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OCR A LEVEL GEOLOGY > Basin Analysis > Flashcards

Basin Analysis Flashcards

1
Q

What is a sedimentary basin?

A

Are subsiding areas of the earth’s crust where sediments have accumulated to greater thickness than in the surrounding areas.

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2
Q

What causes the subsidence to caude the formation of a basin?

A
  1. Attenuation (stretching) at divergent plate margins
  2. Lithosphere contraction due to cooling at convergent margins
  3. Depression of the lithosphere caused by sediment loading or faulting
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3
Q

What is the depositional basin we learn?

A

The Welsh Depositional Basin
Located in the Welsh Borderlands and central Wales, which was formed in the Lower Palaeozoic,. Here there is a great thickness of sediment, which represents a zone which had regular subsidence events

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4
Q

Where was the Welsh basin at the start of the Cambrian (600ma)

A

The supercontinent, Pannotia, was breaking apart. Southern Britain lay on the margins of the supercontinent, on a fragment of crust called Avalonia. This was in the southern hemisphere

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5
Q

What was the environment of the Welsh basin like at the start of the Cambrian?

A
  • Was in a marine transgression
  • Basin on the south side of the Iapetus Ocean
  • Tensional forces caused crustal thinning as Laurentia was moving away from Avalonia.
    -Rapid erosion of surrounding land
  • Rapid transportation and deposition
  • Evidence: Feldspars in clastic sediments, such as greywackes and arkoses, thick beds were deposited.
  • Order of deposition:
    1. Basal conglomerate above unconformity (Beach)
    2. Arkoses, Glauconite, Sandstone, Quartzites (Shallow marine)
    3. Dark shales and mudstones (Deep water)
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6
Q

How was the first Cambrian beds laid in the Welsh Basin?

A

Unconformably on top of eroded Precambrian basement rocks.

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7
Q

What zone fossils were used to identify the age of the rocks in the Cambrian in the Welsh Basin?

A

Trilobites

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8
Q

What was the environment of the Welsh Basin like in the Ordovician?

A
  • Subduction along edges of Iapetus Ocean so it starts to narrow.
  • Island arc of volcanoes form
  • Continued subsidence due to faults at the basin
  • Marine transgression still
  • Black shales (Anoxic water) in the middle of the basin
  • Clastic sediments (on shelf-conglomerates and sandstones)
  • Overlain by more black shales in deepening water
  • Tuff and lavas due to island arc
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9
Q

What had caused a stop in volcanism at the end of the Ordovician at the Welsh Basin?

A

Because the oceanic crust of the Iapetus had all been subducted in this region, as the Iapetus ocean was closing (So less subduction)

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10
Q

What zones fossils were used to identify the age of the rocks in the Ordovician in the Welsh Basin?

A

Graptolites
End of Ordovician was the O/S mass extinction event

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11
Q

What was the environment of the Welsh Basin like in the Silurian?

A
  • Several eustatic marine transgressions and regressions (Polar ice caps growing and shrinking)
  • Some local transgressions (Midland platform faults active allowing for more subsidence)
  • Sediments similar to Ordovician
  • Limestones on the shelf: Reefs eg Wenlock Edge
  • Turbidities thin away from the shelf
  • Tropical conditions
    End of Silurian
  • Caledonian Mountains
  • Red sandstones, desert like, regression
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12
Q

What is the overall tectonic setting of the Welsh Basin?

A
  • Iapetus ocean forms due to continental rifting
  • Formation of MORS
  • As ocean closes again subduction occurs
  • Forms volcanos
  • Ocean closes
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13
Q

What zone fossils were used to identify the age of the rocks in the Silurian in the Welsh Basin?

A

Rugose and Tabulate corals
Graptolites

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14
Q

What are 7 morphological features of graptolites?

A
  1. Nema - A thin tube, an extension of the sicula, possibly to attach to a floating object
  2. Stipe - Basically the stalks of the skeleton where the theca sit
  3. Sicula - Conical tube secreted by the first member of the colony
  4. Theca - Individual cup in which one zooid lived
  5. Aperture - Part where the zooid protruded through the skeleton, in order to filter feed
  6. Virgella - Spine at the end of the sicula
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15
Q

What are the epochs in the Early, Middle and Late of the Cambrian?

A

Early = Caerfai
Middle = St Davids
Late = Merioneth

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16
Q

What are the epochs in the Early, Middle and Late of the Ordovician?

A

Early = Tremaloc, Arenig
Middle = Llavin, Caradoc
Late = Ashgill

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17
Q

What are the epochs in the Early, Middle and Late of the Silurian?

A

Early = Llandovery
Middle = Wenlock, Ludlow
Late = Pridoli

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18
Q

How did graptolites change from the Lower Ordovician to the Early Silurian?

A

Lower - 4 sitpes —> 2 stipes and pointing downwards —-> two stipes curving outwards and up like a bowl
Upper Ordovician - 2 stipes pointing upwards like a V —–> biserial where two stipes are connected into one
Silurian - Single stipe

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19
Q

What is a upside down U shaped graptolite called?

A

Pendant

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20
Q

What is a horizontal graptolite called?

A

Horizontal

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21
Q

What is a vertical graptolite called?

A

Scandent

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22
Q

What is a U/V shaped graptolite called?

A

Reclined

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23
Q

What is a biserial graptolite?

A

Where it has two stipes that are connected

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24
Q

Describe the probable mode of life for graptolites

A

Planktonic - float around in water, filter feeds on plankton

25
Explain why it is difficult to determine the exact mode of life of graptolites
Extinct fast, low preservation, no comparative species
26
When were the Jurassic sediments deposited in the UK
200-145 Ma
27
In the Middle Jurassic, what caused the area between Laurentia and Fennoscandia (where the UK was forming) to decrease by 10 degrees in water temperature?
Uplift of the North Sea dome. It cut off warm water from the Tepthys ocean making the region cooler
28
What was the tectonic setting of the UK during the Jurassic?
UK was a small archipelago with the Boreal ocean to the North, and the Tethys ocean to the South The continent Laurentia to the West and the continent Fennodcandia to the East NORTH SEA DOME TO THE EAST SOMETIMES SUBMERGED
29
Why were their variations in lithofacies in the Jurrassic sediments?
- Uneven flooding of old Triassic rock so water depth varied -"Highs" and "Basins" were created by tension producing horts and grabens -The tension was caused by the North Atlantic opening
30
Modern day, what is the Jurrasdic outcrop like in England
Forms a west-facing, East dipping escarpment across England
31
During the Lower Jurrassic (LIAS) name 2 lithofacies and the environment at the time as well as the sea level.
Lithofacies Mudstone/Shale/Limestone (Cyclothems) Basal Unconformity Environment Transgression over the Triassic surface. Cyclic sedimentation Sea level Deep water is going to shallower water towards the end of the Lias Marine transgression but did decrease slowly
32
Name some lithofacies during the Middle Jurassic (Messy muddle) and the environment at the time, as well as the sea level
Lithofacies Mix of oolites, shallow marine limestone, cross-bedded sandstones, shales, and coals Environment Shallow marine basins (including high energy zones, lagoons, and deltas) Sea level Stable
33
What are three stages of the Upper Jurassic?
Youngest Portlandian Kimmeridgean Oxfordian Oldest
34
During the early part of the oxfordian stage in the upper Jurassic, what is the main lithofacie, the environment and the sea level
Lithofacie Oxford clay Environment Deep water Sea level Increased rapidly overland to deep water Marine transgression
35
During the end of the Oxfordian stage in the upper Jurrassic, what were the main lithofacies, the environment and the sea level
Lithofacies Corallian beds - Sandstone, Limestone, clay (SHOWS SHALLOW WATER ENVIRONMENT) Environment Shallow water environment Sea level Rapidly decreased into shallow water Small regression
36
During the Kimmeridgean in the Upper Jurassoc, name some lithofacies, the environment and the sea level
Lithofacies Clays with high organic carbon content (Kimmeridgean clays) Environment Deep water conditions Sea level Marine transgression
37
During the Portlandian stage of the Upper Jurrassic what were some lithofacies, the environment and the sea level?
Lithofacies Evaporites and limestone Environment Fresh water lagoons which dried up Sea level Marine regression
38
How do we know the sea level was changing during the Jurassic?
Facies changes The rocks provide evidence for cyclic sea level changes such as the corallian beds. Trace fossils in these rocks help determine the sea level
39
What are Arenicolites, and what palaeoenvironments do they show?
A worm burrow (similar to the lugworm) U-shaped burrow, tail end, and head end. Feed at one but excrete at other Show shallow, marginal marine conditions
40
Shat are Diplocraterion and what palaeoenvironments do they show?
Also, a worm. Will dig a U-shaped burrow. This will change in height/depth depending on the environment. Will show past burrows below the current burrow if there is increased deposition and will show deepening burrows if the sea bed is eroding Shows the depth and how much the environment deposition is increasing or decreasing by Has to happen in the lifetime of a single worm - short-term Found in beaches and sandy tidal flats, shallow water
41
What is Rhizocorallium, and how do they show palaeoenvironments?
Are a sort of lobster/crustacean. Will burrow Older previous burrows, which have been covered by sediment (upward movement). They dig burrows on deepening sediment below the one before. Shows deposition in that environment Shallow marginal marine conditions
42
What are Thalassinoides and how do they show palaeoenvironments?
Marine burrowing species making connected tunnels Below normal wave base, but above storm wave base
43
Give some background information on the Corallian formation
-very rapid cyclic sea level changes -no polar ice caps -too fast to be due to MORS -probably isoststic sediment movement -cyclic sedimentation
44
What caused confusion on how the corrallion formation formed?
Cyclothems were a puzzle Repetitions of Sandstone (sub-tidal) Clay (deeper water) oolitic limestone (sub-tidal) This sequence is repeated many times (Cyclothems)
45
What does Walthers law state, and how was that used to help understand the formation of the Corallian beds
Sediment found vertically above one another represents environments of deposition that existed side by side Therefore, this was used to help come up with the conclusion that there were a series of sand dunes or banks migrating across the seabed showing a rapid change in sea level
46
What fossils are found in the Jurassic basin?
Reptiles: Ichythosaurus, Plesiosaurus, Pterosaurs Cephalopods: Ammonites, Belmnites Bivalves: Gryphaea
47
Describe the Ichthyosaur
*Breathed air warm blooded and viviporous (produced live young instead of eggs) *Early Jurassic *Dorsal fin and vertical tail fin *Used end of tail with vertical fin. Flippers used as a direction like a rudder *Reduced from middle to late Jurassic *1-10m in size
48
Describe the Plesiosaurs
*Breathed air, warm blooded, viviporous, worldwide distribution *Late Jurassic *Four flippers *Flippers made flying movement through water *More population from middle Jurassic, some species with longer necks *5-10m long
49
Describe the Pterosaurs
*Earliest vertebrates to possess powered flight *Wings formed of membranes of skin attachable to 4th digit *Wingspan 1m (Pterodactyl) *Coats of hair-like filaments (pucnofibers) covering body and wings *Fish important to diet *Able to swim and land from water
50
Describe Belemnites mode of life
-Marine -nektonic -Carnivores/hunters/predators
51
How did belemnites move?
Jet propulsion Vertical change of buoyancy Walk with tentacles on the floor Swim by rippling of the fins -CO2 and O2 ratio from belmnite show palaeo-temperature and salinity of water at the time. Only guard of belmnites are preserved
52
What 3 types of ammonoida were there and how long did each species last?
Goniatites - from the Devonian to the end of Permian Ceratities - Triassic Ammonites - Jurassic to end of Cretaceous
53
What modern-day marine species are Ammonites related to?
Nautilus
54
How did Ammonites move?
1. Horizontal movment via Jet propulsion through the Hypernome. 2. Vertical movement via filling Chambers up with air/water to rise or sink
55
What is the protoconch on an ammonite?
The first spiral the ammonite started with
56
What is the siphuncle?
Tube going through the chambers if the Ammonite
57
What is the last chamber filled with in an ammonite?
Soft tissue
58
What feature of evolution makes Ammonites superb zone fossils?
Evolution of the septal structure (joined structures on the inside if the shell separating chambers. Nautilus- simple curve Goniatite - zig zag Ceratite- frills on lobes (bottom parts) Ammonite - frilly lobes and saddles bottom and top parts) ALSO SIPHONATE DIRECTION Early Jurassic = retro-siphonate -siphonates point towards proto-shell Middle Jurassic= both pointing towards and away from the protoshell (linked to the two stages of life shallow and deep conditions) Late Jurassic = prosiphonate point away from the protoshell

OCR A LEVEL GEOLOGY (12 decks)

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