Week 7 Flashcards
(32 cards)
Controls on the intertidal/marine environment
WAVES
TIDES
(climate/tectonics)
Waves (control)
Orbital = scour surfaces they pass over
Incompletely closed orbits = Stokes drift = allows to carry small amount of sediment
Swash/backwash = -ve feedback system
- moderates beach steepness
Point where neither dominant = sandbars
Longshore drift
Waves (features)
Symmetrical
High current flow structures e.g. cross-bedding
Higher slope than tides
Strandplains
Bars
Spits
Tides (control)
Wavelength = earth’s diameter
Moon pulls highest tide (gravity on other side)
When water depth = wave depth = break and move sediment around
Tide variations
Earth is not a perfect sphere
Coastline reflect tides = 12 hour lunar day
Amphidomic points
Amphidromic point =
Destructive interference point with no tide, relatively flat water
“Coriolis force moves tidal waves around points”
e.g. close to English channel
Tides (features)
Asymmetrical
Possible environments at the land-water interface
- TIDAL FLAT (linear coast with marine sediment supply; tide dominated)
- DELTA (regressive, elongate lobate coasts; tide/wave fairly balanced)
- ESTUARY (transgressive, embayed coasts; can be tide/wave dominated)
- STRANDPLAIN (linear coast with marine sediment supply; wave dominated)
- Lagoon (transgressive, embayed coasts; wave dominated)
Controls on sediment type
Sediment supply
Pre-existing topography
Wave vs tide magnitude
Accommodation space (tectonics/climate)
Regressive environments
High sediment supply
= decreases SL
Prograding
Coarsens upwards
Transgressive environments
Increasing SL
Fining upwards
Forced regression =
DRIVEN by base level fall
Tidal flat environment
Tide > wave
Fine material washed inshore by tide = plants/algae trap
- stromatolite = algal mat traps sediment in mucus layer
- concentrated seawater = precipitate calcite and cemented
Fine upwards
Why are plants/algae required to trap sediment in tidal flats?
Clastics don’t cement to bind unlike carbonates
Salt flats =
Low relief topography adjacent to shore = evaporitic pan
- saline groundwater recharges
Delta environment =
shoreline protuberance where river flows into an ocean basin/large standing body of water
TRANSPORTS SEDIMENT LADEN WATER FROM CHANNEL TO UNCONFINED ENVIRONMENT
Coarsening upwards sequence
Deltaic sequence
DELTA TOP/PLAIN
- river meets ocean
- sub environments e.g. distributary channels, floodplains, swamps, lakes
= sandstones/mudstones/coal
DELTA FRONT
- sediment carried by distributary channels and deposited
- rapid deposition = seaward migration
= sand with cross bedding/ripples/bioturbation
PRODELTA
- offshore
= organic rich/laminated/bioturbated mudstones
Estuary environment =
Not enough sediment to fill river mouth = deep water near sea
Very productive due to water flowing into sea = nutrients = ecosystems
Salt wedge
N.B. not a lot of evidence in geological record
Beach/strandplain environment =
Think of a bar attached to the side of land
Progrades from shoreline = strand plain with lagoons and spits
- same process as maintenance of lagoon
- refraction is key
- no wave action behind
Lagoon/barrier island environment; siliclastic barriers
When neither swash/backwash dominant = deposition = spits
Not connected PHYSICALLY to water but HYDROLOGICALLY
- no waves/reduced storm effect = bioturbation
- high tide/porous rock
Rock record: mud next to high energy deposit
- protected low energy area = fine grains
- low nutrient input = specific fauna growth e.g. oysters
Lagoon/barrier island environment; carbonate barriers
Due to reef growth = traps muddy sediment = reduces reef permeability
Colonisation of elevated sandbar = promotes carbonate growth
What do tidal flats with carbonates in suggest?
Wouldn’t normal occur
Must be washer deposits
Deposits at the shoreface
Wave ripples
Herringbone cross stratification
Storm deposits
Tempestites
Hummocky cross stratification
Wave ripples
Where oscillating backwards/forwards at water/sediment interface
SHALLOW water depth 1/20 wavelength
Orbital –> elliptical –> side to side
(Wave)
