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G1 : rock forming processes > G1 : Rock Forming Processes KI3 (sedimentary Rocks) > Flashcards

G1 : Rock Forming Processes KI3 (sedimentary Rocks) Flashcards

1
Q

Where does sediment brought to the sea by rivers accumulate

A

On the continental shelf

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2
Q

What happens when sediment at the edge of the continental shelf becomes unstable

A

It will begin to move down the slope under the force of gravity

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3
Q

What causes sediments to become unstable

A

Rapid sedimentation, stress from storm waves and esrthwuakes

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4
Q

What are turbidity currents

A

Submarine gravity flows. It’s density and low friction means it flows with high velocity down the gentle gradient transporting huge volumes of clastic material.

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5
Q

Where is sediment carried in turbidity currents deposited

A

Much is deposited in submarine fans on the continental rise or in ocean trenches. However, they can also spread thousand of km across abyssal planes

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6
Q

What happens as gravity flows in turbidity currents becomes more dilute

A

It’s capacity for water reduces; heavier course grains are deposited first and then the finer sand and silt settled out

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7
Q

What is the back ground sedimentation in the ocean called

A

Pelagic fallout, a gentle rain of fine mud is deposited from suspension to form shale

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8
Q

Bouma layer E

A

Shale, which may contain pelagic marine fossils such as graptolites/parallel laminations in mud/no current. Suspension settling. Interturbidite

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9
Q

Bouma layer D

A

Fine sandstone and siltstone/parallel laminations in sand and silt/low energy current. Suspension settling. Changes in current energy cause alternative lamination of coarser and finer grains

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10
Q

Bouma layer C

A

Sandstone (greywacke)/cross bedding, climbing ripples/sufficient energy to carry sand by saltation. Climbing ripples form when deposition exceeds rate of migration of ripples. Energy insufficient to cause complete erosion of stoss side of ripples

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11
Q

Bouma layer B

A

Coarse, then medium sandstone (greywacke)/flat bed(high velocity): graded bedding, sole marks including flute casts and tool marks/sufficient energy to carry sand grains by traction. Sole structures occur on the base of a bed. They may take the form of pits or grooves that act as moulds for sediment deposited from above. When filled, they produce casts

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12
Q

Bouma layer A

A

Course bed of pebbles or granules (conglomerate) in a sandy matrix/graded bedding, rip up clasts, erosional base/high energy. Coarsest grains in suspension. Erosion at the base of the flow gears up shale from underlying beds, shale rip up clasts are included at the base

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13
Q

Turbidity current (kd)

A

High velocity current that flows down gentle gradients because the sediment within it makes it denser than sea water. These currents are triggered by earthquakes or slope instability.

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14
Q

Turbidites

A

An upward fining deposit of greywacke deposited from a turbidity current

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15
Q

Bouma sequence

A

An idealised sequence of sediments and sedimentary structures seen in a turbidity deposit

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16
Q

Climbing ripples

A

A series of cross laminate formed by superimposing migrating ripples. They form when deposition exceeds the rate of migration of ripples

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17
Q

Tool marks

A

Impressions made of the surface of soft sediment by the dragging of bouncing of an object (tool) in a current

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18
Q

Rip up clasts

A

Prices of shale of mudstone eroded by a current containing suspended sediment. They are preserved when the current deposits it’s sediment

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19
Q

Siliceous ooze

A

Pelagic clay containing >30% biogenic skeletal material made of silica

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20
Q

Diatoms

A

Plankton algae that secrete siliceous material

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21
Q

What happens when plankton organisms die

A

Their tests slowly sink to the sea floor to be preserved as microfossils. The composition of the ooze is dependant on the composition of the planktonic organisms

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22
Q

How do siliceous oozes form

A

Form the skeletons of diatoms in deposits nearer the poles and radiolaria in deposits nearer the equator. Silica devolves at a slow rate In sea water. Oozes accumulate where the rate if deposition is greater than the rate of solution

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23
Q

What are sedimentation rates in abyssal planes

A

Estimated at 1mm to 3cm per 1000 years

24
Q

Facies

A

Included all the characteristics of a sedimentary rock that are produced by its environment of deposition and allow it to be distinguished from rock deposited in an adjacent environment

25
Q

Lithofacies

A

Included all the physical and chemical characteristics of a rock

26
Q

Bio facies

A

Included the palaeontological characteristics of a rock

27
Q

Facies association

A

Describes a group of sedimentary facies that occur together and typically represent one depositional environment

28
Q

What characteristics are included in facies

A

Mineral content, grain size, sorting, structures, fossil content, any other observable property

29
Q

What happens to rocks under glaciers

A

Glaciers scratch striations on underlying rock surfaces

30
Q

What does melting ice deposit

A

Boulder clay, a poorly sorted sediment ranging from boulders to clay size

31
Q

What are sediments produced by meltwater streams from a glacier like

A

Better sorted and show evidence of being transported by water

32
Q

What happens when glacier ice melts

A

Lakes are formed. During the spring thaw, streams transport fine sand, silt and clay. The sand and silt size grains settle to the floor of the lake first. The. Dry fine grains remain in suspension, settling when the lake freezes in winter

33
Q

How do we know that glacier ion has been widespread in the past

A

Using uniformitarianism when examining sedimentary rocks produced in the oast

34
Q

Tillite

A

Rock formed from very poorly sorted and varied material dropped by the ice

35
Q

Varves

A

Annual lake clays and silts in distinctive thin layers

36
Q

Striations

A

Scratches formed by rocks carried in moving ice, they can be used to tell the direction of movement of ancient ice sheets

37
Q

Why does rapid evaporation occur in sabkhas

A

So that as the sea water approaches, the coast salinity increases

38
Q

What do shelled organisms remove in sabkhas

A

Offshore shelled organisms remove most of the calcium carbonate in the sea water

39
Q

What does evaporation of groundwater from the supratidal areas do

A

Draws sea water into the sediment along the shore

40
Q

Sabkha sequence (evaporites and algae)

A

Evaporites develop above the high water line as groundwater is evaporated. Algae grow on the shore between high and low tide levels and are preserved as stromatolites. Evaporites minerals are found above the stromatolites in sabkha sequence

41
Q

What’s the first mineral to crystallise out within the sediments at sabkhas

A

Gypsum

42
Q

What’s the second mineral to crystallise out in sabkhas

A

Anhydride, forms nodules in the sediment

43
Q

Polymictic conglomerate

A

A coarse grained sedimentary rock containing clasts of many different rock tyoes

44
Q

Oligomictic conglomerate

A

A coarse grained sedimentary rock containing clasts of few different rock tyoes

45
Q

Monomictic conglomerate

A

Coarse grained sedimentary rock containing clasts of a single rock type

46
Q

Till fabric analysis

A

A method of tracing the former movement direction of ice by taking compass bearings on the long axes of large clasts in till deposits

47
Q

Fluvio-glacial deposits

A

Sediments produced by meltwater streams flowing from a glacier

48
Q

What processes act during transport to affect the characteristics of a sediments found in any environment

A

Deposition and weathering, some grains are deposited before of after due to their size, shape or density and some grains are destroyed by weathering due to their mineral composition

49
Q

Transport by rivers

A

Sorting by size is the more important process.
Coarser grains are deposited near the source if a river, sand size grains are deposited in the middle course and diner grained minerals such as clay and silt are deposited near the mouth. Some sediments are transported out into shallow seas (marine environment)

50
Q

Transport by wind

A

Small size range and are mineralogical mature. Since transports sand and silt, and the only mineral found in these grain sizes are present in wind transported deposits. Quartz is the most common, but calcite can also survive as solution is rare in arid environments where wind transport is often important. Softer minerals such as mica are destroyed attrition and transported away as fine dust.

51
Q

Glacial transport

A

Weathering is more important. Usually poorly sorted sediments. Include and fine matrix of non resistant rock types and coarse pebbles and cobbles or resistant rocks. Transported over short distances and the characteristics of the sediment change with what rock type over which the ice is moving.

52
Q

Till fabric analysis of boulder clay within till

A

Tend to lie with their long axes parallel to the direction of movement of the ice.

53
Q

Deposition in Shallow marine environments

A

Occurs in the littoral zone and continental shelf. Coarser grained transported for longer distances, for example storm beaches composed of pebbles are on the landward side of a beach. Weathering is important as grains must be able to withstand high energy waves. Deposits contain more resistant grains

54
Q

Walthers law of facies

A

In a sedimentary sequence without unconformaties, a vertical succession of facies represents sedimentary environments that once existed side by side and have migrated over one another through time.

55
Q

Carboniferous delta sequences

A

The facies arranged vertically in a cyclothem are the same as the ones that exist side by side in present day deltas. Cyclotherms are characteristics of Carboniferous delta sequences in areas of Britain once mined for coal e.g the yoredale cycles of the north pennines

56
Q

Fluvial environments and facies

A

Meandering streams migrate across their floodplains due to the bank undercutting on one side of the meander and deposition of point bar deposits on the other side. In between are the channel deposits. As the channel ,ingrates layer sediments are deposited in top of earlier ones

57
Q

Coastal environments and facies

A

Shallow siliciclastic seas and beaches along the coast migrate laterally during marine transgression and regression. The former position of the coastal environments can be seen in the vertical sequence of facies. Here, the time lines cut through facies that were deposited at the same time. The facies boundaries cross the time lines, so in this case they are diachronous

G1 : rock forming processes (3 decks)

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