Chapter 4 Marine Sedimentation

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• SEDIMENTS
• Text Book Chapter 5
• Why do we care about oceanic
• sediments?
• Continents are sites of erosion
• Oceans are sites of depositions
• Therefore oceans retain a more-nearly complete
  and organized record of Earth history.

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Learning Objectives Ocean Sediments

• 1. Understand the origin and classification of
  marine sediments.
• 2. Explain the factors controlling origin and
  deposition of sediment on the continental shelf
  and in the deep ocean.
• Questions
• Why don't the oceans have more sediment in them?
  Where does it all go? Earth is 4.6 billion years
  old and the oceans should have more sediment in
  them.
• Salt composition of the oceans has not changed
  for the last 1.5 billion years. Explain why?

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Marine Sedimentation
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Classification of marine sediments can be based
upon size or origin.
Sediments 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 four
  major categories

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• The 4 main types of sediment
• Lithogenous/terrigenous composed of fragments
  of pre-existing rock material
• Biogenous composed of hard remains of
  once-living organisms
• Hydrogenous formed when dissolved materials
  come out of solution (precipitate)
• Cosmogenous derived from outer space

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Factors that control sedimentation include
particle size and the turbulence of the
depositional environment.
Sediments in the Sea

• Terrigenous sediments strongly reflect their
  source and are transported to the sea by wind,
  rivers and glaciers.

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Streams Rivers
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Glaciers
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Landslide (Gravity)
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ocean environment can be divided intothe shelf
neritic deposits(shallow areas and near a
terrigenous source) the deep ocean basin
pelagic deposits (deep areas and far from a
terrigenous source)
Sedimentation in the Ocean
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Shelf Versus Basin
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Shelf sedimentation is strongly controlled by
tides, waves and currents, but their influence
decreases with water depth.
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.

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

• 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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Influence of Past Sea Level
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Case study The Atlantic Passive Margin
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Case study Pacific Destructive/Subduction Margin
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Carbonate Shelves
If influx of terrigenous sediment is low and the
water is warm, carbonate sediments and reefs will
dominate.
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Ice Rafting
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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.
Sedimentation in the Ocean
Sedimentation in the Deep Sea
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Sedimentation in the Ocean

• Major pelagic sediments in the ocean are red clay
  and biogenic oozes.
• Authigenic deposits are formed in situ (at place)
  by precipitation of submarine weathering (e.g.
  hydrothermal areas, ridges) metal rich sediments
  and manganese nodules

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Biogenic Deposits
Calcium Carbonate (e.g. mollusks, corals,
foraminifers, some algae)
Silica (e.g. diaotms, radiolaria)
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Tiny animals with calcite shells (CaCO3)
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Coccoliths
Tiny plants with a calcite skeleton (CaCO3)
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Diatoms
SiO2
Radiolaria
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Silica Sedimentation
Siliceous ooze found where it accumulates faster
than it dissolves
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Calcium carbonate Sedimentation
CCD Calcite dissolves beneath the calcite
compensation depth (CCD) at 4.5 km
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Nodules Deposits hydrogenous sediment
from precipitation of metal oxides
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1) latitude2) distance from landmasses3) CCD
(carbonate compensation depth)
Distribution of sediments in the deep 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 depth (CCD) while siliceous ooze occur
  below
• 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.
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.
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.
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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1243 sites
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The Drying Up of the Mediterranean Sea

• 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.

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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.

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Learning Objectives
1. Understand the origin and classification of
marine sediments. 2. Explain the factors
controlling origin and deposition of sediment on
the continental shelf and in the deep ocean.