Unit 6 - Depositional Environments Flashcards
What does facies analysis mean?
- the method used to interpret the environment of deposition of a sedimentary succession
What does facies mean?
- it refers to a unit that is identified by having certain specific characteristics.
Depositional facies is defined by?
- lithology (physical characteristics) of the sediment
this includes:
- sedimentary structures
- colour
- diagenetic character
- fossil content.
What is a depositional system?
- three-dimensional association of facies, which are genetically linked by sedimentary processes and environments of deposition
Interpretation of facies tell us what?
the responsible processes for the depostion of the facies but not about the environment of the deposition → to identify that, we need to consider the vertical and lateral association of facies
What is a genetic unit?
- a group of facies associations that were formed by closely related processes in related depositional environments
Example: a delta lobe would form a genetic unit, containing several sandstone facies (channel deposits) as well as mudstone, siltstone and coaly facies deposited in inter-channel areas.
the arrangement of multiple facies is highly dependent on ?
the depostional environment
Where do aeolian sediments mostly occur?
Desert environments
How thick are aeolian dune sets normally?
meters to 10s of meters → barely happens subaqueous → therefore large set size is often taken as aeolian origin
Periods of wetting and drying in aeolian successions can be caused by ?
- rise in sea or lake level or increase in rainfall
What happens to aeolian dunes during wetting periods?
- They are less active and might get eroded
The four types of channels of river forms (fluvial environments)?
What are the two most common types of river systems?
- meandering
- braided
An old meander loop often turns to an ?
ox-box lake
What is the floodplain?
- the surrounding low-lying land of a river, which may be flooded during periods of flooding
What happens, when a river breaks through its banks?
They form temporary crevasse channels
If river go over their banks during a flood, how will they distribute their sediments?
- They will deposit their coarser sediments closer to the river → they produce thin beds, that become finer and thinner, the further away from the river
What is a levee?
- if meandering rivers repeatedly produce floods over many years → elevated ridges of sediment, known as levees, close to the channel will start to form
What is the difference between a meandering river and a braided river?
- Meandering rivers only have one active channel at a time
- braided rivers have number of active channels, separated by sandy or gravelly bars
Where do braided rivers form?
- slightly steeper slopes
→ with a high proportion of sandy or gravelly sediment
In which direction do individual channels of braided river systems migrate?
- laterally, shifting their course frequently
- the intervening bars migrate both downstream and across the streams
→ Braided rivers therefore produce compound sandbodies consisting of a number of mutually-erosive channel bodies
Anastomosing rivers create which kind of sand bodies?
- relatively narrow but thick multi-storey sandbodies
Meandering Rivers produce classically what kind of sandbodies?
erosive-based, upward-fining sandbodies
What do rivers form when they leave the confines of a valley ?
- they usually form cones of sediment known as alluvial fans
Describe alluvial fans
- sediment becomes finer away from fan apex
- migration of fan produces an upwards coarsing trend
When the relative sealevel falls, what happens to the sediment passing through an incised valley?
- it will bypass it
When the relative sealevel rises, what happens to the sediment passing through an incised valley?
Sedimentation will start in the incised valley, which will continue into the highstand period
Beaches form the boundary between …?
Shallow marine and terrestrial environments
Why does a beach produce an upward-coarsening facies sequence, as it progrades offshore?
Because in general, the energy is greatest, and the sediment coarsest, on the higher parts of the beach profile
Why are waters in a lagoon brackish?
Because of the mixing of fresh water and of sea water → brackish means intermediate salinity
How is a delta formed , generally?
major river reaches a standing body of water, such as a lake or the sea.
→ river supplies the sediment faster than the basinal processes in body of water can rework it, then the shoreline will project locally into the basin, forming a delta.
What will a river do on the subaerial part of the delta plain?
Split itself up into two or more little channels causing sediment to be supplied to many points along the delta front
What are reasons for the different delta forms?
climate, tectonic setting, sediment supply, energy of receiving basin
What will fluvial dominated deltas do?
- supply more sediment to coastline than can be reworked by basinal processes → resultant delta will therefore form a pronounced protuberance of the shoreline
What will tidal dominated deltas do?
the tidal processes will most likely produce a radial pattern of distributary channels which become broader towards the basin
What will wave-dominated deltas do ?
- a big amount of the sediment supplied by river mouth will be reworked into beach ridges on either side. The resulting delta will, therefore, often cause only a slight deflection of the coastline
What happens generally at the mouths of channels on fluvial-dominated deltas, where sediment laden river waters interact with the basinal water?
- fluvial flow loses power and starts to deposit sediment → sediment fines away from the apex
- Progradation oft delta mouthbar therefore produces an upward-coarsening sequence dominated by current-generated structures
What will the vertical facies of wave dominated deltas look like?
- Facies sequences will be very similar to those produced by prograding beaches → differing mainly by the presence of associated distributary channels
What is a big difference between the physical processes like tides and waves of coastal and shallow marine environments and deep marine clastic environments ?
tides and waves, which dominate coastal and shallow marine environments are generally absent or ineffective in the deep marine environment
What kind of environments are deep oceans most of the time?
- low-energy environments in which fine grained carbonate or clastic muds are able to accumulate
- coarse-grained sediment is transported into these environments by several infrequent and shortlived processes, of which the sediment gravity flow processes (slumps, debris flows, turbidity currents etc.) are dominant.
What were the earliest deep marine sandstones?
Turbidites
Facies models for turbidites, debris flow deposits (debrites) and slump deposits - it is just a graphik, to check out the succesions!
In deep marine clastic environments, examination of some turbidite successions showed that facies sequences, in which the thickness and/or mean grain size of the sandstone beds increased or decreased upwards what is the explanation for that?
- these trends were attributed to the deposition on different parts of a submarine fan
What is a submarine fan?
- occur offshore from major river systems or off the continental shelf
- they are sourced from a single point (like deltas) ( or multiple sourced) and contain a system of distributary channels supplying lobes
- upward-thickening trends in turbidite sequences are attributed to the progradation of fan lobes
- upward-thinning trends to deposition within a channel.
Multiple source submarine fan (Graphic)
What are the “natural” sediments in marine environments?
Carbonates
What is a big difference between clastic grains and carbonate grains?
- clastic grains have their origin most of the time further away from their site of deposition
- carbonate grains are formed at or very close to their eventual site of deposition
What are bioclasts?
- majority of carbonate grains are precipitated by organisms
or by biochemical processes
- clasts derived from skeletal material are known as bioclasts → most of the time broken pieces of skeletons
What are pellets or pelloids?
- ovoid shapes composed of micrite (lime mud) and microscopic shells
What are ooids?
- small (less than 2mm) near-spherical carbonate grains
- generally form in moderate to high energy shallow marine environments
What are intraclasts?
- common for carbonates to be partially lithified at, or close to, the sediment surface
→ If erosion then occurs, sediment may break into groups of grains lightly cemented together →called intraclasts
Where are the majority of carbonates deposited?
- marine or lacustrine environments
