Week 7

Question 1

Controls on the intertidal/marine environment

Answer 1

WAVES

TIDES

(climate/tectonics)

Question 2

Waves (control)

Answer 2

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

Question 3

Waves (features)

Answer 3

Symmetrical
High current flow structures e.g. cross-bedding
Higher slope than tides

Strandplains
Bars
Spits

Question 4

Tides (control)

Answer 4

Wavelength = earth’s diameter

Moon pulls highest tide (gravity on other side)

When water depth = wave depth = break and move sediment around

Question 5

Tide variations

Answer 5

Earth is not a perfect sphere

Coastline reflect tides = 12 hour lunar day

Amphidomic points

Question 6

Amphidromic point =

Answer 6

Destructive interference point with no tide, relatively flat water

“Coriolis force moves tidal waves around points”

e.g. close to English channel

Question 7

Tides (features)

Answer 7

Asymmetrical

Question 8

Possible environments at the land-water interface

Answer 8

1. TIDAL FLAT (linear coast with marine sediment supply; tide dominated)
2. DELTA (regressive, elongate lobate coasts; tide/wave fairly balanced)
3. ESTUARY (transgressive, embayed coasts; can be tide/wave dominated)
4. STRANDPLAIN (linear coast with marine sediment supply; wave dominated)
5. Lagoon (transgressive, embayed coasts; wave dominated)

Question 9

Controls on sediment type

Answer 9

Sediment supply

Pre-existing topography

Wave vs tide magnitude

Accommodation space (tectonics/climate)

Question 10

Regressive environments

Answer 10

High sediment supply
= decreases SL

Prograding

Coarsens upwards

Question 11

Transgressive environments

Answer 11

Increasing SL

Fining upwards

Question 12

Forced regression =

Answer 12

DRIVEN by base level fall

Question 13

Tidal flat environment

Answer 13

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

Question 14

Why are plants/algae required to trap sediment in tidal flats?

Answer 14

Clastics don’t cement to bind unlike carbonates

Question 15

Salt flats =

Answer 15

Low relief topography adjacent to shore = evaporitic pan

- saline groundwater recharges

Question 16

Delta environment =

Answer 16

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

Question 17

Deltaic sequence

Answer 17

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

Question 18

Estuary environment =

Answer 18

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

Question 19

Beach/strandplain environment =

Answer 19

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

Question 20

Lagoon/barrier island environment; siliclastic barriers

Answer 20

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

Question 21

Lagoon/barrier island environment; carbonate barriers

Answer 21

Due to reef growth = traps muddy sediment = reduces reef permeability

Colonisation of elevated sandbar = promotes carbonate growth

Question 22

What do tidal flats with carbonates in suggest?

Answer 22

Wouldn’t normal occur

Must be washer deposits

Question 23

Deposits at the shoreface

Answer 23

Wave ripples

Herringbone cross stratification

Storm deposits

Tempestites

Hummocky cross stratification

Question 24

Wave ripples

Answer 24

Where oscillating backwards/forwards at water/sediment interface
SHALLOW water depth 1/20 wavelength

Orbital –> elliptical –> side to side

(Wave)

Question 25

Lag =

Answer 25

Where bar will build up

Question 26

Herringbone cross stratification

Answer 26

Larger scale

TIDAL

Lots of sediment supply in system

Question 27

Storm deposits

Answer 27

Waves/tides interact

Most energy, destroys anything not lithified before

Question 28

FWB =

Answer 28

How deeply you get turbulent flow

Question 29

SWB =

Answer 29

No sediment structures beneath ~20-50m

Although N.B. tempestites = “storm beds”

Question 30

Tempestites =

Answer 30

Stuff thrown up comes back down in low energy
Indicative of SWB

Look like turbidites but more lenticular and if HCS further up in cross section = identify

Mass transport downslope = big layers

Question 31

Hummocky cross stratification

Answer 31

Tides/waves interact = small areas with no transport

1-2cm width, cross bedding 10-20cm

Between FWB and SWB

Hummocks preserved at depth

If just swales preserved = at SWB (top of hummocks eroded)

Question 32

Combined deposit overview

Answer 32

Aeolian cross beds
Rootlets

Flat beds
Antidunes
Wave ripples
Well-sorted sand, compositionally mature placer deposits

Lunate dunes

Swaley cross stratification

Hummocky cross stratification

Increasing bioturbation and mud offshore

Graded storm beds and shelf muds