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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
What would be a typical gradient of a coarse-grained delta?
30 degrees
26
What is a sediment plume and what is it caused by?
Suspended load, caused by fresh water having a lower density than saline, and thus remaining more bouyant
27
What is a common sub-marine feature of the pro-delta region?
Turbidite currents
28
What is a common feature of the overall sucession of a delta?
Progradation from deep marine to fluvial
29
Where are mouth bars found on a delta?
At the top of the delta - where flow is still high enough energy
30
What are the destinctive features of delta progradation? (2)
Coarsening up and shallowing up
31
What features would likely be found at the top of a delta succession? (5)
Subaerial deposition: River channel, plant growth, soil formation, desiccation cracks, tracks of land animals
32
What is the key facies required to determine delta succession?
River facies
33
Give an example of a modern day river dominated delta
Mississippi
34
Give an example of a modern day wave influenced delta
Rhone
35
What are the three factors which infuence morphology of delta facies?
Grainsize, water depth, force dominating (river/wave/tide)
36
What would the bedload/suspension/deposition of a fine grained delta look like?
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
What forms are created due to the slope of a fine grained delta?
Turbidity currents at base, plus syndepositional features on the slope
38
What are fan deltas usually fed by? (2)
Pebbly braided rivers or alluvial fans
39
Where are fan deltas often found?
Ajacent to areas of steep relief (e.g. faulted margins of rift basins)
40
What is the delta top environment of a coarse-grained delta usually like?
Coarse braided river or alluvial fan
41
What is the main driving processes which form a coarse-grained delta?
Wave action main energy - tidal range is small
42
What is the main difference in progradation successions between fine and coarse grained deltas?
Top facies may be different
43
What is delta prograde rate directly dependant on? (2)
Water depth and basin size
44
What would be characteristic of delta front facies?
Sandy/gravely deposits that are relatively well sorted, reworked by wave action
45
In a deeper water delta, where is the majority of sediment deposited?
Lower parts of delta
46
What is a likely feature about the top of the delta which protrudes into deep water?
Area of delta top is small - thus plain facies, mires, mouth-bar facies are limited
47
What is a likley feature of the slope of a deep water delta?
Gravity flows prominent - turbidity currents
48
What is a "gilbert-style" delta?
Coarse sediment and steep margin
49
What are the three parts to the structure of a "gilbert-style" delta?
Topset, Foreset, Bottomset
50
What consists of the topset of a delta?
Subaerial to shallow marine, gravel, likely reworked by waves
51
What consists of the foreset of a delta?
Delta front - steep angle (30 degrees) slumping, debris flow, grain flow
52
What consists of the bottomset of a delta?
The base of the foreset - finer deposits, result of turbidites and suspension deposits
53
What is the height of the topset determined by?
Depth of water - 10's to 100's m
54
What defines the succession of a gilbert style delta vs an alluvial fan?
Alluvial fans don't usually reach angle of rest
55
Where are gilbert style deltas often found?
Usually edge of basin margins in tectonically active regions
56
Under what conditions is a delta river dominated?
When the environment is microtidal and wave energy is dissapated before it reaches the coastline
57
What is the common shape formation for a river dominated delta and why?
Crows-foot - unidirectional current continues to build levees into the basin
58
What can occur in an unstable river dominated delta?
Avulsion of the river, and repeated switching giving a pattern of abandoned lobes
59
What is a common feature found between the lobes of a river dominated delta?
Interdistributary bays
60
What would a deltaic succession which included interdistributary bays look like?
There would be small scale coarsening up successions
61
Where are mouth bars located in a river dominated delta?
At at the front of the "toes"
62
Why is progradation on wave dominated deltas limited?
Levees do not form
63
How does wind direction have an effect on a wave dominated delta?
Longshore drift - lateral migration of sediment
64
Which is better sorted, a wave action dominated delta or a river action dominated delta?
Wave action
65
When does progradation occur in a wave dominated delta?
When the waves do not carry all of the material away
66
What forms in a wave dominated delta which has a net supply of bedload from a river?
Shore parallel sand ridges and mouthbars build up and out to form a new beach
67
What features do both wave dominated and river dominated deltas tend to share?
Delta front and prodelta deposits will most likely be very similar between the two
68
What is the most obvious feature about delta top deposits in a tide-dominated deltaic succession? (2 examples)
May have significant reversal of flow - either reverse paleocurrents or mud drapes with stagnation
69
Why is a tide-dominated delta more susecptible to flooding?
High tide mixed with high-stage flow of the river
70
What is characteristic of the sand bars deposited on a tide-dominated delta?
Elonage and perpendicular to shoreline
71
What is a likely feature of the foreset of a tide-dominated delta?
Mud-drapes from the ebb
72
What three processes cause a delta lobe to subside?
Either in a subsiding basin/sea level rise, or gradual compaction of sediment
73
Where and why does intense bioturbation occur in a deltaic facies?
Off the toe of the delta if slow sedimentation rate
74
What are the two keys to recognising deltaic successions?
Both marine and continental facies in the vertical, coarsening upwards
75
Where can coal rich ssuccessions occur in deltaic facies?
Organic growth on the delta plain which becomes flooded and buried
76
Which two parts of a delta have good reservoir potential and why?
Mouth bars and sandy delta plains due to wave reworking making it well sorted
77
How does delta lobe abandonment have good hydrocarbon potential?
Sands topped by fine grained material
78
What are the typical lithologies associated with deltaic deposits?
Conglomerate, Sandstone, Mudstone
79
What would be expected of the mineralogies of deltaic deposits?
Variable - delta front = mature
80
What would be expected of the texture of deltaic deposits?
Moderately mature in delta top - sand/gravels mature in reworked delta front
81
What would be expected of the bed geometry of deltaic deposits?
Lense shaped delta channels, mouth-bar lenses variable/elongate - prodelta thin bedded
82
What sedimentary structures would be expected in deltaic deposits?
Cross-bedding + Lamination in delta top, Mouth bar facies
83
What paleoflow would be expected in deltaic deposits?
Topset facies - direction of progration, Wave/tidal reowrking variable on delta front
84
What fossils would be expected in deltaic deposits?
Terrestrial plants/animals of delta top - marine fauna on delta front
85
What would be expected of the sedimentary colouring of deltaic deposits?
Not diagnostic - delta dop deposits may be oxidised
86
What facies would be expected to be associated to deltaic deposits?
Overlaying shallow marine, overlain by fluvial - progradational pattern

Sedimentology and Stratigraphy (19 decks)

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