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Environmental Science > 3.1- Sedimentary Basins > Flashcards

3.1- Sedimentary Basins Flashcards

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1
Q

What are the stages of the rock cycle?

A

Sediment is compacted into sedimentary rock. This undergoes heat and pressure and turns to metamorphic rock. This undergoes melting in the mantle and turns to magma. The magma crystallises into igneous rock which is then weathered and deposited as sediment.

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2
Q

What makes up siliciclastic rocks?

A
  • Quartz
  • Orthoclase (K) and plagioclase (Na) feldspar
  • Lithics (pre-existing rock fragments
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3
Q

What do granite and arkose have in common?

A

They have the same composition but have different textures
(Contain quartz, feldspars and biotite)

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4
Q

What happens to granite as it matures?

A

It matures though weathering and turns from granite into arkose and then into quartz arenite when it is mature

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5
Q

What is the diagram called that measures the composition of siliciclastic rocks?

A

Ternary plot

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6
Q

What happens to gabbro as it matures?

A

Gabbro turns to lithium arenite and then sublitharenite as it matures. The quartz ratio increases with its maturity.

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7
Q

What do different sediment compositions result from?

A

The weathering and erosion of different parent rocks created in different plate tectonic settings

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8
Q

What type of composition do igneous rocks at a continental craton/hot spot have when weathered?

A

Rich in feldspar

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9
Q

What type of composition do ultramafic rocks have at an ocean basin/divergent ocean plate boundary when weathered?

A

When weathered rocks are rich in pyroxene

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10
Q

What types of rock form at volcanic arcs?

A

Intermediate igneous rocks

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11
Q

What are sedimentary rocks classified by?

A

Their grain size, grain sorting, grain shape (show sediment transport) and their mineral compositions

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12
Q

What are the sedimentary characteristics

A
  • Composition
  • Texture
  • Sedimentary structures (Types of bed forms)
  • Bedding geometry, style and thickness
  • Sharp, wavy or flat bedding contacts
  • Fossils present
  • Colour (can reflect chemistry of pore fluids)
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13
Q

What are sedimentary facies?

A

A distinct set of lithological and fossil characteristics of a given sedimentologic unit, TYPICALLY DEFINED AT THE BED SCALE

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14
Q

What do the sedimentary characteristics provide us with?

A

The sum total of the characteristics provide us with a tool for interpreting the environmental significance of sedimentary deposits

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15
Q

What does the cycle of a sedimentary rock reflect?

A
  • Provenance
  • Transport and deposition
  • Lithification and diagenesis
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16
Q

What is provenance?

A

The source area where rock undergoes weathering and erosion

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17
Q

What happens during transport and deposition?

A
  • Sediment transported by water and wind and is deposited in a marine or non-marine environment
  • Sediments change their compositions, size, and shape as they are transported
  • Sediments become sorted during transport because of size, shape and density
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18
Q

What is lithification and diagenesis?

A

It is alteration of the sediment after deposition

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19
Q

What is a sedimentary basin?

A

It is a low area in the earths crust, of tectonic origin, in which sediments accumulate

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20
Q

What can a basin look like?

A

They range in size from hundreds of metres to large parts of ocean basins.

They don’t have to have a typical basinal shape as sediment can be deposited on a surface with a gentle and uniform slope

21
Q

What is essential for the creation of a sedimentary basin?

A

Tectonic creation of relief, to provide both a source of sediment and a relatively low place for the deposition of that sediment.

22
Q

What is accommodation space?

A

It is the space available within a basin for either marine or non-marine sediment to be deposited.

23
Q

What can affect accommodation space?

A

Changes in sea level and subsidence and the lowering of the crust eg. Due to pressure from a build up of sediment

24
Q

What is subsidence?

A

It is the gradual downward sinking of the land surface during sedimentation. It is required for long-term accumulation and preservation of sediments. Compounded by weight of overlying sediments.

25
Q

What are the causes of subsidence?

A

-The thinning of underlying crust
- Tectonic loading
- Changes in the thickness or density of adjacent lithosphere

26
Q

What are the worlds thickest sediment accumulations and where do they form?

A

The worlds thickest accumulations are 15-20km (filled over 10-100myrs) and often form in enclosed basins supplied by major rivers which transport sediment by fluvial processes.

27
Q

Why is it important to understand the different sedimentological units of basin sedimentation?

A

Each unit will have different lithological characteristics. Can distinguish areas where the sediment is porous where hydrocarbons can be extracted or C02 can be stored in.
C02 cannot be stored in rocks that have no pore space.

28
Q

Why are sedimentary basins important?

A

They contain hydrocarbons such as oil, gas and coal

29
Q

What is carbon sequestration?

A

C02 is captured and pumped down pipes into rocks where it is stored

30
Q

Why are sedimentary basins important?

A
  • They are used to plan and understand water resources
  • They contain ore and mineral resources that can be extracted
  • They contain an archive of earth’s history (can see when life occurred)
31
Q

How are foreland basins formed?

A

The weight of a mountain belt pushes down the crusts surface to form a basin.

As the mountain range uplifts, it creates a depression or basin on the side of the range facing away from the source of uplift. Sediments eroded from the mountain range are deposited in this basin, forming a thick wedge-shaped layer of sediment that progrades outwards towards the adjacent continental margin.

himalayas
alps

32
Q

How are rift basins formed?

A

There is a downward slip on faults which produces narrow troughs.

  • They are formed as a result of the stretching and thinning of the Earth’s crust in response to tectonic forces. When tectonic plates move apart, the tensional stress causes the crust to stretch and thin, creating a series of parallel faults that form a rift valley. As the rift valley deepens, it can fill with sediments that are eroded from the surrounding uplands

Over time, the rift basin may continue to expand as the tectonic plates continue to move apart. This can result in the formation of a new ocean basin, as the rift valley becomes flooded with seawater. Alternatively, the rift may fail to continue opening, in which case it may become a failed rift,

33
Q

How are intracontinental basins formed?

A

The basin forms in the interior of a continent, perhaps over an old rift

  • Intracontinental basins are not directly related to mountain building or rifting, but instead form in response to a variety of processes such as subsidence, sediment loading, or regional stress. They are often more circular or irregular in shape, and their stratigraphy is typically dominated by fluvial and lacustrine deposits rather than volcanic or rift-related sediments.
34
Q

How are passive margin basins created?

A

Subsidence occurs over thinned crust at the edge of an ocean basin (this accumulates huge amounts of sediment)

  • As the plates move apart, the land on one side of the boundary subsides and becomes a basin, while the other side rises to form a mountain range.
35
Q

How can heat from the mantle affect the lithosphere/crust?

A

If the lithosphere is heated from below, it becomes less dense and is able to float higher in the asthenosphere.

This produces crustal uplift.

If the lithosphere cools back to its original temperature, the crust sinks back to its original level (isostatic subsidence).

36
Q

How can thermal processes from the mantle affect the crust to create a sedimentary basin?

A

When the lithosphere is heated from below and the crust is uplifted, erosion takes place when the crust is elevated.

The crust is thinned where erosion takes place and so when the crust is cooled and sinks back down, it ends up in a position lower than where it started.

This created a basin which can then be filled by sediment (accumulation space created).

37
Q

What else apart from erosion of the crust during uplift can create a basin?

A

When the crust is heated and rises, extensional thinning often accompanies the heating. When the crust cools the elevation of the top of the lithosphere is less than before this heating and cooling.

This THINNING PROCESS explains the creation of many sedimentary basins.

38
Q

How can flexural processes create sedimentary basins?

A

This is when a large load on the lithosphere which causes the lithosphere to subside by isostatic adjustment.

Because the lithosphere has considerable flexural rigidity, adjacent lithosphere is bowed down also.

The region between the high standing load and the lithosphere far away is depressed to form a basin.

39
Q

What type of basins is flexural processes associated with?

A

The creation of foreland basins.

These are formed ahead of large thrust sheets that move out from orogenic areas onto previously unreformed cratonal lithosphere.

40
Q

How do rift basins form sedimentary basins?

A

Basins form where tectonic activity creates space.

Rift basins are formed at divergent plate boundaries.

The crust thins by stretching and rotational normal faulting. The thinned crust subsides and sediment fills the down-dropped basin.

41
Q

How are passive margin basins formed?

A

A passive margin is the transition between oceanic and continental crust (it is not an active plate margin but still tectonically created)

The crust is thinned and lifted by previous rifting. When the thinned crust cools it subsides.

This creates a passive margin basin

42
Q

How are intracontinental basins formed?

A

They form in the interiors of continents away from margins.

They initially form because of thermal subsidence over a rift.

They may continue to subside in pulses even millions of years after but the reasons are not well understood.

43
Q

Where does sediment come from at a foreland basin?

A

The mountain belt that pushes crust down to form a basin is weathered and the weathered mountain belt causes sediment to fill the basin

44
Q

What controls accommodation space?

A
  • Global sea level change
  • Tectonic uplift/subsidence (of sea floor)
  • Changes in rates of sediment accumulation

Tectonics are needed to make sedimentary basins but the record of the basin itself is sedimentary

45
Q

How is sea level responsible for accommodation space?

A
  • Depositional belts shift landward (sea rise) or seaward (sea drop) in response to sea level change.
46
Q

What is transgression?

A

Transgression is flooding due to sea level rise which causes sediment belts to shift landward

  • Strata deepens upwards
47
Q

What is regression?

A
  • Regression is exposure due to sea level fall.
  • Depositional belts shift seaward and strata shallows upward.
  • Preserved sediment can give records of past sea level rise and falls

(Regression tied to erosion is less likely to be preserved)

48
Q

How did the Death Valley form?

A

Death Valley formed because of crust that slipped down creating accumulation space. The basin filled with sediment from the weathering of the mountains on each side of the valley - sediment is delivered by subarial fans. (Basin is 10km wide).

Environmental Science (15 decks)

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