Continental environments

Question 1

What are the main depositional environments?

Answer 1

Continental environments
- Fluvial – Alluvial
- Desert
- Glacial
- Lacustrine

Transitional environments
- Marine deltaic
- Estuarine – Lagoonal
- Littoral

Marine environments
- Shallow siliciclastic seas
- Shallow-marine carbonate - Pelagic
- Deep clastic

Volcanic environment

Question 2

What is the general term for all deposits directly deposited by ice?

Answer 2

Unconsolidated = till
Lithified = tillite

Question 3

What is moraine?

Answer 3

Accumulations of till formed directly at the margins of a glacier

Question 4

Continental ice meeting the sea

Answer 4

At continental margins in polar areas, continental ice feeds floating ice sheets that eventually melt releasing detritus to form a till sheet and calve to form icebergs, which may carry and deposit dropstones.

Question 5

What are the lines seen on the sides of a U-shaped valley?

Answer 5

Paleo shore lines

Question 6

What is loess?

Answer 6

A clastic, predominantly silt- sized sediment, that is formed by the accumulation of wind-blown dust

Question 7

Hvad menes der med “Jordens termostat”?

Answer 7

Ændring i graden af forvitring.

Forvitring er den centrale mekanisme til at fjerne CO2 fra atmosfæren.

Question 8

Hvordan kan Snowball Earth opstå?

Answer 8

Snowball Earth = globalt isdække, 800-620 mio. år siden

Hurtig afkøling -> Is-albedo feedback (isen kaster mere og mere lys tilbage uden at opfange stråling (og varme))

Vulkaner afgasser og opbygges stille og roligt = udløser meget vulkanisme = isen smelter igen

Weathering = caption of CO2
Snowball Earth = no weathering

Question 9

What are the characteristics of glacial deposits?

Answer 9

Lithologies: conglomerate, sandstone and mudstone

Mineralogy: variable, compositionally immature

Texture: extremely poorly sorted in till to poorly sorted in fluvio-glacial facies

Bed geometry: bedding absent to indistinct in many continental deposits , glaciomarine deposits may be laminated

Sedimentary structures: usually none in tills, cross-bedding in fluvioglacial facies

Palaeocurrents: orientation of clasts can indicate ice flow direction

Fossils: normally absent in continental deposits, may be present in glaciomarine facies

Colour: variable, but deposits are not usually oxidised

Facies associations: may be associated with fluvial facies or with shallow-marine deposits

Question 10

What is important to remember about aeolian environments?

Answer 10

Remember: wind is a fluid
Ergo: the atmosphere is a thick layer of fluid movement

Evaporation exceed precipitation!

Distribution of modern-day desertic environments is related to global wind pattern and water vapor.

Question 11

What are the aeolian bedforms?

Answer 11

Ripple field (wavelength 1-64 cm)
Dune field (wavelength 20-640 m)
Draa field (wavelength 640+ m)

Question 12

What is true about aeolian sands? How do they move?

Answer 12

The wind makes the grains very nicely rounded and a bit polished.

Concrete made with aeolian sand (very poor) cannot hold itself together.

Sand grains migrate up windward side of the dune by rolling, saltation, and ripple migration
–> grains blown off the crest of the dune fall on to lee slope
–> accumulation of grains in upper part of slope causes oversteepening and results in grain flow

Question 13

What are the most common aeolian dunes?

Answer 13

Barchan dunes
- crescent
- with the ends pointing away from where the wind blows
- uniform winds
- sparse sand supply

Parabolic dunes
- the opposite of barchan dunes (with ends pointing towards the wind)

Transverse dunes
- linear
- perpendicular to direction of wind
- abundant sand supply

Linear dunes
- linear
- oblique thanks to two prevailing wind directions
- sparse sand supply

Star dunes
- multiple arms
- 4 prevailing wind directions (variable winds)
- can become very big
- abundant sand supply

Question 14

What is characteristic for aeolian dunes?

Answer 14

Well-sorted and well-rounded sand grains are mostly deposited by aeolian dunes (wind cannot move pebbles!).

Grain-flow tongues at the foot of aeolian dunes help to distinguished them from subaqueous bedforms.

Question 15

What are the characteristics of aeolian deposits?

Answer 15

Lithologies: sand and silt only

Mineralogy: mainly quartz, with rare examples of carbonate or other grains

Texture: well- to very well-sorted silt to medium sand

Fossils: rare in desert dune deposits, occasional vertebrate bones

Bed geometry: sheets or lenses of sand

Sedimentary structures: large-scale dune cross-bedding and parallel stratification in sands

Palaeocurrents: dune orientations reconstructed from cross-bedding indicate wind direction

Colour: yellow to red due to iron hydroxides and oxides

Facies associations: occur with alluvial fans, ephemeral river and lake facies in deserts, also with beach deposits or glacial outwash facies

Question 16

What are the biggest differences between sea and lake?

Answer 16

• Water type (fresh or salt)
  – general chemistry; varies from lake to lake
• Fossils
  – fauna and flora are distinct
• Lakes too small for tides (=static bodies)

Question 17

What is lake water stratification?

Answer 17

The thermal stratification of fresh lake waters results in a more oxic, upper layer (the epilimnion), and a colder, anoxic lower layer (the hypolimnion).

Sedimentation in the lake is controlled by this density stratification above and below the thermocline.

Question 18

What are the characteristics of lake deposits?

Answer 18

Lithologies: sandstone, mudstone, fine-grained limestones and evaporites

Mineralogy: variable

Texture: sands moderately well sorted

Fossils: algal and microbial plus uncommon shells

Bed geometry: often very thin-bedded

Sedimentary structures: wave ripples and very fine parallel lamination

Palaeocurrents: few with paleoenvironmental significance

Colour: variable, but may be dark grey in deep lake deposits

Facies associations: commonly occur with fluvial deposits, evaporites and associated with aeolian facies

Question 19

What happens if the equilibrium line between degradation and aggradation is crossed?

Answer 19

Degradation = sediment supply
Aggradation = stream power

More sediments = degradation (depositional area flader ud) —> lowers energy in the stream - also in the upper part of the river —> lowers erosion

Question 20

What are the principal physical controls on river type?

Answer 20

• Sediment load
• Slope
• Seasonality of runoff - Baseline

Question 21

What is a riparian strip?

Answer 21

A vegetated area near a stream.

Braided = small area of riparian strip
Meandering = medium area of riparian strip
Anabranching = big area of riparian strip (streams are bigger = riparian strips on either side of every stream)

Question 22

What are river deposits made of?

Answer 22

Sand and pebbles:
- Deposited by flowing waters within or in the vicinity of river channels
- Alternation of sand and pebbles = quite probably a river sediment

Mud (mostly clay):
- Deposited by stagnant water in floodplain (sometimes also in abandoned channels)

Question 23

How is a point bar and cutoff bank created (bends in meandering rivers)?

Answer 23

Point bar = inside, deposition
Cutoff bank = outside, erosion

Water on outside flows faster than inside = difference in velocity –> corkscrew pattern of flow –> erosion on outside bank and deposition on inside –> river moves laterally

Question 24

What are levees and crevasses?

Answer 24

Levees = diger
Crevasses = sprækker

Levees form when flood waters rapidly deposit sediment close to the bank and crevasse splays are created when the levee is breached.

Question 25

What are river bars?

Answer 25

Patches in between the sub-channels

Question 26

How do you differentiate between a braided and a meandering river?

Answer 26

Important: How much sand is there compared to mud?

Braided = amalgamated (sand-deposits are interconnected)

Meandering = non-amalgamated

Question 27

What are alluvial plains?

Answer 27

Characterized by fine (“mud”) deposits and soils development

Question 27

What are the characteristics of fluvial deposits?

Answer 27

Lithologies: conglomerate, sandstone, and mudstone

Mineralogy: variable, often compositionally immature

Texture: very poor in debris flows to moderate in river sands

Fossils: fauna uncommon, plant fossils may be common in floodplain facies

Bed geometry: sheets on fans, lens shaped river channel units

Sedimentary structures: cross-bedding and lamination in channel deposits

Palaeocurrents: indicate direction of flow and depositional slope

Colour: yellow, red, and brown due to oxidising conditions

Facies associations: alluvial fan deposits may be associated with ephemeral lake and aeolian dunes, rivers may be associated with lake, delta or estuarine facies