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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
Q

Explain why it is difficult to determine the exact mode of life of graptolites

A

Extinct fast, low preservation, no comparative species

26
Q

When were the Jurassic sediments deposited in the UK

A

200-145 Ma

27
Q

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?

A

Uplift of the North Sea dome. It cut off warm water from the Tepthys ocean making the region cooler

28
Q

What was the tectonic setting of the UK during the Jurassic?

A

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
Q

Why were their variations in lithofacies in the Jurrassic sediments?

A
  • 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
Q

Modern day, what is the Jurrasdic outcrop like in England

A

Forms a west-facing, East dipping escarpment across England

31
Q

During the Lower Jurrassic (LIAS) name 2 lithofacies and the environment at the time as well as the sea level.

A

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
Q

Name some lithofacies during the Middle Jurassic (Messy muddle) and the environment at the time, as well as the sea level

A

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
Q

What are three stages of the Upper Jurassic?

A

Youngest

Portlandian
Kimmeridgean
Oxfordian

Oldest

34
Q

During the early part of the oxfordian stage in the upper Jurassic, what is the main lithofacie, the environment and the sea level

A

Lithofacie
Oxford clay

Environment
Deep water

Sea level
Increased rapidly overland to deep water
Marine transgression

35
Q

During the end of the Oxfordian stage in the upper Jurrassic, what were the main lithofacies, the environment and the sea level

A

Lithofacies
Corallian beds - Sandstone, Limestone, clay (SHOWS SHALLOW WATER ENVIRONMENT)

Environment
Shallow water environment

Sea level
Rapidly decreased into shallow water
Small regression

36
Q

During the Kimmeridgean in the Upper Jurassoc, name some lithofacies, the environment and the sea level

A

Lithofacies
Clays with high organic carbon content (Kimmeridgean clays)

Environment
Deep water conditions

Sea level
Marine transgression

37
Q

During the Portlandian stage of the Upper Jurrassic what were some lithofacies, the environment and the sea level?

A

Lithofacies
Evaporites and limestone

Environment
Fresh water lagoons which dried up

Sea level
Marine regression

38
Q

How do we know the sea level was changing during the Jurassic?

A

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
Q

What are Arenicolites, and what palaeoenvironments do they show?

A

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
Q

Shat are Diplocraterion and what palaeoenvironments do they show?

A

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
Q

What is Rhizocorallium, and how do they show palaeoenvironments?

A

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
Q

What are Thalassinoides and how do they show palaeoenvironments?

A

Marine burrowing species making connected tunnels

Below normal wave base, but above storm wave base

43
Q

Give some background information on the Corallian formation

A

-very rapid cyclic sea level changes
-no polar ice caps
-too fast to be due to MORS
-probably isoststic sediment movement
-cyclic sedimentation

44
Q

What caused confusion on how the corrallion formation formed?

A

Cyclothems were a puzzle
Repetitions of
Sandstone (sub-tidal)
Clay (deeper water)
oolitic limestone (sub-tidal)

This sequence is repeated many times (Cyclothems)

45
Q

What does Walthers law state, and how was that used to help understand the formation of the Corallian beds

A

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
Q

What fossils are found in the Jurassic basin?

A

Reptiles: Ichythosaurus, Plesiosaurus, Pterosaurs
Cephalopods: Ammonites, Belmnites
Bivalves: Gryphaea

47
Q

Describe the Ichthyosaur

A

*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
Q

Describe the Plesiosaurs

A

*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
Q

Describe the Pterosaurs

A

*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
Q

Describe Belemnites mode of life

A

-Marine
-nektonic
-Carnivores/hunters/predators

51
Q

How did belemnites move?

A

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
Q

What 3 types of ammonoida were there and how long did each species last?

A

Goniatites - from the Devonian to the end of Permian
Ceratities - Triassic
Ammonites - Jurassic to end of Cretaceous

53
Q

What modern-day marine species are Ammonites related to?

A

Nautilus

54
Q

How did Ammonites move?

A
  1. Horizontal movment via Jet propulsion through the Hypernome.
  2. Vertical movement via filling Chambers up with air/water to rise or sink
55
Q

What is the protoconch on an ammonite?

A

The first spiral the ammonite started with

56
Q

What is the siphuncle?

A

Tube going through the chambers if the Ammonite

57
Q

What is the last chamber filled with in an ammonite?

A

Soft tissue

58
Q

What feature of evolution makes Ammonites superb zone fossils?

A

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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