Seds - Intertidal

Question 1

Controls on sedimentation at the shoreface:

Answer 1

Overall controls
o Climate and tectonics
• Waves
• Tides

Question 2

Control on sedimentation: Waves

Answer 2

o A way of transferring energy without necessarily moving particles
o Waves look longitudinal but are orbital
 Open orbital = ability to move mass
 Allows waves to scour surfaces they pass over
 Incompletely closed orbits.
• Allows waves to carry small amounts of sediment
o Swash and backwash
 Swash and backwash set up a negative feedback system for modulating beach steepness.
 Balance between two allows for building up sandbars
 Swash moves sediment up
 Backwash moves sediment down
 Mostly in equilibrium therefore means most beaches have similar angles
o Longshore drift
o Allows for direction of progradation to not be perpendicular to shore.

Question 3

Control on sedimentation: Tides

Answer 3

o Effectively long wavelength waves, that always act like they’re in shallow water.
o Movement of moon means lunar day is 24 hr 50 mins
o Long-wavelength waves
• High tides at red flags – ‘equator poles’
• Moon attraction on right side
• Less attraction to moon on left side so water can move in opposite direction

Question 4

Amphidromic Points

Answer 4

• Destructive interference produces points where there is no tide.
• Caused by the Coriolis force moving tidal waves around the points

Question 5

Tides and Waves combined:

Answer 5

• Can’t consider processes individually. The convolution of processes determines the resulting environment.
• Tide dominated gives more asymmetrical structures.
• Wave dominated more likely to produce strandplains
• Wave dominated gives more current flow structures.
• Wave dominated more likely to produce bars and spits.

Question 6

Shoreface Deposits:

Answer 6

o Occur wherever there is oscillating backward and forwards.
o Orbital wave motion becomes oscillatory at water-sediment interface where water depth < 1/20 wavelength
o Herringbone cross stratification
o Tidal and storm features would be convolved with the wave structures.

Question 7

Storm deposits:

Answer 7

o Note that storm beds/tempestites are found beneath the storm wave base.
o Density current genesis makes tempestites similar to turbidites in many outcrops.

Tempestites:
o Similar appearance to turbidites, but more lenticular form of bedding

Hummocky cross-stratification:
o Produced on the shallow sea floor during storms from current and oscillatory flow action
o Sediment is dropped irregularly on a scoured surface leading to ‘hummocks’

Question 8

Wave dominated environments

Answer 8

Beaches - Typical’ beach
o A beach is a bar, conceptually similar to what you’d find in a river.
o Becomes a strandplain when the beach progrades from the shoreline

Barrier islands -
o Siliclastic barriers formed by:
o Wave convergence bars
o Sea-level rise drowning beach
o Spits
o Carbonate barriers formed by:
o Reef growth
o Need trapping of muddy sediment to reduce reef permeability and properly isolate lagoon from marine water
o Colonisation of elevated sandbar to promote carbonate growth
o Washover deposits.
o Protected area of low energy gives fine grains.
o Undisturbed, but low nutrient input gives specific fauna growth.
o Oysters common.
o Peloids if carbonate sedimentation

Question 9

Tidal dominated environments

Answer 9

Tidal flats -
o Low relief topography adjacent to the shore, acting as evaporitic pan.
o Saline groundwater allows for recharge of evaporite minerals.
Estuaries -
o Extremely productive.
o Water forms ‘salt wedge’ configuration.
o Current abundance unusual
Tidal sand ridges