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Chem 1103: Lecture 9: Marine Sedimentation: Chap 4




Sediments show history of what has happened geologically both on land and under
oceans



Sediments show a record of what climate has been



Sediments produced by weathering of the rocks, granite from continen
ts, basalt from
ocean floor and other sources which will be discussed later



Without plate tectonics, the continents would be long ago eroded away. Tectonics,
which we discussed last week, uplift sediments



Classification of marine sediments can be based up
on size or origin

o

Size classification divides sediment by grain size into gravel, sand, and clay

o

Mud is a mixture of silt and clay

o

Origin classification divides sediment into five categories



Terrigenous sediments


erodes from the land an are mainly sand a
nd
mud



Biogenic


shells and skeletal remains of organisms



Authigenic


chemical and biochemical precipitates that from in place
on the sea floor


manganese nodules



Volcanogenic


volcanic particles ejected during eruptions



Cosmogenic


extraterrestrial p
articles that fall from space



Factors that control sedimentation include particle size and the energy/ turbulence of
the deposition environment

o

Terrigenous sediments strongly reflect their source and are transported to the
sea by wind, rivers and glaciers

o

Rate of erosion is important in determining nature of sediments



Slow erosion means the sediment will be well sorted, consisting of a
limited range of particle sizes



Rapid erosion means the sediment will be poorly sorted and consist of a
mixture of sediment

sizes

o

Average grain size reflects the energy of the depositional environment



High energy allows deposition of only larger grains; smaller grains are
kept in suspension



Low energy allows deposition of smaller particles, but larger particles
are rare becaus
e there is insufficient energy to transport them into the
low energy environment

o

Hjulstrojm’s Diagram graphs the relationship between particle size and energy
for erosion, transportation, and deposition



For particles larger than sand, energy for erosion in
creases with size
because larger particles weigh more



For particles smaller than sand, energy for erosion increases with
decreasing particle size because particles are cohesive



Sand is easily erosed because it is small and not cohesive



As energy decreases,

particles are deposited according to size (weight)
with the largest first and smallest last



More energy is required to erode than transport a particle



Sedimentation in the ocean

2

o

Based upon water depth, the ocean environment can be divided into the shelf,
which is shallow and near a terrigenous source and the deep ocean basin, which
is deep and far from a terrigenous source



Seaward water becomes deeper and more distant from a terrigenous
source

o

Shelf sedimentation is strongly controlled by tides, waves, and

currents, but
their influence decreases with 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



Past fluctuations of s
ea level have stranded coarse sediment (relict
sediment) across the shelf including most areas where only fine
sediments are deposited today



During last ice age ~15,000 yrs ago, sea level was 130 m lower and
rivers dumped their loads further out



60
-
70% of

continental shelf is relict

o

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 mud dominate temperate shelves



Glacial

till and ice
-
rafted sediments dominate polar shelves

o

Geologic controls of continental shelf sedimentation must be considered in
terms of a time frame



For a time frame up to 1000 yrs., waves currents, and tides control
sedimentation



For a time frame up to
1,000,000 yrs, sea level lowered by glaciation
controlled sedimentation and caused rivers to deposit their sediments at
the shelf edge and onto the upper continental slope and to dig canyons;
Glaciation during Pleistocien epoch see Apendix for epochs, age
s



For a time frame up to 1000,000,000 years, plate tectonics have
determined the type of margin that developed and controlled
sedimentation



Atlantic type margin is characterized by a long history of
sedimentation and slow subsidence; balance between subsid
ence
and sedimentation



Pacific type margin is characterized by subduction and the
formation of an accretionary prism of deep sea sediments onto
the seaward edge of the shelf

o

If influx of terrigenous sediment is low and the water is war, carbonate
sediments

will dominate


BREAK

3


o

Deep
-
sea sedimentation has two main sources for sediment: terrigenous material
from the land and biogenic and authigenic from the sea



Major sedimentary processes in the deep sea include:



Bulk emplacement is the large scale down
-
slop
e slump or slope
failure of a largely undisturbed coherent sedimentary mass from
the shelf edge or upper slope



Debris flows (poorly sorted sediment) and mud flows are slurries
(mixes of sediment and water) which flow down slope to the sea
floor



Submarine c
anyons transport lots of sediment and are dug by a
combination of sediment slumps and turbidites



Slump sediment piles that flow downslope intact



Turbidity currents are fast
-
moving, erosive, turbulent slurries
which are largely responsible for the erosion o
f the submarine
canyons, deposition of brad deep
-
sea fans at the canyon mouth
and the formation of the continental rise

o

Turbidites are the deposits of turbidity currents and
display graded bedding



In polar areas ice
-
rafting produces a mixture of sediment s
izes



Pelagic sediment is fine
-
grain terrigenous or biogenic sediment
that slowly sinks to the deep sea floor



Major pelagic sediments in the ocean are red clay and biogenic oozes



Red clay (brown clay or pelagic clay) consists of very fine,
weathered particl
es of wind
-
blown terrigenous clays and
extraterrestrial dust

o

Clay composition is climate controlled consisting mainly
of kaolinite in the tropics and subtropics and chlorite in
the polar and subpolar regions



Biogenic oozes are a fine
-
grain sediment of whic
h at least 30%
are shells of micro
-
organisms
-

zoo
-

and phyto
-
plankton. They
are classified by their composition

o

Calcareous oozes consist of the shells of foraminifera,
ostracods and coccoliths

o

Siliceous oozes consist of the shells of diatoms and
radiolari
a



Carbonate compensation depth (CCD) is the depth at which
water becomes sufficiently acidic to dissolve carbonate shells

o

The depth is usually between 4 to 5 km but is dependent
upon the supply of carbonate material and water acidity,
temperature, and pres
sure

o

Calcium carbonate is white and with CCD looks like
snow on mountain tops under the water



Siliceous ooze dominates in the tropics and polar waters where
the sea floor is below the carbonate compensation depth and
productivity of radiolarians and diatom
s is high

4



Authigenic deposits are chemical and biochemical precipitates that form
on the sea floor and include ferromanganese nodules and phosphorite



Ferromanganese nodules are deep
-
sea deposits consisting of
concentric layers of metallic compounds precipi
tated by bacteria,
foraminifera, or possibly inorganic chemical reactions



Phosphorite is a shelf deposit formed where upwelling of
nutrient
-
rich water generates high biological productivity
resulting in a high concentration of phosphate
-
rich organic debris

within shelf sediments

o

The distribution of sediments in the deep ocean varies greatly, but is strongly
controlled by the compensation depth



Sediments become compacted and turn into sedimentary rock



Mud and clay to shale



Sand to sandstone



Calcium carbonate

to limestone and chalk



Silicates to opal



Surface deposits are the sediments found exposed on the sea floor



Much of the Pacific sea floor is covered by biogenic and
hydrogenic (authigenic) sediments because terrigenous sediment
form the land is largely tra
pped in the trenches



In areas of high productivity, carbonate oozes dominate where the
sea floor is above the carbonate compensation depth. Silicieaous
oozes prevail where the sea floor is below the carbonate
compensation depth



Deep
-
sea stratigraphy refer
s to the broad
-
scale layering of the sediments
of the sea floor. It strongly reflects the sediment types deposited as the sea
floor migrated away from the ridge crest



At the ridge crest the sea floor is above the carbonate
compensation depth and calcareou
s oozes accumulate. These
later become limestone



Away from the ridge crest in waters below the carbonate
compensation depth calcareous material dissolves and mud
accumulates



Because plates migrate across climate zones in the Pacific Ocean
the layering is
more complex

o

An additional layer of limestone above the layer of mud
reflects sufficiently high calcareous sediment production
just north of the equator to have suppressed the carbonate
compensation depth and allowed more carbonate oozed to
accumulate

o

Fart
er north where carbonate production is reduced, fine
carbonate sediment again dissolves before reaching the
bottom and mud accumulates

END

Lab



Note hydrogeneous = authigenic

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