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EART23202 Interpreting the stratigraphic record > Shallow marine clastic environments > Flashcards

Shallow marine clastic environments Flashcards

1
Q

Define shallow water

A

Occuring above the storm wave base (varies)
Influenced by wave/tidal activity and their products
Typically 50-200m

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2
Q

How else can shallow water be defined?

A

Low gradient continental shelf
100-140m

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3
Q

What are the three modes of sediment transport in shallow seas?

A

Dissolved load, suspended load, bedload

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4
Q

What are the three drivers of transport?

A

River-dominated, tide-dominated, wave-dominated

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5
Q

What type of coastlines are wave dominated?

A

Linear coastlines

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6
Q

What type of coastlines are tide dominated?

A

Embayed coastlines

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7
Q

What type of coastlines are river dominated?

A

Oblate coastlines

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8
Q

Give four examples of wave-dominated settings

A

Strandplains, spits, beaches, and lagoons

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9
Q

Give two examples of tide-domianted settings

A

Estuaries and tidal flats

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10
Q

Give an examples of a river-dominated setting

A

Deltas

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11
Q

Describe a mouthbar

A

Where fluvial currents decelerate

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12
Q

Define a funnel

A

Tidal excavation

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13
Q

Where do particle pollutants often end up?

A

The shelf

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14
Q

What are rocky, eroding shorelines analogous to?

A

Bedrock rivers
Unconformity forming

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15
Q

Describe a regressive coastline

A

Building out into the sea
Happens when sea level falls
Overall shallowing

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16
Q

Describe a transgressive coastline

A

Moving back into the land
Happens when sea level rises
Overall deepening

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17
Q

What happens when sediment is deposited in a subsiding basin?

A

Transgression accelerates and regression slows
Causing flooding and slow building out

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18
Q

Describe transgressive deposits

A

Thin or absent in the rock record

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19
Q

Describe the shallowing up succession

A

Characteristic of shallow marine deposits
Repeated coarsening-up, thickening-up, shallowing-up surfaces, separated by sharp flooding surfaces

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20
Q

Describe a delta

A

Protuberances of the land into the sea

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21
Q

How do deltas form?

A

Where the rivers deliver sediment faster than it can be reworked and transported away by wave and tidal processes

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22
Q

How do deltas affect coastlines?

A

They are the main way by which shorelines prograde, deltas are fundamentally regressive

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23
Q

By what mechanism is flow in deltas driven?

A

Pre-exisiting inteetia of the river as it enters the sea (or lake)

24
Q

Describe jet theory in relation to deltas

A

Turbulent ‘jet’ (river) enters the standing body of water, leading to energy diffusion
Jet begins to deposit bedload and suspended load at the river mouth

25
Q

Why do hyperpycal jets not make deltas?

A

Jet is denser than the standing water, hugging the bed
The hyperconc of sediment surpresses turbulence, so inertia of flow is maintained

26
Q

Describe hypopycnal jets

A

The jet has low density (or sea water has high density) and detaches from the bed
The jet cannot drive bedload, coarse load is rapidly deposited at river mouth, forming mouth bars

27
Q

What happpens to the suspended load of a hypopycnal jet?

A

Carried out to the shelf or reworked by tide a wave processes
Forms shelf muds

28
Q

Describe a homopycnal jet

A

Sea is denser than jet
The jet expands in 3d, decelerating and staying in contact with bed
Rapid deposition of bedload and suspended load

29
Q

What are the results of a homopycnal jet?

A

Chocking of the river mouth
Rapid switching and migration of the distributary
Radial ‘fan’ deltas

30
Q

Describe delta evolution

A

Mouth bar formation, bifurcation, channel abandonment

31
Q

What are the two classic characteristics of delta deposits?

A

Clinoforms and coasening upward successions

32
Q

Describe clinoforms in deltas

A

Seaward-dipping surfaces because they build out into deeper water

33
Q

Describe coarsening-upward successions in deltas

A

Form because the coarse bedload is deposited rapidly at mouth bar and the finer suspended load is carried further offshore

34
Q

What characterises delta deposits?

A

Shallowing-upward cycles, both vertically and laterally
Known as ‘manye’
Cross-lamination and cross-bedding

35
Q

What are most non-deltaic coastlines?

A

Wave-dominated

36
Q

When are tidal processes important on a local scale in non-deltaic coastlies?

A

Tidal amplification and absence of wave energy

37
Q

What is the ultimate source of sediment in shorefaces?

A

Rivers and deltas

38
Q

What provides the mechanism for trasporting and depositing sediment along coasts?

A

Waves and associated processes

39
Q

What is the main wave transport mechanism?

A

Longshore drift

40
Q

Describe long shore drift

A

Movement of sediments (the spit) along the coastline by waves that approach the shore at an angle but recede directly from it
Eradicates irregularies in coastline, making them straight

41
Q

When is a spit formed?

A

Longshore drift in transgressive settings carris sediment past the mouths of flooded valleys (estuaries and embayments)

42
Q

How is a barrier-lagoon complex formed?

A

When the embayment is closed by the spit

43
Q

Describe the formation of broad strandplains

A

The build out of the shoreface-beach system in regressive settings (progradational)

44
Q

Define the littoral energy fence

A

The line that sediment from the landward side must cross to be removed from the shoreface
Needs to cross threshold velocity

45
Q

What are the two wave bases of shorefaces (wave-dominated systems)

A

Fair weather wave base
Storm wave base

46
Q

Describe the fair weather wave base

A

Typically 5-15m
Everything above is permanently agitated
No mud deposition
This is the shoreface

47
Q

Describe the storm wave base

A

100m+
Seabed is agitated during storms
Forms the offshore transition zone

48
Q

Describe sediment deposition after a storm

A

Onshore winds and decreasing atm pressure creates pressure gradient
Gradient decreases as storm wanes, directing a current offshore
Flows entrain, wane, and deposit
Unidirectional/oscillatory currents form bedforms

49
Q

What is the product of combined unidirectional and oscillatory currents after a storm in shorefaces?

A

Hummocky cross stratification

50
Q

What causes breaking waves?

A

Acceleration occurs as the flow is shoaled against the inclined bed, velocity reduced by high bed friction

51
Q

Why are waves different heights?

A

Sea-floor irregularities
Cell-like circulation in nearshore (flow fron high-low pressure)

52
Q

Describe flows at the upper shoreface

A

High velocity, unidirectional (or combined)

53
Q

What type of beds are formed at the wave awash zone?

A

Upper stage plane beds formed by laminar flow that shallows rapidly

54
Q

What type of sequences are seen in shoreline progradation deposits?

A

Shallowing-up

55
Q

Describe tide-dominated systems

A

Sediment is reworked by tidal currnets, creating subaeria and subaqueous tidal shoals and islands (parallel to tidal flow direction)

EART23202 Interpreting the stratigraphic record (11 decks)

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