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Morphology of Ocean Basins
Elements
Continental margins
Shelf
Shelf break
Continental slope
Continental rise
Submarine canyons
Abysal plains
Mid-ocean Ridges
Guyots and seamounts
Two types of continental margins:
a) Passive (Atlantic type)
b) Active margins (Pacific type)
Elements of continental margins
Shelf
Shelf break
Continental slope
Continental rise
Submarine canyons
Earth’s interior:
Crust,
lithospere,
upper mantle
active and passive
margins
Physical and geochemical characteristics of continental crust, oceanic crust
and upper mantle: Isostasy
Elevations of Earth’ crust on land and in the oceans
Percent of Earth’s
Total Surface Area
Area Square
Kilometers
Land area
29.2%
148,940,000
Area
covered by
Water
70.8%
361,132,000
Pacific
Ocean
30.5%
155,557,000
Atlantic
Ocean
20.8%
76,762,000
Indian
Ocean
14.4%
68,556,000
Southern
Ocean
4.0%
20,327,000
Arctic Ocean
2.8%
14,056,000
Surface
Continental Margins
The oceans cover 70.8% of the earth surface
Total area of the earth:
Area of oceans:
Area of land
560 x 106 km2
361 x 106 km2 (71%)
149 x 106 km2 (29%)
Continental Margins
Shelf
27 x 106 km2
Continental slope 28 x 106 km2
Continental Rise
19 x 106 km2 (present only at passive margins)
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Total
74 x 106 km2
(20.5% of ocean area)
Continental Shelf
Shelves are large parts of the low-lying areas of
continents covered by the sea. They are submerged during
interglacial periods and most parts of it exposed during
Glacial periods
The edge of the shelf lies at different water depths, usually
between -100 and -200 m
Taking the shelf edge at 200m depth, gives an area of
28 x 106 km2, wich is about 5% of the earths surface,
7-8% of the ocean floor
Terms assoicated with shelf area
Littoral: intertidal
Supralittoral: above the high tide, spray zone
Sublittoral: offshore from the tidal area
Base level
Sea level is the level which defines a base level for erosion and deposition
of sediments.
Sea level is the baseline for continental erosion. It is also the baseline for
marine deposition.
The sediments deposited offshore can only built up to the sealevel.
This is why coastal areas and offshore areas close to the sea level are flat.
SEDIMENTATION ON CONTINENTAL MARGINS
Continental margins are depositional sites.
Especially the shelves are the depositional sites for the detrital
sediments, where thick (10 km or more) wedge of sediment
accumulate.
Margins are also the most fertile parts of oceans with high
organic carbon burial: organic rich sediments, which convert into
HCs. These areas are therefore favourable sites for petroleum
exploration.
Especially the Atlantic-type margins are important for petroleum,
because of high sedimentation rate, gradual subsidence and lack
of tectonic disturbance.
Passive (Atlantic-type) and active (Pacific-type)
continetal margins
Atlantic margins are steadily sinking regions, accumulating great thicknesses
of sediments.
Pacific-Type margins are rising associated with volcanism, earthquakes,
folding, faulting and mountain-building processes.
Atlantic type margins
In the Atlantic, the margins formed initally by rifting apart of continents,
then evolved through subsidence and loading of sediment.
Best example are the Red Sea-East African Rift System
The sediment consists of coarse continental clastic sedimentary rocks
interbedded with alkaline volcanic rocks. Evaporites are common and
deposited in the early phase of rifting.
Reefs may develop on the faulted and submerged blocks and form reef
Carbonates.
Evolution of passive margins
a) Uplift of cotinental crust
above a hot spot in mantle & erosion
a, b) Extension and graben formation
b,c) Coarse clastic sediment
deposition and alkaline volcanism
c) Evaporite deposition
d) Rifting and drifting: seafloor
spreading, new ocean
crust formation by
melting of mantle
beneath
Block medel for contient-ocean transition (Seibold and Berger, 1996)
Development of
triple junction and a new
ocean with passive
margins: Red Sea example
Basins due to thermal subsidence after heating
following rifting– passive margins
Passive continental margins are very important for petroleum potential
•Sedimentation due to primary rifting. Lake and river deposits of shale and clastics
• During drifting evaporites and black shales and marls will deposit (organic rich)
•Post-rift stage offshore deep marine deposits
Typical form of a passive margin
Active (Pacific-type) continental margins
Pacific-Type margins are related to subduction (convergent margins)
which can be classified according to the collosion type:
 Continent-continent (Himalayas)
 Continent-ocean (Peru-Chile margin)
 Island arcs (Marianas and Japan in Pacific)
These margins are characterized by:
 Trenches on the offshore side with steep slopes
 Accretion of sedimentary and magmatic material from down-going plate;
all piled up against the margin as mélange.The sediments include pelagic
deposits and magmatic rocks of ocean-lithosphere origin
Gravitaty controlled sedimentation (turbidites) in the trenches
Deformation of sediments (folding and shearing)
High fluid pressure in acretionary prism: fluids such water, CO2, methane
released by metamorphic reactions
Magmatic and hydrothermal activity on the overlying plate
Association with back-arc spreading and back-arc basins
Three types of
active continental
margins:
Ocean-island type
Ocean-continent type
Continent-Continent
type
Divergent boundaey
Mid-Ocean ridge
Convergent boundary
Active or Pacific type margin
Convergent plate boundary: Acitve (Pacific-Type) margins
Active margins
Peru-type continental
margin
Aubouin (1984)
Island arc (ocean-ocean)
With back-arc basin
spreading
Curray et al. (1974)
Trenches
 Associated with active margins
 100 km wide and several hundred to thousands of km long
 7-11 km deep
 Slopes 8-15 degrees
 Flat floor filled with sediment
 Turbidite sedimentation common and accretionary prisms of
highly deformed sediments
 Eastern Pacific ones bordered by land: high sedimentation rate
 Oceanic ones more deep and with less sediment
Examples in Pacific Ocean: Kermadec, Japan, Marianas, Tonga
Peru-Chile
Mariana Trench
The trench is about 2,550 kilometres
long but has a mean width
of only 69 kilometres. It reachess
a maximum-known depth of about
10.91 km at he Challenger Deep, a small
slot-shaped valley in its floor, at its
southern end, although some unrepeated
measurements place the deepest portion
at 11.03 km.[
Sediments and fluids at active (Pacific-type) margins
Nankai subduction
zone (Japan)
with accretionary
prism of Miocene
sediments over a
decollement surface
Cold seeps
Continental Slopes
The continetal slopes join the shelf with the deep basin
 The slope angle commonly varies between 4 and 6º.
 In some cases it can be more than 15º
 For example, in the Marmara Sea, the northern slope has up to 30º
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
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
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slope angle. In the Bahama Plateau it is more than 40º
It is generally the case that in a specific continental slope the slope
angle decreases with depth.
Width: 20-100 km; rarely 100-300 km wide
Depth: usually from 100-200 m to 1400-3200 m; up to 5000 m
Pacific continental slopes are deeper than Atlantic ones
Covers an area of 28 Million km2
Covered with finer grained sediments than thosee on the shelf
60% mud, 25% sand, 10% gravel, 5% shells
Submarine canyons
Submarine canyons are a common feature of continental slopes. The
smooth suface of the slopes are interrupted by canyons.
Examples: Monterey Canyon, off Central California; Hudson Canyon;
Nazare Canyon in Portugal.
 They are uncommon on gentle slopes and where sediment supply
is low or absent.
 They have tributary systems in shallow parts (shelf area); some
can run several 100s km to 1000s km beyond the continental rise
 Commonly connect to a river mouth on the shore.
 Erosion and mass-wasting processes are common in canyons.
 They are important in transporting sediment to deep basins by
turbidity currents.
Origin:
 Sealevel lowering and subareal erosion (e.g., Mediterranean during
Messinian, or during sealevel glacial lowstands)
 Erosion by turbidity currents that are triggered by storms,
earthquakes, volcanic explosions (current velocities of 10-20 cm/s,
or more)
Submarine canyons and deep sea fans
{
Submarine canyons and
turbidites
Deep sea
fans
Proximal
Proximal
Distal
Distal
Canyons in Sea of Marmara