Transcript Chapter 4 Marine Sedimentation

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.
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:
1. 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.
2. Salt composition of the oceans has not changed for the last 1.5 billion
years. Explain why?
Marine Sedimentation
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Sediments in the Sea
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
four major categories:
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
Sediments 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.
Streams
Rivers
Glaciers
Landslide
(Gravity)
Sedimentation in the Ocean
ocean environment can be divided into:
the shelf – neritic deposits
(shallow areas and near a terrigenous
source)
the deep ocean basin – pelagic deposits
(deep areas and far from a terrigenous
source)
Shelf Versus Basin
Sedimentation in the Ocean
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.
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.
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.
Influence of Past Sea Level
Case study:
The Atlantic Passive Margin
Case study:
Pacific Destructive/Subduction Margin
Carbonate Shelves
If influx of terrigenous sediment is low
and the water is warm, carbonate
sediments and reefs will dominate.
Ice Rafting
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.
Sedimentation in the Deep Sea
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;
Biogenic Deposits
Calcium Carbonate
Silica
(e.g. diaotms, radiolaria)
SiO2
(e.g. mollusks, corals, foraminifers,
some algae)
CaCO3
Tiny animals with calcite shells (CaCO3)
Coccoliths
Tiny plants with a calcite skeleton (CaCO3)
Diatoms
SiO2
Radiolaria
Silica Sedimentation
Siliceous ooze found where it accumulates faster
than it dissolves
SiO2
Calcium carbonate Sedimentation
CCD – Calcite dissolves beneath the calcite
compensation depth (CCD) at ~ 4.5 km
CaCO3
Nodules Deposits – hydrogenous sediment
from precipitation of metal oxides
Distribution of sediments in the deep ocean
1) latitude
2) distance from landmasses
3) CCD (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 depth (CCD); while siliceous ooze occur
below;
• The rate of sedimentation depends on the type of
sediment in deep sea.
Sedimentation in the Ocean
• Deep-sea stratigraphy refers to the broadscale layering of sediments that cover the
basaltic crust. The stratigraphy of the deep
sea is strongly influenced by sea-floor
spreading.
Sedimentation in the Ocean
The Atlantic basin contains a “two-layercake” stratigraphy–a thick basal layer of
carbonate ooze overlain by a layer of mud.
Stratigraphy of the Atlantic Basin
Sedimentation in the Ocean
The Pacific basin contains a “four-layercake” stratigraphy, because unlike the
Atlantic its sea floor as it spreads crosses
the equator where the CCD is lowered to the
ocean bottom.
Stratigraphy and Model of Pacific Basin
Collecting Marine Sediment
There are a number of sampling
techniques for obtaining sediment from
the ocean bottom.
• 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
• 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.
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.