Shallow marine clastic environments

Question 1

Define shallow water

Answer 1

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

Question 2

How else can shallow water be defined?

Answer 2

Low gradient continental shelf
100-140m

Question 3

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

Answer 3

Dissolved load, suspended load, bedload

Question 4

What are the three drivers of transport?

Answer 4

River-dominated, tide-dominated, wave-dominated

Question 5

What type of coastlines are wave dominated?

Answer 5

Linear coastlines

Question 6

What type of coastlines are tide dominated?

Answer 6

Embayed coastlines

Question 7

What type of coastlines are river dominated?

Answer 7

Oblate coastlines

Question 8

Give four examples of wave-dominated settings

Answer 8

Strandplains, spits, beaches, and lagoons

Question 9

Give two examples of tide-domianted settings

Answer 9

Estuaries and tidal flats

Question 10

Give an examples of a river-dominated setting

Answer 10

Deltas

Question 11

Describe a mouthbar

Answer 11

Where fluvial currents decelerate

Question 12

Define a funnel

Answer 12

Tidal excavation

Question 13

Where do particle pollutants often end up?

Answer 13

The shelf

Question 14

What are rocky, eroding shorelines analogous to?

Answer 14

Bedrock rivers
Unconformity forming

Question 15

Describe a regressive coastline

Answer 15

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

Question 16

Describe a transgressive coastline

Answer 16

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

Question 17

What happens when sediment is deposited in a subsiding basin?

Answer 17

Transgression accelerates and regression slows
Causing flooding and slow building out

Question 18

Describe transgressive deposits

Answer 18

Thin or absent in the rock record

Question 19

Describe the shallowing up succession

Answer 19

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

Question 20

Describe a delta

Answer 20

Protuberances of the land into the sea

Question 21

How do deltas form?

Answer 21

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

Question 22

How do deltas affect coastlines?

Answer 22

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

Question 23

By what mechanism is flow in deltas driven?

Answer 23

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

Question 24

Describe jet theory in relation to deltas

Answer 24

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

Question 25

Why do hyperpycal jets not make deltas?

Answer 25

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

Question 26

Describe hypopycnal jets

Answer 26

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

Question 27

What happpens to the suspended load of a hypopycnal jet?

Answer 27

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

Question 28

Describe a homopycnal jet

Answer 28

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

Question 29

What are the results of a homopycnal jet?

Answer 29

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

Question 30

Describe delta evolution

Answer 30

Mouth bar formation, bifurcation, channel abandonment

Question 31

What are the two classic characteristics of delta deposits?

Answer 31

Clinoforms and coasening upward successions

Question 32

Describe clinoforms in deltas

Answer 32

Seaward-dipping surfaces because they build out into deeper water

Question 33

Describe coarsening-upward successions in deltas

Answer 33

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

Question 34

What characterises delta deposits?

Answer 34

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

Question 35

What are most non-deltaic coastlines?

Answer 35

Wave-dominated

Question 36

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

Answer 36

Tidal amplification and absence of wave energy

Question 37

What is the ultimate source of sediment in shorefaces?

Answer 37

Rivers and deltas

Question 38

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

Answer 38

Waves and associated processes

Question 39

What is the main wave transport mechanism?

Answer 39

Longshore drift

Question 40

Describe long shore drift

Answer 40

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

Question 41

When is a spit formed?

Answer 41

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

Question 42

How is a barrier-lagoon complex formed?

Answer 42

When the embayment is closed by the spit

Question 43

Describe the formation of broad strandplains

Answer 43

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

Question 44

Define the littoral energy fence

Answer 44

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

Question 45

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

Answer 45

Fair weather wave base Storm wave base

Question 46

Describe the fair weather wave base

Answer 46

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

Question 47

Describe the storm wave base

Answer 47

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

Question 48

Describe sediment deposition after a storm

Answer 48

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

Question 49

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

Answer 49

Hummocky cross stratification

Question 50

What causes breaking waves?

Answer 50

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

Question 51

Why are waves different heights?

Answer 51

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

Question 52

Describe flows at the upper shoreface

Answer 52

High velocity, unidirectional (or combined)

Question 53

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

Answer 53

Upper stage plane beds formed by laminar flow that shallows rapidly

Question 54

What type of sequences are seen in shoreline progradation deposits?

Answer 54

Shallowing-up

Question 55

Describe tide-dominated systems

Answer 55

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