Chapter 4 Marine Sedimentation

swedishstreakMechanics

Feb 22, 2014 (3 years and 8 months ago)

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Chapter 4


Marine
Sedimentation


©2003 Jones and Bartlett Publishers

Classification of marine sediments can
be based upon size or origin.


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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Sediment in the Sea

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


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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Sediment in the Sea

Hjulstrom’s Diagram


Hjulstrom’s Diagram graphs the relationship
between particle size and energy for erosion,
transportation and deposition.

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

Shelf Versus Basin Depths

Shelf sedimentation is strongly
controlled by tides, waves and currents,
but their influence decreases with water
depth.


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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Sedimentation in the Ocean

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


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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Sedimentation in the Ocean

Shelf Sedimentation Model

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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Sedimentation in the Ocean

Relict Sediment

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

Deep
-
sea Sedimentation has two main sources
of sediment: external
-

terrigenous material from
the land and internal
-
biogenic and authigenic
from the sea.

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Sedimentation in the Ocean

Sedimentation in the Deep Sea


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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Sedimentation in the Ocean

The distribution of sediments in the

deep
ocean reflects latitude, distance from
landmasses, and the calcium carbonate
compensation depth.


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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Sedimentation in the Ocean

The Atlantic basin contains a “two
-
layer
-
cake” stratigraphy

a 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

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

Stratigraphy and Model of Pacific Basin


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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Collecting Marine Sediment

There are a number of sampling
techniques for obtaining sediment from
the ocean bottom.


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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The Drying Up of the
Mediterranean Sea

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.