Chapter 4 Marine Sedimentation

coriandercultureMécanique

22 févr. 2014 (il y a 3 années et 5 mois)

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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

Sand

Very coarse

1
-
2

Coarse

0.5
-
1

Medium

0.25
-
0.5

Fine

0.123
-
0.25

Very fine

0.0625
-
0.125

Mud (silt & clay)



0.0002
-
0.004

Colloid

<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
: CaCO
3

(calcium carbonate) and SiO
2

(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:
CORING
-

preserves 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 (10
3
) 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 (10
6
) 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 (10
8
)
years, sedimentation controlled by:


Plate tectonics and its effect on type of margin.

Plate tectonics & sedimentation on shelf

If influx of terrigenous sediment is low and the
water is warm, carbonate sediments and reefs
will dominate.

Carbonate Shelves

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


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


Modes of sedimentation in deep sea

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, wind
-
bourne, cosmogenous
source


Kaolinite in tropical &
subtropical waters


Chlorite in temperate &
subpolar


Dominate below waters
with little planktonic
production.




Pelagic sedimentation:


Pelagic muds
:


biogenic

oozes


>30% of debris from planktonic organisms


Calcareous oozes (CaCO
3
)

»
Shells of foraminifera & pteropods (zooplankton) and
coccolithophorids (phytoplankton).

»
Accumulate on seafloor
above

CCD.

»
Forms hard limestone under pressure


Siliceous oozes (SiO
2
)

»
Shells of radiolaria (zooplankton) and diatoms
(phytoplankton).

»
Accumulate on seafloor
below

CCD.

»
Accumulate below regions of high diatom production
(equator, poles, upwelling areas)



Modes of sedimentation in deep sea


CaCO
3

(calcite) is a solid material produced
by biological or abiological processes in
seawater:



Ca
2+

+ CO
3
2
-

CaCO
3



The reaction can go both ways, depending on the pH,
pressure.


When the seawater is
undersaturated
with respect to
CaCO
3
, calcite will dissolve:

Ca
2+

+ CO
3
2
-

CaCO
3



But when seawater is
saturated

with respect to CaCO
3
,
calcite will remain in its solid form and not dissolve:


Carbonate Chemistry


Depth in ocean at which seawater becomes
undersaturated

with respect to calcite and rate of
dissolution of CaCO
3

equals its rate of delivery.


CCD ~ 4500 m (or deeper in regions of high surface
productivity).



Depths below CCD:


Seawater undersaturated w.r.t. CaCO
3


Chemical properties of deep water dissolves calcite


CaCO
3
oozes less common than SiO
2

oozes.



Depths above CCD:


Seawater saturated w.r.t. CaCO
3


CaCO
3

remains intact.


CaCO
3

oozes more common than SiO
2

oozes.


Question: Why is the CCD sometimes referred to as the
“snow
-
line”?

Carbonate Compensation Depth, CCD

Foraminifera (zooplankton with CaCO
3

shell)

Size ~ 1mm

Diatoms (phytoplankton with SiO
2

shell)

Size ~ 0.01mm

Deep
-
sea Sediment Distribution

Deep
-
sea Sediment Distribution

TYPE

COMPO
-
SITION

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


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 10
6

y).


Contain economically important metals Cu,
Zn, Co and Pb. (but too expensive to
harvest).


Origin uncertain (biological?)


Authigenic deposits

Ferromanganese nodules

Floor of South Pacific Ocean.

Nodule size 1
-
5 cm diameter

Ferromanganese nodules

Cross
-
section



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

Deep
-
sea stratigraphy

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

2
-
5

Geophysical Surveying

END OF LECTURE 3