ooids and peloids

7/23/2019 Ooids and Peloids

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` Ooids

` Formation of ooids

` Factors Affecting ooids growth` Conclusion

` Peloids

` Depositional environment of Peloids


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` Ooids are small (< 2 mm in diameter), spheroidal,

"coated" (layered) sedimentary grains, usually

composed of calcium carbonate, but sometimes

made up of iron- or phosphate-based minerals.` Pisoids are similar to ooids, but are larger than 2

mm in diameter.

` Oolite (egg stone) is a sedimentary rock formed

from ooids.


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Ooids on the surface of a limestone; Carmel Formation (Middle

Jurassic) of southern Utah


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A thin slice of calcitic ooids from the Carmel Formation, Middle Jurassic,

of southern Utah, USA


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` An ooid forms as a series of concentric layers

around a nucleus. The layers contain crystals

arranged radially, tangentially or randomly. The

nucleus can be a shell fragment, quartz grain orany other small fragment.

Two main types of ooid. An ooidwith tangentially arranged crystals is

shown in the left and an ooidwith radially arranged crystals is on the

right.


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` The formation of these objects has been

speculated from the early 19th Century and ideasfor their origin range from crinoid eggs, insect

eggs to the present day explanation of precipitated

layers of CaCO3.` Recent ooids are forming today in places such as

the Bahamas and Shark Bay, Australia and are all

composed of aragonite.


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The following factors have an affect on ooid

growth:

` Supersaturation of CaCO3` Nuclei

` Agitation

` Location

` Water depth


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` Supersaturation:

the concentration needed to be above 0.002

moles/litre and below 0.0167 moles/litre for ooidsto form successfully. Below 0.002 moles/litre onlyaragonite needles or poor ooids formed. Above0.0167 moles/litre the ooids formed an amorphousmass.

Nuclei:

The type of nuclei affects the rate of growth andthe size of each lamination (Davies et al., 1978).Organic coating on the nuclei give faster and

longer precipitation, while using oxidised quartzshow much slower and shorter precipitation.


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` Agitation:

The agitation an ooid undergoes must be enoughto keep it in suspension for the growing phase

followed by removal to a non-supersaturated fluid.

Agitation may also control ooid size.

Location:The location off ooid formation is important.

They must be kept in the same area throughout

the formation, in order that their life cycle can be

completed.


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` Water Depth:

Most ooids form in water less than 2m deep, but this

may have more to do with wave agitation and tidalmovements than water depth itself.

Conclusion

Ooids are spherical accumulations of carbonate grainsas either aragonite or calcite. They require a high level

of supersaturation with respect to carbonate, thepresence of nuclei and agitation. There is arelationship between the amount of ooids formed andthe water depth, but this may be due to shallowerwater having a higher level of agitaiton. Ooids go

through several stages during formation, including asleeping phase, which may involve the use of organicmaterial in order to initiate a new layer.


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Peloids are fecal pellets that are

produced by organisms thatconsume calcium carbonate mud.

Formed from organismexcretions of undigested calcium

carbonate mud. Tend to be oval in shape anduniform in size.

Unlike ooids, peloids lack concentricand radial internal structures, making them easy todistinguish from ooids and other non-skeletal grains.

Peloids can be identified by their dark color due tothe organic matter from which they are formed.


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` Peloids collect in lagoons and shallow intertidal

zones where they are protected from rough ocean

currents.

` However, peloids will not remain protected foreverand will eventually break down into micrite.

` Through burial processes, peloids will become

cemented and preserved.


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ooids and peloids

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