Dynamic Surface L9

Question 1

What is a delta?

Answer 1

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.

Question 2

Deltas are progradational - what does this mean?

Answer 2

The river delta grows further out into the sea over time.

Question 3

In which direction do deltas coarsen?

Answer 3

Upwards

Question 4

What environments are found at the delta top zone?

Answer 4

Channel sands
Delta plain muds
Distributary channels

Question 5

What feature is found at the mouth of the delta and when does it form?

Answer 5

Mouth bar sands are formed at the mouth of the delta when bedload is dumped

Question 6

Where do prodelta muds form?

Answer 6

Prodelta muds form further out into the basin in deeper waters

Question 7

What is the upwards progression of material at an ancient delta facies (starting from the bottom)?

Answer 7

Marine shelf deposits (deepest)
Prodelta muds
Mouth bar sands
Delta top deposits

Question 8

What is outflow dispersion and where does this occur in the delta?

Answer 8

Outflow dispersion occurs when channelised flow of sediment-laden currents (river) meets the receiving basin (ocean), dispersing the flow at the distributary mouth

Question 9

What is homopycnal flow?

Answer 9

Incoming water is the same density as the water in the receiving basin

Question 10

What is the effect of homopycnal flow?

Answer 10

Mixing occurs rapidly

Question 11

During homopycnal flow, what is the effect of a sudden decrease in flow velocity?

Answer 11

The coarser sediment is dumped immediately

Question 12

What type of delta does homopycnal flow produce?

Answer 12

Classic Gilbert delta

Question 13

What are the 3 sections of a Gilbert delta?

Answer 13

Topset
Foreset
Bottomset

Question 14

What is the topset of a Gilbert delta composed of?

Answer 14

Flat lying gravels

Question 15

What is the foreset of a Gilbert delta composed of and at what angle do these beds dip?

Answer 15

Beds of sand and gravel dipping at 10-25 degrees

Question 16

What is the bottomset of a Gilbert delta composed of and how are these beds inclined?

Answer 16

Fine grained sediments that are gently inclined

Question 17

What is hyperpycnal flow?

Answer 17

Inflowing water is much more dense than the water in the receiving basin

Question 18

What is the effect of hyperpycnal flow?

Answer 18

Much less mixing
Less spread as sediments move out of the channel
Incoming water hugs the bottom of the basin

Question 19

What is hypopycnal flow?

Answer 19

Inflowing water is much less dense than the water in the receiving basin

Question 20

What is the effect of hypopycnal flow?

Answer 20

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

Question 21

What are the corners of the triangle in the Galloway Model?

Answer 21

Fluvial processes (top)
Wave processes (bottom left)
Tidal processes (bottom right)

Question 22

What is a feature of wave dominated deltas?

Answer 22

Ridges parallel to the coastline formed by waves

Question 23

What is a feature of tidal dominated deltas?

Answer 23

Offshore bars of either mud or sand parallel to the movement of the tide

Question 24

What causes the cyclicity of coarse, to fine, to limestone deposits in delta environments?

Answer 24

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.