lecture notes


22 Φεβ 2014 (πριν από 7 χρόνια και 6 μήνες)

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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

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


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


Mud is a mixture of silt and clay


Origin classification divides sediment into five categories

Terrigenous sediments

erodes from the land an are mainly sand a


shells and skeletal remains of organisms


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

manganese nodules


volcanic particles ejected during eruptions


extraterrestrial p
articles that fall from space

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


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


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



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


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



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


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

70% of

continental shelf is relict


Worldwide distribution of recent shelf sediments by composition is strongly
related to latitude and climate

Calcareous biogenic sediments dominate tropical shelves

supplied sands and mud dominate temperate shelves


till and ice
rafted sediments dominate polar shelves


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

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

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

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


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

will dominate




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
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

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


Turbidites are the deposits of turbidity currents and
display graded bedding

In polar areas ice
rafting produces a mixture of sediment s

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


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


and phyto
plankton. They
are classified by their composition


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


Siliceous oozes consist of the shells of diatoms and

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


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


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


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
rich water generates high biological productivity
resulting in a high concentration of phosphate
rich organic debris

within shelf sediments


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

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

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


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


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



Note hydrogeneous = authigenic