750Ma (Laxfordian)
o History of Outer Hebrides different: no ca.1750 event recognized & final juxtaposition with mainland may be as late as Grenvillian (along OHFZ?)
o Down-temperature evolution reflects progressive exhumation of complex through time: at or near surface by 1200Ma
Rhinns & Annagh Gneiss Complexes
o Poorly exposed basement rocks of Central Highlands Terrane in Scotland & Ireland
o Metamorphosed & weakly deformed, calc-alkaline arc rocks intruded ca. 1780Ma (Rhinns) – 1900Ma (Annagh Gneisses)
o Correlated with belt of ca. 1900-1600Ma rocks formed along Andean-type plate margin in N Atlantic region: Ketilidian of S Greenland, Svecofennian of Scandanavia
Torridonian: Stoer Group
o 2 distinct, undeformed units of non-marine sedimentary rocks, mostly fluvial/alluvial red beds with marked irregular basal u/c’s
o Stoer Group (ca 1200Ma): localised syn-rift seds related to early Laurentia-Baltica split; pre-Grenville orogeny (ca. 1100Ma)
Torridonian: Torridon Group
o Torridan/Sleat Groups (ca 1040Ma): extensive syn- to post-rift alluvial fan deposits derived from Minch Fault scarp
o Both ~ same age as Moine, but different sources (detrital zircons) & sedimentary environments suggest deposition in different basins (see below)
Moine Supergroup
o Thick sequence of strongly deformed/ metamorphosed sedimentary rocks originally laid down in shallow marine environment on Lewisianoid basement ca. 1000Ma
o Shallow marine sandstones + mudstones which are very seriously deformed
o Morar, Glenfinnan, Loch Eil groups
o 3 orogenic events recognised: Knoydartian (820-725Ma), Grampian (475-450Ma) & Scandian (435-415Ma)..complex overprinting
o Main regional structures Scandian (eg MTZ)
Moine = Torridonian?
o Moine basin hard to reconstruct due to complexity of deformation & rather monotonous, unfossiliferous nature of successions…here is one suggested correlation
o Collectively part of the foreland basin fill to the Grenvillian orogen?
o 4 main rock units preserved in Scotland and Ireland
o Lewisian Complex (3100-1500 Ma)
o Rhinns Complex (ca. 1700-1900 Ma)
o Torridonian (ca. 1200-1040 Ma)
o Moine Supergroup (ca. 900-1040 Ma)
Three are metamorphosed rocks that form likely basement of Hebridean, N. Highland and Central Highland terranes, whilst Torridonian rocks are relicts of an undeformed cover sequences
Links between these units are partially known, but incomplete due to burial beneath younger strata, younger orogenic overprinting and fragmentary nature of geological record
Palaeoproterozoic regional setting well understood, but key Mesoproterozoic problem is extent of ca. 1100Ma Grenvillian event which enigmatically preserved in British Isles
The British Isles divided: the margins of Iapetus
- During the early Palaeozoic, the Iapetus Ocean separated Laurentia (including Scotland+NW Ireland) from the Avalonian microcontinent related to Gondwana (including England, Wales & SE Ireland)
- The two margins of the ocean preserved in the British Isles had very different origins & histories up until ocean closure in the Silurian-Devonian (rift/passive margin vs subduction/strike-slip/non-volcanic)
The main evidence for Iapetus
- Major differences in the age & character of the Precambrian basement
- Contrasts in Camb-Ord sedimentary facies: warm carbonate-rich (Scotland) vs cooler carbonate poor (SE Britain)
- Faunal contrasts (trilobites/brachiopods) over same time period
- Palaeomag data: low S hemisphere (NW Britain) vs high S hemisphere (SE Britain) lattitudes
- Other evidence ambiguous (ophiolites, arcs, accretionary complexes, etc)
- Arcs at the edges of the ocean
The Laurentian Margin
• A passive margin formed by rifting apart of Pannotia in latest Neoproterozoic time (580-550Ma)
• N Britain on Laurentian promontory
o N Britain on Laurentian promontory - A bend in the margin
• Ocean opening followed long period of episodic rifting which began ca 750Ma (when Rodinia broke up) & continued until the early Ordovician…2 main successions
• Dalradian Supergroup (outboard, SE)
• Cambro-Ordovician shelf succession (inboard, NW)
• Two successions now closer due to crustal shortening during later orogenies
The Dalradian Supergroup
- Thick (up to 25km) Neoproterozoic-L. Ordovician succession of sedimentary rocks located in C. Highland Terrane
- Similar successions elsewhere along Laurentian margin (East Greenland, Newfoundland)
- Deformed (folding, shear zones) & metamorphosed (greenschist-amphibolite facies) mainly during Grampian Orogeny (ca. 470-460Ma)
