Chapter 4 Marine Sedimentation

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Classification of marine sediments can be based
upon size or origin.
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Sediment in the Sea

• Size classification divides sediment by grain
  size into gravel, sand, silt and clay.
• Mud is a mixture of silt and clay.
• Origin classification divides sediment into five
  categories terrigenous sediments, biogenic
  sediments, authigenic sediments, volcanogenic
  sediments and cosmogenic sediments.

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Factors that control sedimentation include
particle size and the turbulence of the
depositional environment.
4-1
Sediment in the Sea

• Terrigenous sediments strongly reflect their
  source and are transported to the sea by wind,
  rivers and glaciers.
• Rate of erosion is important in determining
  nature of sediments.
• Average grain size reflects the energy of the
  depositional environment.

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Hjulstroms Diagram

• Hjulstroms Diagram graphs the relationship
  between particle size and energy for erosion,
  transportation and deposition.

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Based upon water depth, the ocean environment can
be divided into the shelf, which is shallow and
near a terrigenous source, and the deep ocean
basin, which is deep and far from a terrigenous
source.
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Sedimentation in the Ocean
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Shelf Versus Basin Depths
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Shelf sedimentation is strongly controlled by
tides, waves and currents, but their influence
decreases with water depth.
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Sedimentation in the Ocean

• Shoreline turbulence prevents small particles
  from settling and transports them seaward where
  they are deposited in deeper water.
• Particle size decreases seaward for recent
  sediments.
• Past fluctuations of sea level have stranded
  coarse sediment (relict sediment) across the
  shelf including most areas where only fine
  sediments are deposited today.

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Worldwide distribution of recent shelf sediments
by composition is strongly related to latitude
and climate.
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Sedimentation in the Ocean

• Calcareous biogenic sediments dominate tropical
  shelves.
• River-supplied sands and muds dominate temperate
  shelves.
• Glacial till and ice-rafted sediments dominate
  polar shelves.

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Shelf Sedimentation Model
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Geologic controls of continental shelf
sedimentation must be considered in terms of a
time frame.
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Sedimentation in the Ocean

• For a time frame up to 1000 years, waves,
  currents and tides control sedimentation.
• For a time frame up to 1,000,000 years, sea level
  lowered by glaciation controlled sedimentation
  and caused rivers to deposit their sediments at
  the shelf edge and onto the upper continental
  slope.
• For a time frame up to 100,000,000 years, plate
  tectonics has determined the type of margin that
  developed and controlled sedimentation.

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Relict Sediment
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If influx of terrigenous sediment is low and the
water is warm, carbonate sediments and reefs will
dominate.
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Sedimentation in the Ocean
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Deep-sea Sedimentation has two main sources of
sediment external- terrigenous material from the
land and internal-biogenic and authigenic from
the sea.
4-2
Sedimentation in the Ocean
Sedimentation in the Deep Sea
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Sedimentation in the Ocean

• Major sedimentary processes in the deep sea
  include, Bulk emplacement, Debris flows,
  Turbidity currents
• Major pelagic sediments in the ocean are red clay
  and biogenic oozes.
• Authigenic deposits are chemical and biochemical
  precipitates that form on the sea floor and
  include ferromanganese nodules and phosphorite.

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The distribution of sediments in the deep ocean
reflects latitude, distance from landmasses, and
the calcium carbonate compensation depth.
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Sedimentation in the Ocean

• Glacial marine sediments occur in the high
  latitudes.
• Pelagic clays occur far from land and in the
  deepest water.
• Calcareous oozes occur above the calcium
  carbonate composition depth.
• The rate of sedimentation depends on the type of
  sediment in deep sea.

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Sedimentation in the Ocean

• Deep-sea stratigraphy refers to the broad-scale
  layering of sediments that cover the basaltic
  crust. The stratigraphy of the deep sea is
  strongly influenced by sea-floor spreading.

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The Atlantic basin contains a two-layer-cake
stratigraphya thick basal layer of carbonate
ooze overlain by a layer of mud.
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Sedimentation in the Ocean
Stratigraphy of the Atlantic Basin
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The Pacific basin contains a four-layer-cake
stratigraphy, because unlike the Atlantic its sea
floor as it spreads crosses the equator where the
CCD is lowered to the ocean bottom.
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Sedimentation in the Ocean
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Stratigraphy and Model of Pacific Basin
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There are a number of sampling techniques for
obtaining sediment from the ocean bottom.
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Collecting Marine Sediment

• Bottom dredges scrape the sediment and collect
  material in a wire or canvas bag.
• Grab samplers take a bite out of the sediment
  covering the bottom.
• Gravity and piston corers use a weight to drive a
  core barrel into a soft bottom. A piston corer
  takes a much longer core than a gravity corer
  because of the piston in the core barrel.

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The Drying Up of the Mediterranean Sea
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• The Mediterranean basin is located where plates
  are colliding as Africa moves northward relative
  to Europe.
• Anhydrite and stromatolites of Miocene age
  indicate that the Mediterranean sea dried out
  between 5 and 25 million years ago.
• Two models have been suggested to account for
  this emptying of the Mediterranean Sea of its
  water.
• The Uplift Model
• The Drying-Out Model

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Refilling the Mediterranean Sea

• After drying out, seawater from the Atlantic
  Ocean cascaded down the face of the Gibraltar
  Sill, refilling it in about 100 years.