Page 41 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 75, 76, 77
P e l o i d s a n d

I n t r a c l a sts

A large proportion of the allochems in limestones arc
grains composed partly or entirely of micrite, but having
no concentric laminae in their outermost zones. Various
terms have been used to classify these grains and most
depend on an interpretation of their origin.
Those grains composed of micrite and lacking any
rccognitable internal structure are called peloids. 15
�hows a limestone in which the allochems arc mainly
peloids. circular to elliptical in cross-section and averag­
ing about 0.1 mm in diameter. Such peloids arc generally
interpreted as faecal in origin and are called pellets. The
photograph shows pellets at the lower end of the size
range for typical pellets. which extends up to 0.5 mm.
76 shows larger. less regular peloids, some of which
have a trace of internal structure although its nature can­
not be identified. In the lower part of the photograph arc
speckled echinoderm plates, and midway up the right
hand edge arc segments of the dasycladaccan alga
Koninckopora (sec 1 1 3 ). Both echinoderms and algae
�how signs of replacement by micrite around their
margins (micritization. p. 54). It is probable that the
p�:loids were formed by intense micritization of bioclasts,
thus accounting for their vague relict structures.
lmracla.\tS arc sediment which was once incorporated
on the sca-Aoor of the basin of deposition and was later
reworked to form new sediment grains. 77 shows a large
grain which might be described as a 'coated bioclast'. It
comprises a nucleus, which is a fragment of a brachiopod
�hell, surrounded by a coating of microcrystalline calcite.
The coating is not laminated. so the grain cannot be called
an oncotd (see p. 38); it is external to the shell and has a
�harp contact with it so the coating was not formed by
mtcri!lntion (sec p. 54). It is therefore likely that it is a
fragment of locally-reworked sediment, the brachiopod
o,hcll having once been incorporated in a fine-grained
'>Cdtmcnt which was later eroded to produce intraclasts.

75: Stamc>cl thin section. Upper Jurassic. C a p Rhir,
Morocco: 1111/KIIi/ ic a tion x 33, PPL.
76. Lmtained thin section, Woo Dale Limestone, Lower
Carhoni/(orous. Lo11g Dale, Derbyshire, England; magnific­
ation x 1 1, PPL.
77: Stained thin section. Urswick Limestone, Lower
Carlwn(f' erow. Trowbarrmr. Cumbria. England; nwgllijiC­
alion x 15. PPL
Peloids are also shown in 86, 123, 130. 134, 147, !58 and
162.

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