Download Chapter 4 Marine Sedimentation

What is the Seafloor like?
The Sea floor is made of oceanic crust, which is both thinner and
more dense than continental crust.
Major features of the sea flor include the continetal margins and
slopes, the abyssal plains, the mid-ocean ridges, seamounts and
trenches.
Near the edge of the continents most ocean sediments are sands,
mud and gravels from the continent. The composition depends on
latitude and climate: calcite in the tropics, sands in the midlatitudes and glacial deposits in the high latitudes.
Out in the middle of the ocean, Sedimentation rates are very slow.
Sediments are either biogenic oozes, wind-blown clays or
authigenic minerals
Biogenic oozes are the remains of calcareous (calcite) or siliceous
(silica) shells of microorganisms living at the surface of the ocean
Authigenic minerals are those formed in place and include
manganese nodules and many other minerals that precipitate in
the sediments
Although most volcanic activity in the ocean is concentrated at the
ridges, isolated volcanoes can form seamounts and guyots
Basic terms and ideas
Bathymetry — the measurement of water depths and mapping of sea
floor features
The sea floor has two distinct regions: continental margins and deep ocean
basins.
Continental margins are the relatively shallow areas of the ocean
floor near shore. Geologically they are part of the adjacent continent.
Passive vs. active continental margins are distinguished by their plate
tectonic setting.
Deep-ocean basins differ from the continental margins in tectonic
origin, history, and composition. Most important features of the deep ocean
basins were formed by plate tectonic processes.
Major features: mid-ocean ridge, transform faults & fracture
zones, hydrothermal vents, abyssal plain, abyssal hills, seamounts
& guyots, oceanic trench & volcanic arc.
Compositi
on
Physical
Properties
Crustal Properties
Crust
Density
Composition Thickness
continental ~2.8 g/cm3
Felsic
Thick:
20-70 km
~3.2 g/cm3
Mafic
Thin:
2-10 km
oceanic
Age
Old:
up to
4 Byrs
Young:
<200 Mys
Bathymetry: Measuring Ocean Depths
As late as 1870 bathymetric studies were often performed
using a weighted line dropped to measure depth.
Advances in
Bathymetry
• Echo sounding
(1922)
• Multibeam systems
• Satellite altimetry
Echo sounding is a method of measuring depth using
powerful sound pulses. The time it takes for the sound pulse
to travel to the sea bed and bounce back is a measure of the
depth.
Distance = Rate x Time
Multibeam systems can provide more accurate
measurements than echo sounders. Multibeam
systems collect data from as many as 121 beams to
measure the contours of the ocean floor.
Fig. 4-17a, p. 90
Model of Oceanic Ridge
Fig. 12-12, p. 370
Age of the sea floor
Ridges subside and are covered
with sediment
Size comparison of various volcanic features
Continental Margins
Components:
1. Continental shelf = the shallow, submerged edge
of the continent
2. Shelf break = the abrupt transition from
continental
shelf to the continental slope
3. Continental slope = the transition between the
continental shelf and the deep-ocean floor
4. Continental rise = thick accumulations of
sediment
found at the base of the continental slope
Continental
Crust
Oceanic Crust
Continental margin = the submerged outer edge of
a continent; really just an extension of the continental
crust, which has an average composition of granite.
Deep-ocean basin = the deep sea floor beyond the
continental margin; made up of oceanic crust, which
is composed mostly of volcanic basalt.
Types of continental margins
Passive margins (= ―Atlantic-type‖ margins)
" They face the edges of diverging tectonic plates
" Very little volcanic or earthquake activity
Active margins (= ―Pacific-type‖ margins)
" Located near the edges of converging plates, where one plate subducts
beneath another at an oceanic trench
" Extensive volcanic and earthquake activity
Trenches
Oceanic trenches are the deepest parts of the sea floor. They are
formed by subduction.
Notice that most trenches (and therefore most subduction) occur in the
Fig. 12-11b, p. 368
Fig. 4-14, p. 87
Atlantic Coast
Continental slope
Continental rise
Continental slope: California
Fig. 4-11, p. 86
Fig. 4-10, p. 85
Submarine Canyons
Underwater landslides or avalanches called turbidity currents
commonly flow down submarine canyons. The debris settles out to build
up a submarine fan at the base of the canyon.
An
underwa
ter
debris
flow, i.e.
a
turbidite.
Fig. 12-9b, p. 367
Turbidites
graded bedding
Turbidites
graded bedding
Turbidites
Turbidites
Ocean Sediments
• The stuff that covers the basaltic ocean crust
How do we collect sediment
• 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.
Grab samples
Drilling cores
Deep Ocean Basins
Major features:
• Mid-ocean ridges
• Fracture zones and transform faults
• Hydrothermal vents
• Shallow earthquakes
• Abyssal plains
• Abyssal hills
• Seamounts and guyots
• Oceanic trenches
• Volcanic arcs
• Deep earthquakes
Ocean sediment is thickest
over continental
margins and thinnest over
active oceanic ridges.
Based upon water depth, the ocean can be
divided into 2 major depositional
environment s:
the shelf, which is shallow and near a
terrigenous source, and
the deep ocean basin, which is deep and far
from a terrigenous source.
Shelf sediments
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.
Relict Sediments
and glacial/interglacial cycles
Shelf sediments are controlled
by 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.
High latitudes
Midlatitudes
Tropics
High latitudes
Midlatitudes
Tropics
Equator
Deep Sea
Sediments
Sedimentation in the Deep Sea
Sources of Deep-Sea Sediment:
Terrigenous (Rivers and Wind)
Biogenic (Organisms)
Authigenic (Minerals forming in the mud
Biogenic "oozes‖
The remains of ‘plants’ and ‘animals’
Plankton - floaters
Zooplankton and phytoplankton
–Calcareous ooze (CaCO3)
–Siliceous ooze (SiO2)
Coccolithophores: calcareous phytoplankton
(photosynthetic/autotrophs)
Foramifera:
calcareous
(CaCO3)
heterotrophs
http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html
Foramifera:
calcareous (CaCO3)
heterotrophs
http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html
Foramifera:
calcareous
(CaCO3)
heterotrophs
http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html
Diatoms: siliceous (SiO2) phytoplankton
(photosynthetic/autotrophs)
http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html
Diatoms:
siliceous (SiO2)
phytoplankton
(photosynthetic
/autotrophs)
http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html
Radiolarians: siliceous
(SiO2) heterotrophs
http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html
Radiolarians
http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html
Chalk
(Coccolithophores)
Biogenic "oozes‖
> 30% of biogenous sediment
–Calcareous ooze (CaCO3)
–Siliceous ooze (SiO2)
seawifs-6year.mpg
GlobalProd_Seawifs.mpg
OceanProd_SeaWifs.mpg
Other sources
of sediment to
the deep sea
1. Volcanic ash
2. Rivers
3. Wind blown dust
4. Glaciers
5. Cosmogenic
microtektites
The distribution of sediments in the deep
ocean reflects latitude, distance from
landmasses, and the calcium carbonate
compensation depth.
• Terrestrial sediments close to shore
• 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 over areas of
moderate productivity, (warm and no dilution).
• Siliceous oozes where productivity is the
greatest (cool and no dilution)
Fig. 12-20a, p. 377
Biogenic "oozes‖
> 30% of biogenous sediment
–Calcareous ooze (CaCO3)
–Siliceous ooze (SiO2)
seawifs-6year.mpg
GlobalProd_Seawifs.mpg
OceanProd_SeaWifs.mpg
Oceanic sediment patterns
Areas of the seafloor dominated by calcareous (shallow spots) and siliceous (high
productivity spots) biogenous ooze. The remaining deep seafloor accumulates
(very slowly) only pelagic clay. Finally, areas near continents may be dominated
1
by terrigenous or periglacial clastic sediments.
Authigenic sediments:
Manganese nodules
Fig. 12-19a, p. 376
Fig. 12-19b, p. 376
SUMMARY
Distribution of sediment on the sea floor
•Terrigenous:
continental margins and adjacent abyssal plains.
•Calcareous oozes:
wide-spread in relatively shallow areas of the deep sea.
•Siliceous oozes:
polar and equatorial bands where surface waters are rich in
nutrients due to upwelling.
•Manganese nodules:
deep basins, especially the Pacific.
•Red Clay:
deep ocean regions where not diluted
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
Distribution of calcite sediments on a globe
http://geosci.uchicago.edu/~archer/classes/GeoSci238/
Glaciers are dirty
Fig. 12-18, p. 375
Normal marine sediment:
calcareous (CaCO3 i.e.
Calcite) microfossils
Marine sediments during
Heinrich events: only lithic
fragments, i.e. gravel
Dropstones
Norway
5. ‘Cosmogenic’
Shocked grains
Abyssal Plains
Abyssal plains = broad flat areas of sedimentcovered
ocean floor found between the continental margins
and
the mid-ocean ridges
• Typically 4-6 km below sea level
• The flattest surface on Earth
• Sedimentation rates are very slow -millimeters/1000 years!
• Mostly very fine clay, windblown dust, and shells
of
microscopic organisms
Passive Continental Margin
Thick layers of sediment on
top of submerged mountains
Fig. 12-13, p. 371
Formation of Guyots and
submerged mountain chains
Fig. 4-23a, p. 95
Fig. 12-17a, p. 374
Fig. 12-17b, p. 374
Fig. 12-17c, p. 374
Fig. 12-17d, p. 374
Concept Art, p. 380
Fig. 12-7, p. 365
Fig. 12-11a, p. 368