Bob Flashcards
(108 cards)
1
Q
The rules of stratigraphy
A
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
2
Q
Geological Historical Record
A
- Lithostratigraphy (lithology)
- Biostratigraphy (fossils)
- Chronostratigraphy (radiometric dating)
- Sequence stratigraphy (unconformities & relative sea level changes)
3
Q
Overall Geology of Britain & Ireland
A
• 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
4
Q
Caledonian orogeny
A
• 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
5
Q
Variscan orogeny
A
• 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
6
Q
Post-Variscan cycle
A
• 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
7
Q
Global climate & sea-level controls
A
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
8
Q
Biological evolution
A
- 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
9
Q
Geological Influence in Britain
A
- 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
10
Q
What are resources?
A
• 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
11
Q
Construction materials
A
• 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
12
Q
Industrial & metallic minerals
A
• 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
13
Q
Petroleum: processes
A
• 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
14
Q
Petroleum: North Sea plays
A
• 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
15
Q
Coal
A
• 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
16
Q
Geothermal energy
A
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
17
Q
Water: the NW-SE divide
A
- 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
18
Q
Geological hazards
A
• 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
19
Q
Geological legacy of British Isles
A
• 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?
20
Q
Global Plate Tectonics: Back to the Jurassic
A
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
21
Q
Pangea to present:
A
• 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
22
Q
The tools of palaeogeographic reconstruction
A
• 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
23
Q
Global palaeogeographic history (pre-550Ma)
A
- 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
24
Q
British Isles: Palaeocontinental setting
A
• 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
25
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
26
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
27
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
28
Post-Permian
no major orogenic episodes - subsidence dominant - a uniform tectonic history INTRAPLATE location
29
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
30
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
31
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
32
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
33
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
34
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
35
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
36
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)
37
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)
38
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?
39
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
40
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)
41
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
42
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
43
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)
44
Basement & long-lived basins
* Often complex sequences of marine sands, silts, muds & limestones deposited on basement of Moine (NE Scotland) & Rhinns Complex (SW Scotland-NW Ireland)
* Sub-divided into Grampian*, Appin*, Argyll & S. Highland groups (* = largely unfossiliferous)
* Absolute age range: ca 750-470Ma?
* Thick, complex sedimentary succession, mainly marine
* First two are not very fossiliferous
* Broadly a series of superimposed, fault-controlled rift basins/sub-basins
* Many resulting fluctuations in water depth & lateral thickness/ facies variations due to faulting, especially in Argyll Group
* Failed ocean opening event is proceeded by large scale faulting as it prepared to rift open
45
From the icehouse to Iapetus
Key events:
• Port Askaig Tillite: formed during mid-Neoproterozoic Sturtian glaciation ca. 710 Ma – globally recognised event (‘Snowball Earth’)
• During Argyll Gp times, basin became increasingly unstable, culminating with eruption of Tayvallich basaltic volcanics & associated feeder dykes/sills (SW Scotland)
• Dated at ca. 600 Ma: thought to correspond to failed ocean opening event; Iapetus opens later & to SE
• Tayvallich basaltic volcanics due to lithosphere preparing for ocean rifting but it failed and eventually opened further SE
46
Iapetus passive margin: outboard
* S Highland Group are late Neoproterozoic-L. Ordovician deep-water marine turbidites laid down on outer rifted margin of Laurentia
* Fault-bounded rift basins reflect tectonic stretching of the continental lithosphere
* Basaltic volcanism reflects partial melting of upwelling asthenosphere during continental rupture…like many Phanerozoic passive margin sequences
47
Iapetus passive margin: inboard
* Meanwhile, Cambro-Ordovician shallow marine shelf sequence Very planar basal u/c
* Qtz Sst, shale & thick upper unit of Lst up to 1.2km thick (1km is Lst): Arenig-Llanvirn fauna, Lst suggests low lattitudes
* Recognised along entire Laurentian margin
* Overall transgressive sequences, with partial retrogressive event (Salterella Grit)
* V little relief: no rifting?
* Thin succession of clastic rock and then a thick succession of limestones – limestones suggest low lattitudes
* Not rift related, just gradual thermal subsidence
48
The Late Neoproterozoic: world in turmoil?
* About 2.3 to about 0.55 Ga –
* Previous to this, reducing atmosphere, no real tectonic plate resemblance until 4.2 Ga
* Uranium deposition and anoxic palaeosols and BIFs
* After this:
* Huge oxidation event, appearance of red beds and global ice ages
* 2 large ice ages =
* Cryogenian = continuous snow
* Tillites and carbon isotope data suggest ice ages
* Continents were together twice, Rodinia and Panotia = very little spreading ridges, very little volcanism = thin atmosphere – lots of continental crust = lots of mountains = lots of weathering = lots of CO2 inake = global cooling =snow = more albedo
* Probs some Milankovich cycles playing a role
* Runaway? Earth is a semi-self-regulating system
* Supercontinents broke up twice – releasing lots of CO2 and maybe methane = warming + less weathering of continents
* Glaciations drove Cambrian explosion
* Close relationship between tectonics, climate and biology
* Cambrian explosion – modern planet emerging
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The SE Margin of Iapetus
* This margin preserves fragments of long-lived, subduction-related Neoproterozoic volcanic arc (plutons, volcanics, sedimentary rocks) on Pannotia supercontinent margin
* SE Britain (Avalonia) lies along-strike of Armorica, which includes Channel Islands
* Margin faced open ocean as far back as ca 750Ma where it lay on periphery of Rodinia
50
Eastern Avalonia
* 2 Neoproterozoic crustal blocks: Avalon & Monian-Rosslare terranes separated by a major fault zone – the Menai Strait Line
* Avalon Terrane: seen as series of inliers along long-lived fault zones:rocks generally formed ca 700-560Ma – no basement seen
Monian-Rosslare Terrane: metamorphic rocks (inc blueschists) & sediments (Monian Supergroup) spliced-up by ss faults
Ages (670-550Ma??), inter-relationships & origins very unclear – as is link to Avalon Terrane
51
Long-lived subduction model
* Generalized subduction model for Avalonian (-Cadomian) belt
* 680-600Ma: main phases of arc magmatism
* 570-550Ma: convergence increasingly oblique, eventually terminating subduction & slicing-up of forearc into sinistrally offset slivers with associated strike- slip basin development
* Analogues with late Palaeozoic-Cenozoic evolution of W Cordillera of N America?
* Strike-slip continues into Cambrian?
* After this the BI stop being a margin and volcanism stops
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All quiet on the Avalonian margin
• For all of Cambrian*, margin was quiescent, founded on dismembered, eroded & subsided Neoproterozoic arc (*ca 542-492 Ma): a ‘non-volcanic continental margin’
o Arc stops and gradually subsides – non volcanic continental margin
• In earliest Ordovician (Tremadoc) times, the appearance of volcanism reflects onset of subduction of Iapetus
• In general, Cambrian was a time of rising sea-levels due to break-up of Pannotia supercontinent & global warming
• This helped facilitate rapid diversification of marine invertebrates: ‘Cambrian explosion’
53
Basins & (old) faults
* 4 distinct regions separated by old faults: Midland Platform – Welsh Borderland Fault System – Welsh Basin – Menai Strait Fault System – Irish Sea Platform* – Wexford Boundary Line – Leinster/Lake District Basin (* = no Cambrian seds – structural high)
* NW deepening towards Iapetus
* Palaeogeography: rising sea-level & localised rifts, with at least three phases now recognised
* Significant Cambrian (?) & later strike-slip movements along bounding faults
54
The ‘Caledonian Orogeny’: summary
A complex plate collision involving NW-SE foreshortening + sinistral shear affecting area from the Outer Hebrides to the Welsh Borders….4 components:
• Ordovician Grampian event: marginal arc collision with Laurentia on Iapetus margin
• Scandian thrusting (mainly in N Highland Terrane): Laurentia-Baltica collision)
• One or more phases of major Sil-Dev orogen-parallel strike-slip along Laurentian margin
• Ord-Silurian accretion in SUT (Iapetan events) followed by Sil soft-docking & emergence
55
The British Isles divided: the margins of Iapetus
* Early Palaeozoic Iapetus Ocean separated Laurentia (NW Britain) from Avalonia, which was part of Gondwana (SE Britain)
* Max width of Ocean ~3000km? (early Ord)
* The two margins of the ocean preserved in the British Isles had very different origins & histories up until ocean closure in the Ordovician-Silurian: the ‘Caledonian Orogeny’
56
Avalonia crosses the ocean
• In early Ord. (Tremadoc), the appearance of volcanism reflects onset of southward subduction of Iapetus
o Calc-alkaline volcanism at subduction zone – arc magmatism
• Then, E. Avalonia rifts away from Gondwana & migrates towards Laurentia, closing Iapetus & opening Rheic Ocean in its wake
o Often the case – as one ocean closes, another opens
• Faunal/palaeomag evidence suggests E. Avalonia proximal to Baltica by late Ord. & to Laurentia by early Silurian
• Iapetus closure involved subduction at both margins & periods of arc magmatism
57
Ordovician Avalonia: volcanoes
• Marine margin, with fault-controlled basins & platforms - like Cambrian, NW deepening towards Iapetus
• Tremadoc-Caradoc: 2 types of volcanism in Welsh, Leinster & Lakes basins
o Calc-alkaline (andesitic) arc-type
o Bimodal tholeiitic/acidic marginal basin-type reflecting lithospheric extension in back-arc setting
• Centres shift around in space & time, but cease in mid-Caradoc: E. Avalonia over-ran Iapetus spreading ridge or Baltica‘collision’??
58
Ordovician Avalonia: basins
* Patterns of crustal uplift & subsidence driven by lithospheric stretching or by thermal effects adjacent to volcanic centres in transtensional, supra-subduction setting
* Superimposed upon global sea-level changes, including regression in Ashgill due to ice age
* Overall – transtensional supersubduction zone (over a subduction zone)
59
Ordovician Laurentia
* By Early Ord, SE-dipping, intra-oceanic subduction zone & volcanic (Midland Valley/Taconic) arc developed outboard of Dalradian passive margin
* Arc terrane fragments, with ophiolites exposed below younger rocks in Midland Valley Terrane & forms basement of whole terrane
* Collision of this arc terrane with Laurentian margin caused Grampian Orogeny (ca 475-460)
* Associated with regional ophiolite obduction (Shetland, Tyrone etc)
* More complex – why more fragmentary
* Intra-oceanic subduction zone
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The Grampian Orogeny
* Orogeny caused extensive NW-SE foreshortening (folds, shear zones) & associated greenschist-amphibolite Barrovian metamorphism in Central Highlands (Dalradian) & N. Highlands terranes (Moine)
* Collision followed by switch to NW-subduction beneath Laurentia in Late Ord.
* Analogue: Banda Arc-Australia collision
61
(Post-Grampian) Midland Valley Terrane
* Grampian mountains formed source region for basins to SE along active plate margin…but….
* Reconstruction during Ord-Sil is difficult, as much is buried, strongly deformed or sliced-out by later strike-slip faulting
* Also N. Highlands/Hebridean terranes lay along strike & to the NE, facing Baltica
* Midland Valley: Ord-Sil marine sediments infill fault-controlled intra-arc basins founded on Taconic arc: new arc location?
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Southern Uplands Terrane (SUT)
Thick sequences of deep marine sedimentary rocks
3 fault-bounded tectonostratigraphic units: Northern, Central & Southern belts
• Northern belt: Ord seds & rare volcanics
• Central belt: Ord & Sil sedimentary rocks
• Southern Belt: Silurian sedimentary rocks
• SE structures show theres an arc
• Tracks – long pieces of seds bounded by faults – separated into belts
• Sequences of cherts, shales and tubidites in that order
• Southern belt least well preserved – buried under young rock
• Rocks getting younger toward south
* All 3 belts are sub-divided into a series of laterally persistent tracts bounded by major top-to-the-SE thrusts &/or sinistral faults
* Stack of thrust sheets which get younger to SE: bedding in each individual tract youngs NW
* Regionally younger to SE but in each thrust sheet they young to NW – accretionary prism
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SUT: Accretionary prism model
* Main model to explain gross stratigraphic & structural characteristics of SUT is based on modern accretionary prisms
* Form in forearc region by progressive off-scraping of ocean floor sediments at top of subduction zone
* Tracts internally deformed during progressive accretion from NW to SE: diachronous defm.
* In Central & Southern belts, deformation becomes (sinistrally) transpressive: change to more oblique subduction in mid-Sil?
* Based on modern accretionary prisms in modern subduction zones
* Within each scraped off thrust sheet the tracts young away from the subduction zone
* Tracts get deformed by scraping off process – diachronous defm
* Transpressive – collisional deformation and sinistral strike-slip – due to it becoming oblique
* South Uplands is blue prism
* Arc unknown location – maybe sliced out by strike slip, maybe buried below southern uplands
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Meanwhile…inboard & along-strike
* In mid-Silurian, sinistral-oblique Laurentia-Baltica collision produces major Scandian orogenic event (ca 435-415Ma)
* Recognised in Greenland, Scandanavia & N Highlands + Hebridean terranes
* At this time, N Highland/Hebridean terranes faced Baltica whilst terranes to SE did not..later juxtaposed by sinistral ss!
* Scandian orogenic event - hard collision – significantly oblique – did not affect Eastern side of Scotland
* Scandian collision in the West and then siliurian strike slip brought Scotland together
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Meanwhile: in Eastern Avalonia
* In Sil, E. Avalonia underwent soft-collision with Laurentia: unlike hard collisions of W. Avalonia & Baltica along strike
* Fault-controlled Ord-Sil marine rift basins on Avalonian margin experience a diachronous influx of northerly-derived turbidites due to deepening
* Reflects crustal flexure as it was over-ridden by edge of Laurentia
* Followed in latest Sil by diachronous basin shallowing leading to non-marine conditions & emergence during final docking
* Turbidities migrate SE due to crustal flexure
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The end of Iapetus
* Early Palaeozoic Iapetus Ocean separated Laurentia (inc. Scotland+NW Ireland) from Avalonian microcontinent related to Gondwana (inc. England, Wales & SE Ireland)
* Iapetus closed by Ord-Sil (485-420Ma) ‘Caledonian Orogeny’
* Polyphase event involving ‘hard’ collisions (Grampian, Scandian), marginal accretion (Iapetan), soft-collision (Laurentia-E Avalonia) & major orogen-// sinistral strike-slip
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Early-Mid Devonian: NW Britain
• Iapetus-related marine basins largely uplifted by shortening during Caledonian orogeny: part of Laurussia (‘ORS Continent’)
• Arid environment: 0-20S latitude
• ORS – Old Red Sandstone
• Britain is mainly in Laurussia
• Fluvial & alluvial sedimentation occurred in 2 distinct intramontane basins:
o Strathmore/Midland Valley Basin: bounded by Highland Boundary & Southern Uplands faults
o Orcadian Basin: formed due to collapse of Scandian orogen? Significant lacustrine deposits
• Sinistral transtension & granitic plutonism/volcanism very important in early Devonian
• Strike slip displacement along the major basins
• Volcanism = slab breakoff
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Early-Mid Devonian: S Britain
* Fluvial & alluvial sedimentation in Anglo-Welsh Basin sited over old Midland Platform
* Pass S into marine Cornubian terrane of SW England: rifted continental & oceanic crust (Lizard-Start ophiolites)
* Bounded to N by Carboniferous dextral ss Bristol Channel-Bray faults: terrane located ~400Km SE of current position?
* Only region with continuous Devonian sedimentation history
* Fluvial-marine transition obscured by later Variscan deformation
* Bristol Channel-Bray fault zone – completely buried, not even well imaged – must have been there – SW England used to be approx. 400 km south-east – significant strike slip displacement to where it is now
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Early Devonian: Acadian collision!
* Major early Devonian sinistral transpression forming folds & cleavage during the Acadian Orogeny (ca 400-390Ma). Low metamorphic grades
* Regional uplift & unconformity development, but not in NW Scotland or Cornubia
* Wales and Northern England transpression
* Very short orogeny – very low metamorphic grades
* Far north of UK – Orcadian + SW England in the Cornubia Basin was not affected – no evidence – supports that SW England was elsewhere and there was significant displacement – deformation came from south it would have to have been affected
* Arcuate deformation patterns reflect basin inversion & ‘moulding’ of structures around basement structures
* NOT related to Iapetus…may result from collision of ocean ridge in Rheic Ocean or of Armorica? Requires N subduction
* Acadian - Sinistral transpression
* Also, the Brabantian but not seen in the UK
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Late Devonian: sea-level rise & rifting
Acadian topography peneplaned, with fluvial-alluvial sedimentation in some mid-Devonian emergent areas
Mid-Late Devonian rifted marine margin in Cornubian basin to SE
Thrust stacking of S Cornubian basins underway, including emplacement of Lizard ophiolite (ca 390-365Ma)…onset of the Variscan orogeny (…see next lecture…)
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Early Carboniferous: back-arc extension & basin formation
N-dipping subduction zone established on N margin of Palaeotethys
• Ocean
• Subsidence in N Britain due to back-arc rifting (N-S extn) + global sea level rise
• SW England & Europe: next week
• Blue box this week
• Subduction leads to back arc extension
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Early Carboniferous: back-arc extension & basin formation
* 3 elongate regions of Carb. sedimentation (‘provinces’) separated by structural highs:
* Scottish Province – Southern Uplands high – Pennine/Irish Province – Wales/London/ Brabant High – Southern Province/Culm Basin (to SE)
* Broadly, Earlier Carb. rifting → Later Carb. thermal subsidence (‘sag’) & filling-in
* Shelves with deeper basins – caused by structures in the basement
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Basin structure, infill & magmatism
* Provinces: shallow shelves & deeper fault-bounded basins controlled by basement structures & old plutons at depth
* Regionally, clastic sediment supply from surrounding landmasses initially low, so carbonates dominate Early Carboniferous
* Persistent intrusive/ extrusive magmatism related to rifting & asthenospheric melting
* Shallower regions held up by basement Caledonian granites that make them more buoyant
* Sediment supply was scarce – dominated by carbonates
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Basin structure, infill & magmatism
* In Late Carboniferous, uplift of landmasses led to development of major southward-advancing fluvio-deltaic systems
* Deltas prograded from NE to SW
* Whole area was filled in and became essentially a swamp
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Cycles & controls on sedimentation
* Carboniferous carbonates/siliclastics characterised by cyclicity: cyclothems
* Mixture of sedimentary processes such as delta switching, river avulsion, etc + external factors such as tectonics, climate, sea-level change & sediment supply
* Important ice sheet developments in S hemisphere in Gondwana: cylcic growth & melting may cause sea-level fluctuations
* Equatorial location of British Isles also means that seasonal factors are also likely to be important, e.g. monsoonal cycles?
* Coal cyclothem on bottom right
* Britain equatorial – monsoonal cycles and sea level fluctuations from deglaciation and glaciations affecting this
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Old King Coal
* Carboniferous is first period where hydrocarbon rocks of major economic importance formed in British Isles: coal
* Important solid fuel & source for natural gas accumulations in the S North Sea
* Deltaic environments ideally suited to development of coals
* Location of coalfields is controlled by Carb. palaeogeography (i.e. in the main basins)
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So….the (messy) end of Iapetus……
* Iapetus closed during Ord-Sil (ca 485-420 Ma): the so-called ‘Caledonian Orogeny’
* Not a very meaningful term as it was a complex polyphase process involving ‘hard’ collisions, marginal accretion, soft-collisions & major orogen-// sinistral strike-slip faulting: different places, different times
* Followed by enigmatic Acadian event (ca 400-390Ma)
* During Dev-Carb, SW England lay to the SE, adjacent to N France; only later juxtaposed by dextral strike-slip faulting
…but not the end of orogenies
• Not the final stages in assembly of Pangaean supercontinent
• This occurs in end-Carboniferous Variscan orogeny
• HOWEVER, Variscan events are not well-preserved large part of the British geological record, where Devonian-Carboniferous is a period of sediment accumulation in basins
• But collisions still happening in NW Europe from 390-300Ma with fragmentary evidence preserved in SW England
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Caledonian-Variscan: when does one end & the other start?
* Separation of these ‘orogenies’ is a geographic accident
* Both were related to the assembly of Pangaea & involved a long-lived, progressive cycle of terrane amalgamation
* Post-Carboniferous fault displacements are small & geological relationships are coherent compared to pre-Permian times
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Variscan (Hercynian) Orogeny
* The Great Divide: Carb-Permian boundary
* Lasted 100Ma, began in Devonian, climax in Late Carb. & ended in Early Permian, with final assembly of Pangaea supercontinent
* S of Variscan Front in Britain, produced fold & thrust belt at low metamorphic grades: higher-grade ‘core’ in Spain, France, Germany & Czech Republic
* Part of much bigger orogen stretching from Gulf of Mexico to Eastern Europe
* Broad, curvilinear belt with elongate zones separated by steep faults
* Steeply dipping strike slip fault
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Sutures & structures: regional
• Some zone boundaries are marked by Devonian/Carb. ophiolite assemblages & arc volcanics = sutures marking presence of small oceanic tracts between microplates
• Locally, relicts of older (Cambrian-Silurian) ophiolites occur related to microplate rifting
• Series of collisions – microplates collide before final collision between A and Laurussia
• Other key features:
o Outward tectonic transport
o metamorphic symmetry
o major ss faults (dextral & sinistral)
o weakly deformed foreland (or Sub-Variscan) zone in the N
o Alps to S
o Late Variscan granitoid plutons
• Major ss faults (dextral & sinistral) – as microplates collide, they strike slip
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Variscan events: summary
Progressive amalgamation (ca 415-300Ma) of Gondwanan microcontinents onto Laurussia; ended by Gondwana collision
• Iberia impinges with Armorica in Early Dev: dextral transtension & local spreading in S Cornwall, extn rifting elsewhere in SW Eng
• Armorica collides with Avalonia in Late Devonian: dextral transpression in S Cornwall, extn continues in Devon
• Further microplate collisions through Carb & final closure in late Carb with oroclinal bending: N advancing thrusting, basin inversion, uplift, granite emplacement & extn collapse in SW England
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Mid- to Late Devonian events
* N of BCBFZ, region uplifted following Acadian orogeny; SW England unaffected, lying >400km to SE?
* Here, pass southwards into increasingly deep marine Cornubian basins deposited in part on rifted continental crust & to S oceanic crust (Lizard-Start ‘ophiolites’) formed ca 397Ma
* NNW thrusting of ophiolite from ca390Ma-365Ma
* Advancing nappes shed debris N & over-rode this material
* Acadian Event – Northward shunting of Midlands micro-continent – lots of compressional deformation north of Bristol Channel Fault – SW not affected as not part of Isles
83
Carboniferous events
* In Early to Mid-Carb., NNW-thrusting & folding advances N causing tectonic inversion of pre-existing basins
* In Late Carb., advancing thrust wedge flexed lithosphere forming a series of N-ward migrating, syn-tectonic foreland basins in Cornwall, S Wales & Eire
* S-type Cornubian granites (ca 293-270Ma) regional orogenic collapse phase
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Minerals & foreland deformation
* Economically important hydrothermal mineralisation associated with granites (ca 286-265Ma): Sn, Cu, W, Pb, Zn, china clay
* In Late Carb., effects of NW-SE compression superimposed over Foreland undergoing regional thermal subsidence
* Deformation localised along pre-existing faults: style controlled by orientation of old structures relative to shortening
* Dextral transpression dominates
* Fault-hosted mineralisation along faults (e.g.Pennines): Pb & Zn
* Main economic accumulations in Cornwall
* Modern potential accumulations – lithium
85
The Rheic Ocean problem…
* Much uncertainty surrounds the age & longevity of Rheic ocean that separated Avalonia & Armorica: lack of evidence
* Origins of Lizard-Start ophiolites?
* At least 3 possible models:
* Late Dev closure of Rheic Ocean
* Sil-Dev ‘soft’ closure of Rheic Ocean & opening of narrow Rheno-Hercynian sea (as series of dextral pull aparts?) which closed in late Dev-early Carb times
* Rheic ocean lay to the S of marginal RH sea & both closed late Dev-early Carb times
86
A modern analogue for Variscan?
Several analogies with India-Asia collision:
I. Collage of microplates accreted prior to terminal collision event, with complex reactivated suture zones & overprinting
II. Development of curvilinear oroclinal belt with salients & re-entrants formed mainly during terminal collision phase
III. Large volumes of crustally-derived granites
IV. Broad foreland deformation zone
V. Late-tectonic extensional collapse
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Pangaea & a new tectonic regime
* Late Palaeozoic-early Mesozoic: major plate tectonic reorganization associated with the consolidation of Pangaea
* In Europe, onset of megamonsoonal arid/semi-arid climate of low lattitude regions: red beds, evaporites
* Destruction of Palaeotethys by end Triassic & creation of Neothethys in early Permian + in UK extn related to Greenland-Fennoscandia rifting
* Form Pangea and then start rifting it
* Following Variscan, Britain was uplifted leading to regional unconformity development
* Orogenic collapse gave way to thermal subsidence & pulsed rifting, with sedimentation resuming during E. Permian: arid climate
* New rift systems reflecting NW-SE & NE-SW extn: fault-bounded graben/half-graben, e.g. Rockall-Faeroe troughs, Viking & other N Sea graben
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Permian salty seas & volcanoes
* Permian desert Ssts & muds passing up into evaporites* & Lsts formed (after Late Permian sea-level rise) by evaporation in land-locked shallow to marginal marine basins (* economically important!)
* From 305-260Ma: intense igneous activity: basic sills & dykes (e.g. Whin Sill) of mantle origins, whilst in SW England, Cornubian granites (293-275Ma) related to crustal melting following Variscan orogenic collapse
* Evaporites = seal in North Sea
* Trilete rift systems
89
Global extinction & beyond
* At end Permian, a series of the most important global extinction events occur with >90% loss of species
* ‘Supergreenhouse’ effects following major flood-basalt events (China, Siberia) &/or methane release from sediments?
* Into Triassic, Pangaea starts to break up (prior to opening of Central Atlantic): renewed rifting in British Isles
* Initially, basins fill with desert Sst, muds & evaporites similar to Permian
90
Change is in the air…
* Pulsed rifting across Pangaea leads to a continent criss-crossed by shallow seaways
* Late Triassic Britain sees widespread transgressions due also to global sea-levels rise & change to more humid climate as Britain difts N: marine mudstones & Lsts into Jurassic
* Extn pauses, but subsidence continues in sag basins due to thermal subsidence
* Thermal dome in North Sea in mid Jurassic – so little preservation in North Sea
91
Jurassic volcanism & the Mid-N. Sea High
* In mid-Jurassic, alkaline volcanism & uplift of so-called ‘Mid-North Sea high’ reflects limited mantle upwelling event (?plume), with dramatic effect on deposition in N. Sea
* Extension resumes in Late Jurassic as Central Atlantic opens
* Collapse of thermal dome leading to trilete rift system
* Pangea just starting to break up
92
Rift & sag basins: what happens when you stretch continental lithosphere?
* Brittle deformation of continental crust and lithospheric thinning
* Post rifting = thermal subsidence and sag basins
93
The Great Divide
* Carb -Permian boundary marks fundamental change in British geological history = to final assembly of Pangaea
* BI passes from plate boundary to intraplate setting
* Complex, fragmentary early history passes up into period where crustal extn, sedimentation & magmatism dominant
* Very broadly: Mesozoic rifting, Cenezoic sag
94
The Atlantic influence
These events were related to marginal rifting & eventual opening of modern Atlantic Ocean…key events:
• Opening of Central Atlantic in Jurassic along rifts created in Triassic
• Opening of South Atlantic in Cretaceous, causing Africa to rotate ACW towards Eurasia & begin to close Tethys ocean
• Opening of North Atlantic in Late Cretaceous, development of Iceland plume followed by opening of Arctic Ocean leading to separation of N. America
• By the end Britain no longer on plate margin but sill influenced
95
Post-Variscan Template
* Post-Variscan template dominated by normal faults defining Mesozoic rifts that persisted as Cenozoic thermal sags
* Less significant displacement – only a few kms
* Less deformation that pre-Permian
96
Complicating uplift events
* Evidence from onshore & offshore regions that NW Britain has undergone large amount of epeirogenic uplift & exhumation (locally up to 3km!) during Paleogene: related to Iceland plume underplating?
* Mesozoic-Cenozoic basins in SE Britain inverted in Neogene: related either to Alpine Orogeny OR salt movement in basins
* Uplift – exhumation and Outer Hebrides = up to 3km
* Thulean unconformity – related to Iceland plume
97
Structural inheritance?
* Is post-Variscan template influenced by older structural trends?
* In theory, old faults/fabrics can reactivate if weak & suitably oriented
* Long accepted as fact, but in reality not always the case
* Role of old, deep mantle fabrics?
* Old structures are influencing the new sediments
* Note how Mesozoic normal faults control modern physiography in Scotland
98
Cretaceous: a greenhouse Earth
* Atlantic tectonic influences matched by rising global temps & sea-levels, peaking mid Cret.
* Reflects high magmatic CO2 emissions (production of Pacific ocean floor + continental flood basalt events)
* Oceanic anoxic events (black shales) due to high organic productivity, rising CO2 & T
* Early Cretaceous rifting & later Cretaceous flooding, with development of ‘chalk sea’
* Main tectonic influence on Britain at the time is the rifting of the north Atlantic
* CO2 from volcanism is significant + many MORs + flood basalts – Decan traps
* Black shales = high organic productivity – seen in isotopes – due to lots of co2
99
Lower Cretaceous basins & fill
* Early Cretaceous: rifting related to Atlantic opening focuses along Triassic/Jurassic rifting templates
* Marine sedimentation in deeper N Sea & Faeroe/Rockall Troughs, with non-marine deposition in Weald & adjacent basins in S Britain & Ireland
* Later Cretaceous: most land flooded, cutting sediment supply
* Deposition of fine-grained biogenic Lst: CHALK in huge thermal subsidence basins
* Water-world – very little land – very little sediment supply
100
Cretaceous-Tertiary transition
* Towards the end of Cretaceous, sea-levels & temperatures start to fall
* Related to decreasing CO2 levels as chemical weathering affects rising mountain chains in N America & Asia?
* Mass extinction event: bolide impact at Chicxulub, Mexico? Or extrusion of Deccan traps?
* British geological record sheds little light on this controversy
101
Tertiary: Paleogene plume & tilt
* Globally falling sea-levels lead to marine regression, enhanced in NW BI by thermal uplift above asthenospheric plume
* SE-tilting: uplift & erosion in NW, with deposition in major turbidite fans offshore
* Thulean igneous province develops (ca. 60-50Ma), with basic & acid intrusive centres, volcanoes, extrusive basaltic lava flows (in NW) & NW-SE basic dyke swarm – part of a much larger igneous province
102
The Palaeocene-Eocene Thermal Maximum (PETM) & why it matters…..
* Occurred during the high volcanism period – could be related
* 55.5myr - lasted for 100-200 thousand years
* Consistent in a rise in temp of 5 degrees in less than 10ka
* Would need to put 2000 giga tonnes of co2 – equivalent to half all the known fossil fuel reserves
* Thermal dissociates of gas hydrates – methane in arctic tundra is heated and dissociates and turns to methane gas – very effective greenhouse gas – very rich in isotopic carbon which would explain the C13 shift – oxidises in air to form more co2 – clathrates? – runaway positive feedback loop which releases more methane
* Significant evidence of arctic tundra dissociation – Siberian pits opened and are releasing methane
103
Tertiary: Neogene inversions
* Paleogene sedimentation in southern Britain terminated by N-S compression & basin inversion at ca. 43Ma
* Weak foreland manifestation of Alpine Orogeny to the SE
* Folds form above reactivated Mesozoic normal faults
* Global cooling initiated in late Paleogene continues, with the arrival of major ice sheets in N hemisphere by 2.6 Ma, the base of the Quaternary
* Alpine orogeny = southern Europe - England = foreland weak deformation
104
Quaternary: why is it important?
* It includes the present which we use to interpret the past
* Period is persistently effected by advance & retreat of major polar ice sheets: example of an icehouse world
* In UK, it is first time since Vendian that UK has been inundated by major ice sheets
* Includes period during which Homo sapiens evolves – ice sheets may have played an important role in driving this evolution
* Human activity is now beginning to impact on Quaternary environments locally & globally
105
Quaternary: into the icehouse
* Most complete record of events preserved in oceans where oxygen isotope measurements from sedimentary cores are used to measure global ice sheet volumes & sea level: correlate palaeomag. Reversals
* Periodicity related to Milankovitch cycles where Earth’s climate affected by perturbations in orbit around sun
* Onshore record less complete & difficult to correlate: no tills in UK until 0.45Ma (450,000yrs)
106
Ice sheets & periglacial areas
• At least 4 ice sheet advance maxima:
o Anglian (450,000yrs): the most extensive
o Wolstonian (250,000yrs): poorly preserved
o Devensian (20,000yrs): dominate record
o Loch Lomond (10,000yrs): only in uplands
• Devensian event destroys most evidence of Wolstonian
• Adjacent land experienced periglacial conditions, with deposition of fluvioglacial, lacustrine & aoelian seds
• Pre-20Kyrs history not well preserved
• Cold periods separated by warm interglacials
• Major river system in English Channel carrying melt-waters to shelf edge
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The Holocene Interglacial
* 0-10,000yrs: complex shifting coastlines
* Due to interactions of global sea level rise + isostatic rebound of British Isles following melting of ice sheets, unloading lithosphere
* Latter superimposed on continuing thermal subsidence of N Sea basin (isobase map)
* Measurements of present-day crustal stress suggest NW-SE compression due to ridge-push from N Atlantic spreading ridge
* Small earthquakes in NW Scotland & central Britain…poorly understood
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…and the future ?
• Question of scale here:
o On Ma scale, likely to remain quiescent for 10s of Ma until subduction of N Atlantic begins: another plume event unlikely?
o On 100,000yr scale: Reglaciation equivalent to Devensian likely (Milankovitch cycle), with less intense events on 10,000yr scale
o Of most concern is the 100yr-scale & the likely consequences of human-induced global-warming..greenhouse gas emissions are rising at unprecedented rates – how will ocean-atmospheric system respond ? Shutdown Gulf Stream leading to marked cooling in NW Europe?
• The past may not be the key to the future
