Week 6 - Shallow marine clastic environments

Define shallow water

Above the storm wave-base

Influenced by wave and tidal activity and their products

50-140m deep

Low gradient (<0.05degres) continental shelf

100s of m - 100s of km wide

Forms of sediment transport in shallow seas

Dissolved load

Suspended load

Bedload

Dissolved load

Ions in solution

Suspended load

Fine sand, silt and clay particles and turbidity currents

Bedload

On/near the bed, rolling and saltating

List the 3 (not mutually exclusive) types of shallow water system

River dominated

Tidal dominated

Wave dominated

Key features of river dominated systems

Lobate coastlines

Deltas (bulges)

Channels branching and thinning

Key features of Tidal dominated systems

Embayed coastlines

Estuaries and tidal flats

Channels widen towards the coast

Key features of wave dominated features

Linear coastlines

Strandplains, spits, beaches and lagoons

Regressive shorelines

Coast which progrades → shifts towards the sea

Transgressive shorelines

Coast which retrogrades → shifts towards the land

Key features of deltas

Protuberance of the land and sea

Deliver sediments faster into the sea than can be transported and reworked

Regressive → causes shorelines to prograde further out

Form coudening upwards successions with courser grains deposited rapidly at the mouth bar and diner load carried further offshore

How is the sediment deposited out of a delta

Flow driven by the inertia of the river as it enters the sea

Turbulent diffusion at the outlet → flow mixes with ambient water

Loses energy and rapidly deposits sediment

Hyperpycnal flows

When ρ jet > ρ ambient (e.g. high suspended load entering fresh water), the jet hugs the bed

Hyperconcentration sediment supresses jet turbulence

Doesn’t mix instead travels along the bottom

Hypopycnal flows

When ρ jet < ρ ambient (e.g. sluggish, low suspended load entering salt water), the jet detaches from the bed

Cannot drive the bedload

Suspended load carried out to the shelf, or reworked by tide and wave processes

Homopycnal flows

When ρ jet = ρ ambient

Jet expands and decelerates rapidly in 3D, but remains in contact with the bed

Rapid deposition of bedload and suspended load

Choking of the river mouth and rapid switching and migration of the distributary → Radial ‘fan‘ deltas

Stages of delta evolution

What is a clinoform

Surface dipping seaward

Formed by deltas

Summary facies model for deltas

Coarsening-up, thickening-up from prodelta mud → delta front (mouthbar) sandstones

Tractive sedimentary structures (cross-lamination, cross-bedding)

Seaward dipping clinoform

The littoral energy fence

No change in energy, but velocity coming in > going out

 

Threshold velocity only exceeded on the way in, not on the way out so more grains are moved on the way in -> sediment is piled up onto the land