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EART27201 Sedimentary rocks and fossils > Ichnology > Flashcards

Ichnology Flashcards

1
Q

What are ichnofossils and how are they formed?

A

Trace fossils formed by the movement of organisms

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2
Q

What do ichnofossils show and what can they be used to determine?

A

They represent activity of soft-bodied organisms and can be used to determine current velocity, sedimentation rate, and strength of substrate.

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3
Q

Where are boring ichnofossils usually found?

A

High energy, rocky coastlines

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4
Q

Where are simple burrow ichnofossils usually found?

A

Sandy shores

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5
Q

Where are organisms that live and graze on the surface usually found?

A

Low energy offshore and deepwater environments

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6
Q

How are ichnofossils classified?

A

Ichnogenus, ichnospecies (Linnean binomials)

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7
Q

How can one organism produce different types of ichnofossils in its life?

A

Different activities and different substrates

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8
Q

What is the name for locomotion traces?

A

Repichnia

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9
Q

What is the name for resting traces?

A

Cubichnia

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10
Q

What is the name for dwelling traces?

A

Domichnia

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11
Q

What is the name for feeding burrows?

A

Fodinichnia

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12
Q

What is the name for grazing traces?

A

Pascichnia

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13
Q

Describe dwelling traces

A

Cylindrical tubes with minor branching

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14
Q

Name and describe the two different types of dwelling traces

A

Borings made into hard or semiarid substrate (rock). Burrows made into soft substrate (unconsolidated sand).

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15
Q

What can feeding burrows do and what can they show?

A

They can cross bedding surfaces and the complex patterns represent mining behaviour

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16
Q

What do repeating patterns in grazing traces show?

A

A systematic feeding strategy

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17
Q

FOSSIL ROOTS ARE NOT TRACE FOSSILS

A

Got that?

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18
Q

What are fossil roots diagnostic of?

A

The terrestrial depositional environment

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19
Q

Describe the morphology of fossil roots

A

Highly irregular and downward branching

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20
Q

What can bright colours in sedimentary rocks indicate?

A

Clays (broken down micas and feldspars) and organic material

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21
Q

Define diagenesis

A

The process of loose sediment becoming rock.

22
Q

What does diagenesis destroy?

A

Rock

23
Q

Describe the conditions of diagenesis

A

Begins at surface temperature and pressure and continues as T and P increases.

24
Q

Give the names for early and late diagenesis

A

Eodiagenetic and mesodiagenetic

25
Q

What is bioturbation?

A

The disturbance of sedimentary structures by organisms

26
Q

Give an example of bioturbation and what it results in

A

The consumption and excretion of sediment that can change the composition of some clays

27
Q

Describe a eodiagenetic bioturbation

A

The mechanical and chemical activities of organisms buried near the surface sediment

28
Q

What does the churning and homogenising of sediment result in?

A

The reduction of deposition barriers to flow in reservoirs

29
Q

What causes mechanical compaction?

A

Overburden

30
Q

What are two eodiagenetic processes that occur in mechanical compaction?

A

Grains are rotated to reduce volume and water is expelled upwards

31
Q

Describe the formation of sandstone dykes

A

Water is explosively expelled upwards after a sudden, large compaction

32
Q

Why types of grains can be highly compacted and why?

A

Silts and clays have platy morphology and can be compacted up to 90%.

33
Q

What type of grain cannot be highly compacted and why?

A

Sand because it is mostly quartz, which is often round.

34
Q

What mesodiagenetic process allows further compaction after the rotation of grains?

A

Mechanical fractioning of the grains

35
Q

What type of process is pressure solution?

A

Mesodiagenetic

36
Q

Describe the process of pressure solution

A

The material at the grain contacts (perpendicular to compaction) dissolves. This reprecipitates as cement at contacts that are perpendicular to compaction. Porosity is lost.

37
Q

What causes dissolution?

A

Changes in temperature and pressure and/or composition of porewater

38
Q

When is a mineral more stable?

A

In a solution

39
Q

Where does dissolution without reprecipitation occur?

A

In systems that are subject prolonged pumping of exotic porewater with an abundant fresh supply

40
Q

Describe the process of cementation

A

Chemical precipitates, forming new crystals in the mores of a sediment/rock, binding the grains together

41
Q

What types of sources can the precipitate have during cementation?

A

Local or external

42
Q

What are concretions?

A

Spherical masses of sediment that are more strongly cemented than the surrounding volume

43
Q

What grain size do concretions have compared to the surrounding?

A

The same

44
Q

What are concretions associated with?

A

Organic material that reacts with the surrounding chemistry

45
Q

Give an example of a concretion produced by organic material

A

Iron found in organic matter that forms iron carbonate concretions

46
Q

What is recrystallisation?

A

The reorientation of the same crystal lattices in mineral grains

47
Q

What processes are involved in recrystallisation?

A

Solution and reprecipitation of the mineral phase already present

48
Q

Where is the process of recrystallisation commonly seen?

A

Carbonate shells

49
Q

What is mineral replacement?

A

Where a newly formed minerals replace preexisting ones in situ

50
Q

How can the new minerals be different to the original mineral?

A

It can be a polymorph

51
Q

How does mineral replacement affect pore space?

A

The new structure can have a different volume

EART27201 Sedimentary rocks and fossils (10 decks)

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