3.3 - Deltaic Sediments

Question 1

What is a delta?

Answer 1

Deltas are discrete shoreline protuberances formed where rivers enter ocean, semi-enclosed seas, lakes or lagoons, and supply sediment more rapidly than it can be redistributed by basinal processes.

They are a sink for physical weathering

Question 2

Where are deltas found?

Answer 2

25% of the worlds deltas are found on passive margins of continents (need standing body of water) = broad, shallow continental shelf’s

• Not found in rift basins = as the steep sides of the Rift Valley means if a river flows over, it will be fast paced and less likely to deposit material at the mouth of the river

Question 3

What are the importance of deltas?

Answer 3

• They are the site of substantial fossil fuel resource accumulation (coal, oil, and natural gas)
• They contain high amounts of carbon
• They are also a source of sand and gravel that is used in highway, building and infrastructure construction.
• Their high biological activity makes them vital nursery grounds for fisheries

Question 4

How are deltas important when it comes to agriculture?

Answer 4

Delta land is important in agriculture use.

• Sacramento-San Joaquin Delta in California is one of the most agriculturally diverse productive areas in the US, supporting numerous crops such as kiwis.

Question 5

What is the importance of the Sacramento-San Joaquin Delta in California?

Answer 5

The delta responsible of CA’s statewide water system.

• Half the total river flow in the state passes through this region, from which water is exported to the San Joaquin Valley, Southern California and portions of the Bay Area to supply around 4,600km2 of farmland and 23 million people in central and Southern California. The delta provides an estimated 8.6km3 of water per year.

Question 6

How many people rely on deltas around the world?

Answer 6

Around 300 million people live on 40 deltas
(They are organically productive and create transport routes for shipping)

• Risk of drowning and people displacement due to sea level rise associated with climate change

Question 7

What are the 3 geomorphological elements in a delta?

Answer 7

1. Delta plain
2. Delta front
3. Pro delta

Question 8

What is a delta plain?

Answer 8

• A sub Ariel, flat swampy plain
• Fluvial processes generally predominate
• Incised by fluvial distributary channels

Question 9

What is a delta front?

Answer 9

• A shallow submarine platform
• Where most sand accumulates (mouth bars)

Question 10

What is a pro delta?

Answer 10

• Has huge sediment deposits
• Most distal, water depth >10-15m
• Bulk suspension settles out

Question 11

What is an upper delta plain?

Answer 11

An upper delta plain is above tidal influence. Sedimentation occurs in distributary channels and on the floodplain.

Question 12

What is a deltaic plain?

Answer 12

It lies between low-tide mark and the upper limit of tidal influence. Sedimentation is mostly associated with tidal channels, levees, embayments, marshes and swamps

Question 13

What is a subaqueous delta plain?

Answer 13

It is seaward of low tide level. Seaward fining notable on the delta front through to the pro delta.

Question 14

What are the 3 types of deltas?

Answer 14

1. River dominated
   - Small tidal range, weak storms and large sediment flux builds delta out into basin
2. Tide dominated
   - Large tidal ranges dominates transport, deposition and morphology
3. Wave dominated
   - Strong and repeated storms rework delta sediment

Question 15

In a river dominated delta what happens when there are changes in upstream fluvial processes and accretion?

Answer 15

It means that wave domination will increase

Question 16

What is an example of a mixed wave/tide dominated delta?

Answer 16

The Niger

• Tidal processes creates elongated islands aligned with tidal processes

Question 17

What is an example of a mixed river/wave dominated delta?

Answer 17

The Nile

Question 18

What is an example of a tide dominated delta?

Answer 18

Bangladesh delta

Question 19

How did major large deltas develop?

Answer 19

• Around 100,000 years ago there was a period of glaciation
• Around 80,000 years ago this caused sea level to decrease as ice sheets increased
• Around 20,000 years ago, huge amounts of erosion occurred to the exposed coastal area
• When the glaciers melted, sea levels rose around 7,000 years ago

This formed deltas and so they are all roughly of the same ages (7,000 years ago)

Question 20

Cross section across a delta lobe

Answer 20

The sediment down a delta slope fines as it goes down and out into the ocean. This is because velocity decreases and the courser material is deposited first but finer material is suspended in the water longer.

Lateral accretion can also occur. This generates a coarsening upward trend (the slope decreases and the coarser material can travel longer and deposits on top of older finer material)

Question 21

Bottomset bed in a delta lobe

Answer 21

• Lightest suspended particles settle farthest away from the active delta front as the river flow diminishes into the standing body of water and loses energy
• This suspended load is deposited by sediment gravity flow, creating a turbidite
• These beds are laid down in horizontal layers and consist of the finest grain sizes

Question 22

What are forest beds

Answer 22

• Deposited in inclined layers over the bottomset beds as the active lobe advances
• The sediment particles within foreset beds consist of larger/more variable sizes, and constitute the bed load that the river moves downstream by rolling and bounding along the channel bottom. When the bed load reaches the edge of the delta front, it rolls over the edge, and is deposited in steeply dipping layers over the top of the existing bottomset beds
• Under water, the slope of the outermost edge of the delta is created at the angle repose of these sediments. As the foresees accumulate and advance, subaqueous landslides occur and readjust overall slope stability. The forest slope, thus and maintained, extends the delta lobe outward

Question 23

What is the angle of repose?

Answer 23

The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping.

• for rice it’s about 34 degrees, but its less for sand, the angle can only build up so much before goes around sides

Question 24

What are topset beds

Answer 24

• Deposited in turn over the previously laid foresets, truncating or covering them.
• Topsets are nearly horizontal layers of smaller-sized sediment deposited on the top of the delta and form an extension of the landward alluvial plain
• As the river channels meander laterally across the top of the delta, the river is lengthened and its gradient is reduced, causing the suspended load to settle out in nearly horizontal beds over the deltas top

Question 25

What is Walther’s law

Answer 25

• Beds that occur in conformable vertical successions of strata were deposited in laterally adjacent environments
• Overall relative sea level rise causes vertical succession of increasingly deeper marine environments
• Overall relative sea level fall causes vertical succession of increasingly shallower marine and non-marine environments

Question 26

Where is the majority of oil extracted from?

Answer 26

Deltas?

Question 27

How can you distinguish a delta in the geological record

Answer 27

They have a classic coarsening upward cycle such as Cretaceous, Wyoming

• The oils industry puts money into understanding the rock record of deltas to map and predict where oil is before they start drilling

Question 28

What is the midland valley of Scotland

Answer 28

It is a Rift Valley made during the Carboniferous period

Fife is an old delta and is useful for understanding oil deposits eg. In north sea

Question 29

What is eustatic sea level

Answer 29

• global sea levels
• changes in sea level influences the effects of wave and tide processes and also affects the overall geometry of the delta body.
• A delta will spread further out into the basin if it is building out into a shallow water than in deeper water.

Question 30

What is lobe switching

Answer 30

Bays such as in Mississippi delta are dynamic and can change over time due to factors such as erosion, sedimentation, and sea level rise

Question 31

What is the size of a delta dependant on?

Answer 31

It is dependant on the size of drainage basin, the elevation of the land and climate