Chapter 4 Marine Sedimentation

Report
Chapter 4
Marine
Sedimentation
©2003 Jones and Bartlett Publishers
EXAM and QUIZ #1
•In 2 weeks, September 30.
•Exam on chapters 1-5 in Pinet’s text.
•1 - 1.5 h length
Multiple choice, short answer.
Study material at end of chapters:
keywords
Review of basic concepts questions
Critical thinking essays questions
Discovering with numbers questions
•Quiz questions taken directly from laboratory
exercises
Classification of marine sediments based upon size.
Sediment
Type
Diameter (mm)
Gravel
Boulder
>256
Cobble
65-256
Pebble
4-64
Granule
2-4
Very coarse
1-2
Coarse
0.5-1
Medium
0.25-0.5
Fine
0.123-0.25
Very fine
0.0625-0.125
Sand
Mud (silt & clay)
Colloid
0.0002-0.004
<0.0002
Classification of marine sediments based upon Mode
of Formation.
•Terrigenous: Sands and mud produced by weathering and
erosion of rocks on land.
•Biogenic: CaCO3 (calcium carbonate) and SiO2 (silica)
muds and oozes composed of hard parts of organisms.
•Authigenic: formed by precipitation of minerals in seawater
(Manganese (Mn) and Phosphorus (P) nodules).
•Volcanogenic: ejected from volcanoes (ash).
•Cosmogenous: pieces of meteorites that survive trip thru
atmosphere.
Sediment Sampling Methods
Sediment Sampling Methods: CORINGpreserves deep stratigraphy, or layering.
• Hjulstrom’s Diagram graphs the relationship
between particle size and energy for erosion,
transportation and deposition.
Shelf Versus Basin Depths
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
Shelf sedimentation is strongly controlled by
tides, waves and currents, but their influence
decreases with water depth.
Sea-level rise and fall
Sea-level rise and fall
Geologic controls of continental shelf
sedimentation must be considered in
terms of a time scales.
• For a time scale up to 1000 (103) years, sedimentation controlled by:
– Waves
– Wind-induced currents
– Tidal currents
(all related to water depth)
Million-year time scale.
• For a time scale up to
1,000,000 (106) years,
sedimentation controlled
by:
– Glaciation and its effect on
position of coastline
Relict Sediment: deposited in the past under
conditions that are no longer present.
Shelf Sedimentation Model
Hundred-million year time scale…
• For a time frame up to 100,000,000 (108)
years, sedimentation controlled by:
– Plate tectonics and its effect on type of margin.
Plate tectonics & sedimentation on shelf
Carbonate Shelves
If influx of terrigenous sediment is low and the
water is warm, carbonate sediments and reefs
will dominate.
Passive (Atlantic) vs. Active (Pacific) Type Margins.
• Atlantic (passive) type margin:
Passive boundary
Long history of sedimentation
Sedimentation rate = subsidence rate
Broad, smooth continental shelf
• Pacific (active) type margin:
Convergent boundary
Sedimentary layers compressed and deformed
Volcanic sediment
Seismic activity causes slumps and slides of sediments to
deep-sea trenches.
Passive (Atlantic) vs. Active (Pacific) Type Margins.
Passive (Atlantic) Type Margin.
Active (Pacific) Type Margin.
Deep-sea Sedimentation
The Deep sea has two main sources of sediment:
1. External- terrigenous material transported to oceans via
rivers and wind,
2. internal-biogenic and authigenic from the sea.
River transport of sediment
Wind transport of sediment
Deep-sea sedimentation processes
Modes of sedimentation in deep sea
• Bulk emplacement:
– Slumps: sediment transport by mass with little
deformation or folding of layers
– Slurries: debris flows and mud flows- destroy any
previous bedding or layering.
• Turbidity currents
– Deep-Sea canyons formed by these processes.
– Ice Rafting
• Polar latitudes, debris from melting icebergs.
• Glacial marine sediment
Bulk Emplacement: Slumps and Slurries
Bulk Emplacement: Ice Rafting
Deep-sea sedimentation processes
Modes of sedimentation in deep sea
• Pelagic sedimentation:
– Pelagic muds:
• Inorganic red or brown
clays and silt
– Fine-grained (0.0002 –
0.0004 mm)
– Quartz, feldspar, kaolinite
& chlorite minerals
– Terrigenous, windbourne, cosmogenous
source
– Kaolinite in tropical &
subtropical waters
– Chlorite in temperate &
subpolar
– Dominate below waters
with little planktonic
production.
Modes of sedimentation in deep sea
• Pelagic sedimentation:
– Pelagic muds:
• biogenic oozes
– >30% of debris from planktonic organisms
– Calcareous oozes (CaCO3)
» Shells of foraminifera & pteropods (zooplankton) and
coccolithophorids (phytoplankton).
» Accumulate on seafloor above CCD.
» Forms hard limestone under pressure
– Siliceous oozes (SiO2)
» Shells of radiolaria (zooplankton) and diatoms
(phytoplankton).
» Accumulate on seafloor below CCD.
» Accumulate below regions of high diatom production
(equator, poles, upwelling areas)
Carbonate Chemistry
• CaCO3 (calcite) is a solid material produced
by biological or abiological processes in
seawater:
Ca2+ + CO32-
CaCO3
– The reaction can go both ways, depending on the pH,
pressure.
– When the seawater is undersaturated with respect to
CaCO3, calcite will dissolve:
Ca2+ + CO32CaCO3
– But when seawater is saturated with respect to CaCO3,
calcite will remain in its solid form and not dissolve:
Carbonate Compensation Depth, CCD
• Depth in ocean at which seawater becomes
undersaturated with respect to calcite and rate of
dissolution of CaCO3 equals its rate of delivery.
– CCD ~ 4500 m (or deeper in regions of high surface
productivity).
– Depths below CCD:
• Seawater undersaturated w.r.t. CaCO3
• Chemical properties of deep water dissolves calcite
• CaCO3 oozes less common than SiO2 oozes.
– Depths above CCD:
• Seawater saturated w.r.t. CaCO3
• CaCO3 remains intact.
• CaCO3 oozes more common than SiO2 oozes.
Question: Why is the CCD sometimes referred to as the
“snow-line”?
Foraminifera (zooplankton with CaCO3 shell)
Size ~ 1mm
Diatoms (phytoplankton with SiO2 shell)
Size ~ 0.01mm
Deep-sea Sediment Distribution
Deep-sea Sediment Distribution
TYPE
COMPOSITION
ATLANTIC
(%)
PACIFIC
(%)
INDIAN
(%)
GLOBAL
(%)
Foram. ooze
Carbonate
65
36
54
47
Pteropod
ooze
Carbonate
2
0.1
-
0.5
Diatom ooze
Silica
7
10
20
12
Radiolarian
ooze
Silica
-
5
0.5
3
Red clay
Aluminum
silicate
26
49
25
38
Authigenic deposits
• Formed by chemical or biochemical
reactions on ocean floor
• Nodules of ferromanganese (Fe and Mn) or
phosphorite (P).
• Concentric layers of metal oxides accrete
on particles over millions of years (1-4 mm
per 106 y).
• Contain economically important metals Cu,
Zn, Co and Pb. (but too expensive to
harvest).
• Origin uncertain (biological?)
Ferromanganese nodules
Floor of South Pacific Ocean.
Nodule size 1-5 cm diameter
Ferromanganese nodules
Cross-section
Deep-sea stratigraphy
• Broad-scale layering of sediments that
cover the basaltic crust.
• Strongly influenced by sea-floor spreading
and direction of spreading centers with
relation to latitude.
Deep-sea stratigraphy
The Atlantic basin contains a “two-layercake” stratigraphy–a thick basal layer of
carbonate ooze overlain by a layer of mud.
Pacific plate moves across latitudes…
The Pacific basin contains a “five-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.
Geophysical3Surveying
END 2-5
OF LECTURE

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