SedimentsSediments

Chapter 5Chapter 5

SedimentsSediments

• Particles of organic or inorganic matter that accumulate in a loose, unconsolidated form.

• Sediments are classified by particle size.– Waves and currents transport

smaller particles further than the larger ones

SedimentsSediments

• Well-sorted sediments Layer of sediment that is composed of particles of one size

• Poorly-sorted sediments Layer of sediment that is composed a mixture of particle sizes

Classification of SedimentsClassification of Sediments

Terrigenous SedimentsTerrigenous Sediments• Sediments that come from the land.

• The sediment cycle: – Over geological time, mountains rise as plates collide,

fuse, and subduct. The mountains erode and the sediments are transported to the sea by wind and water, where they collect on the seafloor. The sediments travel with the plate and are either uplifted or subducted. The material is made into mountains and the cycle begins again.

The Sediment CycleThe Sediment Cycle

Biogenous SedimentsBiogenous Sediments

• Made up of siliceous and calcareous compounds that were originally brought to the ocean in solution by rivers or dissolved in the ocean at oceanic ridges.

• Used to build protective shells and skeletons

Hydrogenous SedimentsHydrogenous Sediments

• Minerals that have precipitated from seawater.

• Sources: Submerged rock and sediments, leaching of the fresh crust at oceanic ridges, material issuing from hydrothermal vents. And substance flowing into the ocean in river runoff

Cosmogenous SedimentsCosmogenous Sediments

• Extraterrestrial origin sediments that come from:– Interplanetary dust that falls constantly into

the top of the atmosphere – Rare impacts by lager asteroids and comets

BREAKBREAK

Distribution of Marine SedimentsDistribution of Marine Sediments

• Neritic—Continental shelf sediment – Consist primarily of

terrigenous material• Pelagic—Sediments of

the slope, rise and deep ocean– Consist primarily of

biogenous material

Sediments of Continental MarginsSediments of Continental Margins

• Sediments can accumulate to a great thickness on continental margins

• Shelf sediments can be converted into sedimentary rock in the process of lithification.• Pressure-induced compaction or by

cementation• Once lithified, sediments can be thrust above

sea level by tectonic forces • Forming mountains or plateaus

Sediments of Deep-Ocean BasinsSediments of Deep-Ocean Basins• Classification of Sediments

– Turbidites—deposits made by turbidity currents (graded layers of terrigenous sand interbedded with pelagic sediments)

– Oozes—deep ocean sediment containing at least 30% biogenous material

• Siliceous• Calcareous

– Hydrogenous—originates from chemical reactions that occur in the existing sediment

– Evaporites—salts that precipitate as evaporation occurs

oozesoozes

• Ooze—classified by the type of life from which it was derived– Siliceous ooze—formed by organisms that

contain silica in their shells. Diatoms are one type of organism whose remains contribute to siliceous ooze.

– Calcareous ooze—formed by organisms, such as foraminifera, which contain calcium carbonate in their shells or skeletons. Also formed by pteropods and coccolithophhores

Calcareous OozesCalcareous Oozes

• Calcium Carbonate Compensation Depth (CCD)– The depth at which the rate of accumulation of

calcareous sediments equals the rate of dissolution of those sediments.

– Below this depth, sediment contains little or no calcium carbonate.

– Calcareous sediments dominates the deep-sea floor at depths less than about 4,500m.

Hydrogenous SedimentsHydrogenous Sediments

• Usually a result of chemical reactions– Found in the form of nodules containing

manganese and iron oxides

EvaporitesEvaporites

• Precipitates that form as water evaporates or as the conditions in the water change

• Currently form in the Gulf of California, the Red Sea, and the Persian Gulf

Oolite SandOolite Sand• Hydrogenous sediment formed when

calcium carbonate precipitates from warmed seawater as pH rises

• Forms rounded grains around a shell fragment or other particle (resembling fish eggs)

Studying SedimentsStudying Sediments

• Grab Sampler—can be used to obtain a relatively undisturbed sediment sample.

Studying SedimentsStudying Sediments

• Corer—allows a cylinder of sediment to be taken for analysis to determine the age of the material, as well as the density, strength, molecular composition and radioactivity of the sediment.

Studying SedimentsStudying Sediments

• What do we learn from studying sediments?– Historical information– Location of natural resources

• Crude oil • Natural gas

Questions:Questions:

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