Dynamic Surface L9 Flashcards
What is a delta?
A protuberance of the shoreline that forms when a river enters a basin whose indigenous processes, waves and tides are unable to redistribute the sediment load.
Deltas are progradational - what does this mean?
The river delta grows further out into the sea over time.
In which direction do deltas coarsen?
Upwards
What environments are found at the delta top zone?
Channel sands
Delta plain muds
Distributary channels
What feature is found at the mouth of the delta and when does it form?
Mouth bar sands are formed at the mouth of the delta when bedload is dumped
Where do prodelta muds form?
Prodelta muds form further out into the basin in deeper waters
What is the upwards progression of material at an ancient delta facies (starting from the bottom)?
Marine shelf deposits (deepest)
Prodelta muds
Mouth bar sands
Delta top deposits
What is outflow dispersion and where does this occur in the delta?
Outflow dispersion occurs when channelised flow of sediment-laden currents (river) meets the receiving basin (ocean), dispersing the flow at the distributary mouth
What is homopycnal flow?
Incoming water is the same density as the water in the receiving basin
What is the effect of homopycnal flow?
Mixing occurs rapidly
During homopycnal flow, what is the effect of a sudden decrease in flow velocity?
The coarser sediment is dumped immediately
What type of delta does homopycnal flow produce?
Classic Gilbert delta
What are the 3 sections of a Gilbert delta?
Topset
Foreset
Bottomset
What is the topset of a Gilbert delta composed of?
Flat lying gravels
What is the foreset of a Gilbert delta composed of and at what angle do these beds dip?
Beds of sand and gravel dipping at 10-25 degrees
What is the bottomset of a Gilbert delta composed of and how are these beds inclined?
Fine grained sediments that are gently inclined
What is hyperpycnal flow?
Inflowing water is much more dense than the water in the receiving basin
What is the effect of hyperpycnal flow?
Much less mixing
Less spread as sediments move out of the channel
Incoming water hugs the bottom of the basin
What is hypopycnal flow?
Inflowing water is much less dense than the water in the receiving basin
What is the effect of hypopycnal flow?
The incoming flow shoots out over the surface of the water in the receiving basin
Very little deposition - sediment will take a long time to settle out through the water column
What are the corners of the triangle in the Galloway Model?
Fluvial processes (top)
Wave processes (bottom left)
Tidal processes (bottom right)
What is a feature of wave dominated deltas?
Ridges parallel to the coastline formed by waves
What is a feature of tidal dominated deltas?
Offshore bars of either mud or sand parallel to the movement of the tide
What causes the cyclicity of coarse, to fine, to limestone deposits in delta environments?
The abandonment and reactivation of delta lobes.
When a delta lobe is abandoned it sinks and marine deposits form on top of it however it may be reactivated meaning fluvial deposits will form. This process repeats.
