Turbidites Flashcards
What is a turbidity current?
A high velocity current that flows down gentle gradients because the sediment dispersed within it makes it denser than sea water
What are turbidity currents a result of?
Triggered by earthquakes or slope instability
What is an easier way to imagine a turbidity current (not to be used in an exam)?
It’s like an underwater avalanche
What are turbidites?
Sediment deposited after a turbidity current. Forms a sequence of sediment types
How do turbidites form?
Sediment brought to sea (by rivers) accumulate on the continental shelf.
This sediment then becomes unstable. Submarine gravity flows (turbidity currents) cause turbidite deposits.
What are turbidite sequences?
Form when the turbidite is deposited. They have an upward fining deposit of greywacke
What is a Bouma sequence?
An idealised sequence of sediments and sedimentary structures seen in a turbidite deposit
How many layers are in the Bouma sequence?
5 (E-A)
The top of the Bouma sequence is what layer?
Layer E, deposited last.
A is bottom
What is the first layer deposited in a Bouma sequence?
Layer A - The bottom of the sequence.
Coarse bed of pebbles/granules (conglomerates) in a sandy matrix.
There is graded bedding, rip up clasts and an erosional base (unconformity).
What leads to the formation of Layer A in a Bouma sequence?
High energy. Coarsest grains carried in suspension until energy lost to deposit coarse grains.
Erosion at the base of the flow tears up shale from underlying beds (rip up clasts at base)
What is the second layer deposited in a Bouma sequence?
Layer B.
Coarse then medium sandstone (greywacke), with less pebbles and rip up clasts.
Parallel sands. Flat beds from high velocity and graded bedding. Sole marks including flute casts and tool marks. Sole structures can be 2m long
What leads to the formation of Layer B in a Bouma sequence?
Sufficient energy to carry sands by traction. Energy high enough to keep beds flat/parallel
What are the sole structures in Layer B?
they occur on the base of the bed.
They may take the form of grooves or pits. These can act as moulds for when further sediment is deposited, when filled they produce clasts
What is the third deposited layer of the Bouma sequence?
Layer C.
Sandstone (greywacke). Climbing ripples (poss. cross bedding). Less coarse grains with unidirectional ripple marks (asymmetrical).
What leads to the formation of layer C in the Bouma sequence?
Sufficient energy to carry sand by saltation.
Climbing ripples form when deposition happens rapidly and erosion is insufficient to erode the stoss side of ripples
How do climbing ripples form?
When deposition exceeds the rate of migration of ripples. Energy insufficient to cause complete erosion of the stoss side.
More deposition before the ripples could migrate fully
What is the fourth layer deposited in the Bouma sequence?
Layer D.
Fine sandstone and siltstone. Parallel laminations in sand and silt.
What leads to the formation of Layer D in the Bouma sequence?
Low energy.
Sediment carried in suspension. Changes in current energy cause alternating laminations of coarser (still fine) and finer grains.
What is the fifth layer to be deposited in a Bouma sequence?
Layer E, the last layer. Top layer.
Shale which may contain pelagic (mud) marine fossils such as graptolites.
Parallel laminations in the mud
What leads to the formation of layer E in the Bouma sequence?
No current. Suspension settling. Interturbidite (happens between turbidity currents). This is basically back to normal now. Pelagic and hemipelagic
What is the boundary between the bottom of layer A and the older underlying sediment?
Erosional boundary - unconformity.
Where there has been erosion of sediment and in sequences, it is a gap in time.
Erosion of older rock and decomposition of newer rock
What are tool marks?
Impressions made on the surface of soft sediments by the dragging or bouncing of objects in a current. Appear (usually) as long grooves. Found in the original older deep marine sediment (below layer A)
What are rip-up clasts?
Pieces of shale or mudstone eroded by a current, containing suspended sediment. They are deposited when the current deposits its sediment . Shale/mudstone ripped up by turbidity flows are deposited into layer A
Where is calcareous ooze found?
Deep water.
E.g. coccolithophores
What are siliceous oozes composed of?
Depends on the composition of planktonic organisms.
30% made from skeletons of diatoms (silica-rich) and radiolarians
Where are siliceous oozes relevant to turbidite deposits?
Siliceous oozes make up the backround material before turbidite deposition
What is a delta?
Where a river floats into the sea or a large river. This deposits sediment being carried by the rivers (distributary channels)
Why is sediment deposited where rivers meet the sea?
Energy is lost
What shape do deltas deposit sediment?
A fan shaped deposit
How is delta sediment lay out?
It is coarsest closest to the mouth of the river and fines out
How would delta sediments be deposited?
There must be low tidal range and energy (little wave action)
How is sediment deposited at deltas?
As a river meets the sea, energy is lost and sediment deposited. This fills the river channels until they overflow and spread elsewhere (hence the fan shape). This causes distributary channels to the sea
Where are the upper layers of a delta?
Just above sea levels, between some distributary channels
What are the three parts to a delta deposit?
Delta top, delta front, prodelta
What is the highest part of a delta deposit?
The delta top/delta plain
What is the environment like at the delta top?
Dominated by distributary channels, with swampy areas, or flood plains in between. (anaerobic)
What is delta switching?
Happens in the delta top. When distributary channels change course, this happens frequently. Lateral changes in sedimentation
What is sediment called which is deposited in the delta top?
Topsets
What are topset sediments like?
Coarse grains and gravel
What is deposited between the channels in the delta top?
clays
What is seat earth and why is it important?
Soils in which trees grew may preserve as seat earth. Diagenetic processes lead to the formation of coal
What is the middle layer of a delta deposit?
Delta front. As the river meets the sea
What is sediment deposited in the delta front called?
Foresets
What are foreset deposits like?
Coarsest sands deposited first, finer silts deposited last (grading). Sand is often cross bedded. Marine fossils may be found.
finer sands and silts further down
What is formed when the delta front advances into the sea?
A vertical succession
What is sediment like at the crests of sandbars in the delta front?
Contain very clean sand due to wave/currents which remove finer material
What is often a key sign proving an environment is deltaic?
Coal presense.
This differs delta deposits from fluvial deposits
What is the final/deepest layer of a deltaic deposit called?
Prodelta.
What is the environment of deposition like in the prodelta?
Low energy. Deeper water
What are sediments deposited in the prodelta called?
Bottomsets
What are bottomset deposits like?
Consist mainly of silts and clays. These are thinly bedded and lack sedimentary structures. These lithify to form shales. May contain marine fossils
Why does the deltaic sequence form?
Because the delta progrades (always moving out into sea).
What are distributary mouth bars?
Crescent shaped deposits of sand and silt forming below sea level where distributaries meet the sea
What does BIFs stand for?
Banded Iron Formations
What are banded iron formations?
Unit of sedimentary rocks. Consist of repeated thin layers of dark iron oxides (few mms)
When were banded iron formations deposited?
In the archaen (4000Ma) paleoprotarozoic (3500Ma)
What are most of the iron minerals deposited in banded iron formations?
Magnetite and Haematite
Why is the iron red in banded iron formations?
Oxidation cycles
When did banded iron formations form?
When oxygen was first produced. The first photosynthesis in oceans
What was it called when oxygen nearly killed everything?
The Great Oxidation Event
What is the name of the bacteria responsible for producing the first oxygen in oceans?
Cyanobacteria and Halobacteria
What colours were oceans likely to be before the Great Oxidation Event? Why?
2-3% of the World was land.
The rest was ocean, likely green from anaerobic rust
Banded iron formations have alternating layers of grey and red, what rocks are these?
Grey - Chert, fine sedimentary
Red - Iron formations
Why did oxygen put the biosphere at risk at first?
Most organisms were anaerobic at this time and couldn’t survive the new conditions
What did the introduction of oxygen do to the ocean colour?
Turned them red, from new oxidation - causing aerobic rust
The increase of oxygen resulted in a 300 million year long … called …?
Ice age. called the Huronian glaciation
Which creatures died out from the Great Oxidation Event?
Most of them, including photosynthetics.
When the glaciers retreated after the Huronian Glaciation, what happened?
New oxidised conditions. The creation of the ozone layer
What is a photoferrotroph?
Photosynthetic bacteria that oxidise ferrous iron (Fe2+) into ferric iron (Fe3+) using sunlight and hydroxides instead of molecular oxygen
How did photoferrotrophs result in banded iron formations?
Fe2+ originated in at hydrothermal vents.
They converted the iron ions to Fe3+, which oxidised to form minerals such as magnetite and haematite (iron in BIFs)
Why are banded iron formation deposits important?
Valuable iron deposits which we mine today. We mine today in Western Australia and Minnesota
What does Facies mean?
Includes all the characteristics of a sedimentary rock produced by its environment of deposition and allow it to be distinguished from a rock deposited in an adjacent environment
What is a facies association?
A sequence
e.g. deltas and beaches
What do we count as ‘rock characteristics’?
Sedimentary structures, colour, grain size, sorting, fossils, etc
What is Walther’s law of facies?
Any vertical progression of facies is the result of a succession of depositional environments that are laterally juxtaposed to each other
What does Walther’s law actually mean?
In a sedimentary sequence (no uncomformities), a vertical succession of facies represents sedimentary environments that once existed side by side
What is the facies association for beaches as a log?
Moving down
Top: conglomerate/pebbles.
Next down: Coarse to fine sand
Next down: Shale
Bottom: Limestone
What is the facies association for hot deserts as a log?
Moving down
Top: Evaporites of playa lakes.
Next down: siltstone from playa lakes.
Next down: Sands + siltstones, small scale cross bedding.
Next down: Coarse sandstones, large scale cross bedding.
Next down: Wadi conglomerate and arkose or alluvial fan
Bottom: Evaporites of playa lakes again.
What is the facies association for turbidites as a log?
Moving down.
Top: Shale laminations, possible pelagic marine fossils
Next down: Laminations of silts and sands
Next down: Climbing ripples sandstone
Next down: Parallel laminations
Next down: Conglomerate, sandy matrix, erosional base. rip up clasts
Bottom: Shale again
What is the facies association for deltas as a log?
Moving down.
Top: coal
Next down: Sandstones, seat earth
Next down: Sandstones coarse
Next down: Siltstones
Bottom: marine shales
What is the facies association for fluvial environments as a log?
Moving down.
Top: Plant roots, flood plain clay
Next down: small scale cross bedding,
Next down: Medium scale cross bedding
Next down: Large scale cross bedding, channel gravels at base
Bottom: Mudstones/clay
