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Sedimentology and Stratigraphy > (S7) Deltaic Environments and Successions > Flashcards

(S7) Deltaic Environments and Successions Flashcards

1
Q

What are the three variables which determine the presence of a estuary/delta/neither?

A

Morphology, Sediment supply, processes acting

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2
Q

What is the definition of a delta?

A

Discrete shoreline protuberance formed at point where a river enters the ocean or another body of water

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3
Q

What is the definition of an estuary?

A

Accumulation of sediment without any buildup of sediment into the sea

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4
Q

What occurs at a river mouth where an estuary or delta does not form?

A

No significant mixing and sediment supply is carried away, not deposited at the mouth

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5
Q

What is the smallest form of delta and what scale does it occur?

A

Stream into a lake - tens to hundreds m’s across

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6
Q

What scale can deltas reach?

A

thousands of sq km

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7
Q

How did deltas establish their name?

A

Named after the Nile delta that looks like a triangle, hence greek “delta”

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8
Q

What are the three processes which vary the form of a delta?

A

River current, Wave action, Tide action

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9
Q

Give two modern examples of mud/silt delta deposits

A

Mississippi and Ganges

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10
Q

Give a modern example of a sandy delta deposit

A

Rhone delta

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11
Q

Give a modern example of a pebbly stream deposit

A

Skeidarsandur supply

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12
Q

How is sediment supply to a delta determined by the hinterland climate? (2)

A

Weathering/erosion

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13
Q

How is sediment supply to a delta determined by hinterland tectonics? (3)

A

Topography, river gradient, grainsize

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14
Q

How is sediment supply to a delta determined by water-basin shape?

A

Tidal range

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15
Q

How is sediment supply to a delta determined by water-basin climate?

A

Wave activity

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16
Q

How does the depth of the waterbasin influence delta form?

A

Shallow water body - delta will build out further

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17
Q

What can complicate the recognition of marine deltas in succession?

A

Flora and fauna of land environments found in proximity to animals exclusively from marine conditions

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18
Q

What two variables do delta channels usually have?

A

Meandering or Braided, and Single or Multiple channels

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19
Q

What is a common feature of delta overbank/floodplain deposits in wet tropical regions?

A

Vegetate swamps and thus accumulation of peat

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20
Q

What type of mud may be common on a delta overbank and why?

A

Mixture of organic and clastic material forming carbonaceous mud

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21
Q

What are interdistributary bays?

A

Shelted areas of shallow water protected from higher energy tidal/wave influence

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22
Q

What is a common feature of an interditributary bay?

A

Brackish water due to low energy environment

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23
Q

What is the technical location of the beginning of a delta?

A

Immediately forward of the channel mouth

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24
Q

What would be a typical gradient of a fine-grained delta?

A

1-2 degrees

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25
Q

What would be a typical gradient of a coarse-grained delta?

A

30 degrees

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26
Q

What is a sediment plume and what is it caused by?

A

Suspended load, caused by fresh water having a lower density than saline, and thus remaining more bouyant

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27
Q

What is a common sub-marine feature of the pro-delta region?

A

Turbidite currents

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28
Q

What is a common feature of the overall sucession of a delta?

A

Progradation from deep marine to fluvial

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29
Q

Where are mouth bars found on a delta?

A

At the top of the delta - where flow is still high enough energy

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30
Q

What are the destinctive features of delta progradation? (2)

A

Coarsening up and shallowing up

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31
Q

What features would likely be found at the top of a delta succession? (5)

A

Subaerial deposition: River channel, plant growth, soil formation, desiccation cracks, tracks of land animals

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32
Q

What is the key facies required to determine delta succession?

A

River facies

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33
Q

Give an example of a modern day river dominated delta

A

Mississippi

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34
Q

Give an example of a modern day wave influenced delta

A

Rhone

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35
Q

What are the three factors which infuence morphology of delta facies?

A

Grainsize, water depth, force dominating (river/wave/tide)

36
Q

What would the bedload/suspension/deposition of a fine grained delta look like?

A

Bedload would be sandy, with mud in suspension, sandy mouth bars would form at mouth with mud fallout on delta slope and pro-delta +bays

37
Q

What forms are created due to the slope of a fine grained delta?

A

Turbidity currents at base, plus syndepositional features on the slope

38
Q

What are fan deltas usually fed by? (2)

A

Pebbly braided rivers or alluvial fans

39
Q

Where are fan deltas often found?

A

Ajacent to areas of steep relief (e.g. faulted margins of rift basins)

40
Q

What is the delta top environment of a coarse-grained delta usually like?

A

Coarse braided river or alluvial fan

41
Q

What is the main driving processes which form a coarse-grained delta?

A

Wave action main energy - tidal range is small

42
Q

What is the main difference in progradation successions between fine and coarse grained deltas?

A

Top facies may be different

43
Q

What is delta prograde rate directly dependant on? (2)

A

Water depth and basin size

44
Q

What would be characteristic of delta front facies?

A

Sandy/gravely deposits that are relatively well sorted, reworked by wave action

45
Q

In a deeper water delta, where is the majority of sediment deposited?

A

Lower parts of delta

46
Q

What is a likely feature about the top of the delta which protrudes into deep water?

A

Area of delta top is small - thus plain facies, mires, mouth-bar facies are limited

47
Q

What is a likley feature of the slope of a deep water delta?

A

Gravity flows prominent - turbidity currents

48
Q

What is a “gilbert-style” delta?

A

Coarse sediment and steep margin

49
Q

What are the three parts to the structure of a “gilbert-style” delta?

A

Topset, Foreset, Bottomset

50
Q

What consists of the topset of a delta?

A

Subaerial to shallow marine, gravel, likely reworked by waves

51
Q

What consists of the foreset of a delta?

A

Delta front - steep angle (30 degrees) slumping, debris flow, grain flow

52
Q

What consists of the bottomset of a delta?

A

The base of the foreset - finer deposits, result of turbidites and suspension deposits

53
Q

What is the height of the topset determined by?

A

Depth of water - 10’s to 100’s m

54
Q

What defines the succession of a gilbert style delta vs an alluvial fan?

A

Alluvial fans don’t usually reach angle of rest

55
Q

Where are gilbert style deltas often found?

A

Usually edge of basin margins in tectonically active regions

56
Q

Under what conditions is a delta river dominated?

A

When the environment is microtidal and wave energy is dissapated before it reaches the coastline

57
Q

What is the common shape formation for a river dominated delta and why?

A

Crows-foot - unidirectional current continues to build levees into the basin

58
Q

What can occur in an unstable river dominated delta?

A

Avulsion of the river, and repeated switching giving a pattern of abandoned lobes

59
Q

What is a common feature found between the lobes of a river dominated delta?

A

Interdistributary bays

60
Q

What would a deltaic succession which included interdistributary bays look like?

A

There would be small scale coarsening up successions

61
Q

Where are mouth bars located in a river dominated delta?

A

At at the front of the “toes”

62
Q

Why is progradation on wave dominated deltas limited?

A

Levees do not form

63
Q

How does wind direction have an effect on a wave dominated delta?

A

Longshore drift - lateral migration of sediment

64
Q

Which is better sorted, a wave action dominated delta or a river action dominated delta?

A

Wave action

65
Q

When does progradation occur in a wave dominated delta?

A

When the waves do not carry all of the material away

66
Q

What forms in a wave dominated delta which has a net supply of bedload from a river?

A

Shore parallel sand ridges and mouthbars build up and out to form a new beach

67
Q

What features do both wave dominated and river dominated deltas tend to share?

A

Delta front and prodelta deposits will most likely be very similar between the two

68
Q

What is the most obvious feature about delta top deposits in a tide-dominated deltaic succession? (2 examples)

A

May have significant reversal of flow - either reverse paleocurrents or mud drapes with stagnation

69
Q

Why is a tide-dominated delta more susecptible to flooding?

A

High tide mixed with high-stage flow of the river

70
Q

What is characteristic of the sand bars deposited on a tide-dominated delta?

A

Elonage and perpendicular to shoreline

71
Q

What is a likely feature of the foreset of a tide-dominated delta?

A

Mud-drapes from the ebb

72
Q

What three processes cause a delta lobe to subside?

A

Either in a subsiding basin/sea level rise, or gradual compaction of sediment

73
Q

Where and why does intense bioturbation occur in a deltaic facies?

A

Off the toe of the delta if slow sedimentation rate

74
Q

What are the two keys to recognising deltaic successions?

A

Both marine and continental facies in the vertical, coarsening upwards

75
Q

Where can coal rich ssuccessions occur in deltaic facies?

A

Organic growth on the delta plain which becomes flooded and buried

76
Q

Which two parts of a delta have good reservoir potential and why?

A

Mouth bars and sandy delta plains due to wave reworking making it well sorted

77
Q

How does delta lobe abandonment have good hydrocarbon potential?

A

Sands topped by fine grained material

78
Q

What are the typical lithologies associated with deltaic deposits?

A

Conglomerate, Sandstone, Mudstone

79
Q

What would be expected of the mineralogies of deltaic deposits?

A

Variable - delta front = mature

80
Q

What would be expected of the texture of deltaic deposits?

A

Moderately mature in delta top - sand/gravels mature in reworked delta front

81
Q

What would be expected of the bed geometry of deltaic deposits?

A

Lense shaped delta channels, mouth-bar lenses variable/elongate - prodelta thin bedded

82
Q

What sedimentary structures would be expected in deltaic deposits?

A

Cross-bedding + Lamination in delta top, Mouth bar facies

83
Q

What paleoflow would be expected in deltaic deposits?

A

Topset facies - direction of progration, Wave/tidal reowrking variable on delta front

84
Q

What fossils would be expected in deltaic deposits?

A

Terrestrial plants/animals of delta top - marine fauna on delta front

85
Q

What would be expected of the sedimentary colouring of deltaic deposits?

A

Not diagnostic - delta dop deposits may be oxidised

86
Q

What facies would be expected to be associated to deltaic deposits?

A

Overlaying shallow marine, overlain by fluvial - progradational pattern

Sedimentology and Stratigraphy (19 decks)

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