Turbidities

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Turbidities

• A tubidite is defined as the deposite of
  turbidity currents. Density currents flow-down
  slope to basin floor due to suspended sediments,
  the flow is termed a turbidity currents. The
  concept of turbidities has been used to account
  for gradded sandstone bed that lack evidence of
  shallow water reworldng, and also to explain the
  transport of shallow water organisms within
  sandstone which are interbedded with shales
  containing abathyal, abyssal or pelagic fauna.

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• Turbidity currents typically form on the
  continental slope, and are dense currents of
  water and sediment, often caused by
  destabilization of slope sediments during an
  earthquake

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Grand Banks 1929

• An 7.2 earthquake (star) triggered a turbidity
  current, with cables up to 600km away broken up
  to 13 hours later.
• The area of the deposit is roughly the size of
  Quebec.

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Turbidity Currents

• Animation of the turbidity current seen as a
  longitudinal profile of the sediment
  concentration. The current is animated with 3
  seconds between each picture. The part of the
  flume shown in the figure is 10 meters long.

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Facies classification

• 1- Classical turbidities facies there are a
  number of features which can be taken as a set of
  descriptors for Classical turbidities (Bouma
  sequence)
• A- Sandstones and shales are monotonously
  interbedded through many tens or hundreds of
  meters of stratigraphic sections.
• B- Sandstone beds have sharp, abrupt bases, but
  tend to grade upwards into finer sand, silt or
  mud.
• C- The undersurface of the sandstones carry
  abundant marking, now classified into three types
• 1) tool mark, carved into underlying mud by rigid
  objects (sticks, stones) carried in 2) Scour
  marks cut into the underlying muds by fluid
  scour.
• 3) Organic markings representing trails and
  burrows filled by deposition from turbidity

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Generalized sequence through a turbidite unit.
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Bouma Sequence
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Turbidite Bouma Sequence

• Laminated muds
• Cross-laminated sands
• Planar laminated sands
• Graded bedding from a coarse lag base

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complete Bouma sequence
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Complete Bouma sequence ,beginning with a graded
division A, overlain by parallel laminated
division B and cross-laminated division C.
Division D and E broke off this specimen ,which
is from the cote Frechette road cut, Levis
Formation (Cambrian), Québec.
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• 2- Massive sandstone fades consist of thick
  sandstones with thin or no interbedde shale.
  Individual sandstone bed range from about 50cm to
  many meters thick, and the Bouma division
  normally present is division A. Channeling is
  fequent, but the one sedimentary structure
  commonly found is the dish structure.

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Massive sandstone facies ( About 180 m of section
can be seen in the photograph. Note thickness of
individual sand stone beds, and absence of
mudstone interbeds
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• 3- Pebbly sandstones facies tend to be well
  graded and stratification is fairly abundant, and
  associated with or interbedded with classical
  turbidities.

Pebbly sandstones facies showing medium scale
cross bedding.
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• 4- Conglomerates facies they are an important
  fades in deep water environments, and probably
  intergarditionalwith character relating to
  relative position down-flow.

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• 5- Slump, slides. debris flow and other exotic
  facies Group of rocks which are generally poor
  to unstratified, and which are commonly poorly
  sorted. Blocks and boulders rest in fine grained
  matrix which may show evidence of sedimentary
  deformation.

Model of resedimented facies, for slumps,
debrites and turbidities. showing the idealized
structural sequence for debrites and for coarse-,
medium- and fine-grained turbidities.
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Summary of sedimentary structures associated with
turbidites.