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.
What are river bars?
Patches in between the sub-channels
How do you differentiate between a braided and a meandering river?
Important: How much sand is there compared to mud?
Braided = amalgamated (sand-deposits are interconnected)
Meandering = non-amalgamated
What are alluvial plains?
Characterized by fine (“mud”) deposits and soils development
What are the characteristics of fluvial deposits?
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
