Shallow marine clastic environments Flashcards
Define shallow water
Occuring above the storm wave base (varies)
Influenced by wave/tidal activity and their products
Typically 50-200m
How else can shallow water be defined?
Low gradient continental shelf
100-140m
What are the three modes of sediment transport in shallow seas?
Dissolved load, suspended load, bedload
What are the three drivers of transport?
River-dominated, tide-dominated, wave-dominated
What type of coastlines are wave dominated?
Linear coastlines
What type of coastlines are tide dominated?
Embayed coastlines
What type of coastlines are river dominated?
Oblate coastlines
Give four examples of wave-dominated settings
Strandplains, spits, beaches, and lagoons
Give two examples of tide-domianted settings
Estuaries and tidal flats
Give an examples of a river-dominated setting
Deltas
Describe a mouthbar
Where fluvial currents decelerate
Define a funnel
Tidal excavation
Where do particle pollutants often end up?
The shelf
What are rocky, eroding shorelines analogous to?
Bedrock rivers
Unconformity forming
Describe a regressive coastline
Building out into the sea
Happens when sea level falls
Overall shallowing
Describe a transgressive coastline
Moving back into the land
Happens when sea level rises
Overall deepening
What happens when sediment is deposited in a subsiding basin?
Transgression accelerates and regression slows
Causing flooding and slow building out
Describe transgressive deposits
Thin or absent in the rock record
Describe the shallowing up succession
Characteristic of shallow marine deposits
Repeated coarsening-up, thickening-up, shallowing-up surfaces, separated by sharp flooding surfaces
Describe a delta
Protuberances of the land into the sea
How do deltas form?
Where the rivers deliver sediment faster than it can be reworked and transported away by wave and tidal processes
How do deltas affect coastlines?
They are the main way by which shorelines prograde, deltas are fundamentally regressive
By what mechanism is flow in deltas driven?
Pre-exisiting inteetia of the river as it enters the sea (or lake)
Describe jet theory in relation to deltas
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
Why do hyperpycal jets not make deltas?
Jet is denser than the standing water, hugging the bed
The hyperconc of sediment surpresses turbulence, so inertia of flow is maintained
Describe hypopycnal jets
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
What happpens to the suspended load of a hypopycnal jet?
Carried out to the shelf or reworked by tide a wave processes
Forms shelf muds
Describe a homopycnal jet
Sea is denser than jet
The jet expands in 3d, decelerating and staying in contact with bed
Rapid deposition of bedload and suspended load
What are the results of a homopycnal jet?
Chocking of the river mouth
Rapid switching and migration of the distributary
Radial ‘fan’ deltas
Describe delta evolution
Mouth bar formation, bifurcation, channel abandonment
What are the two classic characteristics of delta deposits?
Clinoforms and coasening upward successions
Describe clinoforms in deltas
Seaward-dipping surfaces because they build out into deeper water
Describe coarsening-upward successions in deltas
Form because the coarse bedload is deposited rapidly at mouth bar and the finer suspended load is carried further offshore
What characterises delta deposits?
Shallowing-upward cycles, both vertically and laterally
Known as ‘manye’
Cross-lamination and cross-bedding
What are most non-deltaic coastlines?
Wave-dominated
When are tidal processes important on a local scale in non-deltaic coastlies?
Tidal amplification and absence of wave energy
What is the ultimate source of sediment in shorefaces?
Rivers and deltas
What provides the mechanism for trasporting and depositing sediment along coasts?
Waves and associated processes
What is the main wave transport mechanism?
Longshore drift
Describe long shore drift
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
When is a spit formed?
Longshore drift in transgressive settings carris sediment past the mouths of flooded valleys (estuaries and embayments)
How is a barrier-lagoon complex formed?
When the embayment is closed by the spit
Describe the formation of broad strandplains
The build out of the shoreface-beach system in regressive settings (progradational)
Define the littoral energy fence
The line that sediment from the landward side must cross to be removed from the shoreface
Needs to cross threshold velocity
What are the two wave bases of shorefaces (wave-dominated systems)
Fair weather wave base
Storm wave base
Describe the fair weather wave base
Typically 5-15m
Everything above is permanently agitated
No mud deposition
This is the shoreface
Describe the storm wave base
100m+
Seabed is agitated during storms
Forms the offshore transition zone
Describe sediment deposition after a storm
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
What is the product of combined unidirectional and oscillatory currents after a storm in shorefaces?
Hummocky cross stratification
What causes breaking waves?
Acceleration occurs as the flow is shoaled against the inclined bed, velocity reduced by high bed friction
Why are waves different heights?
Sea-floor irregularities
Cell-like circulation in nearshore (flow fron high-low pressure)
Describe flows at the upper shoreface
High velocity, unidirectional (or combined)
What type of beds are formed at the wave awash zone?
Upper stage plane beds formed by laminar flow that shallows rapidly
What type of sequences are seen in shoreline progradation deposits?
Shallowing-up
Describe tide-dominated systems
Sediment is reworked by tidal currnets, creating subaeria and subaqueous tidal shoals and islands (parallel to tidal flow direction)
