Bob Flashcards
(108 cards)
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