Continental environments Flashcards
What are the main depositional environments?
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
What is the general term for all deposits directly deposited by ice?
Unconsolidated = till
Lithified = tillite
What is moraine?
Accumulations of till formed directly at the margins of a glacier
Continental ice meeting the sea
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.
What are the lines seen on the sides of a U-shaped valley?
Paleo shore lines
What is loess?
A clastic, predominantly silt- sized sediment, that is formed by the accumulation of wind-blown dust
Hvad menes der med “Jordens termostat”?
Ændring i graden af forvitring.
Forvitring er den centrale mekanisme til at fjerne CO2 fra atmosfæren.
Hvordan kan Snowball Earth opstå?
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
What are the characteristics of glacial deposits?
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
What is important to remember about aeolian environments?
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.
What are the aeolian bedforms?
Ripple field (wavelength 1-64 cm)
Dune field (wavelength 20-640 m)
Draa field (wavelength 640+ m)
What is true about aeolian sands? How do they move?
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
What are the most common aeolian dunes?
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
What is characteristic for aeolian dunes?
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.
What are the characteristics of aeolian deposits?
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
What are the biggest differences between sea and lake?
- 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)
What is lake water stratification?
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.
What are the characteristics of lake deposits?
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
What happens if the equilibrium line between degradation and aggradation is crossed?
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
What are the principal physical controls on river type?
- Sediment load
- Slope
- Seasonality of runoff - Baseline
What is a riparian strip?
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)
What are river deposits made of?
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)
How is a point bar and cutoff bank created (bends in meandering rivers)?
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
What are levees and crevasses?
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.
