Page 59 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 123, 124, 125
N o n - s keletal a l g a e

Micritization

In the shallow marine environment, some non-skeletal
blue-green algae may bore into skeletal material. These
are called endolithic algae. The borings, around I 0 J.lm in
diameter, arc filled with micrite after the death of the
algae. If the process continues, the margin of a shell
fragment may become completely replaced by micrite.
The process is known as micritization and the replaced
shell margin as a micrite envelope.
123 shows micrite envelopes developed on brachiopod
shells (the foliated structure) and echinoderm fragments
(the speckled plates). Note the irregularity of the contact
between the micrite envelope and the unaltered skeleton.
This enables micrite envelopes formed by micritization by
blue-green algae to be distinguished from micritic coat­
ings around the exterior of skeletal fragments (77).
Repeated micritization may lead to the production of a
grain with no remaining recognizable structure. This
would then be called a peloid (76). Skeletal algal frag­
ments arc often susceptible to this total micritization and
it is possible that some of the micritic grains in 123, with
their irregular shape and trace of internal structure, were
formed by this process.
124 shows the importance of micrite envelopes in
preserving molluscan fragments during diagenesis. The
original aragonite molluscan shell has been completely
dissolved and the mould, outlined by a thin micrite
envelope. was then filled by a sparry calcite cement.
Although the sparitc is mainly blue-stained fcrroan
calcite, there arc thin zones of pink-stained non-ferroan
calcite. This is clearly seen in the shell fragment to the left
below the centre.
Allochems other than bioclasts may become micritized.
125 shows a number of grains with varying degrees of
prc�crvation of radial and concentric ooid textures (p.
35). It is possible that the texture was partially lost b)
micritiLation, although it might also have been lost during
inversion of an original aragonite ooid to calcite (neomor­
phism. p. 60).
123: Unstained thin section, Woo Dale Limestone, Lower
Carhon((erous, Peak Forest. Derbyshire, England; m a g n if ­
ication x 25. PPL.
124: Stained thin section, In f erior Oolite, Middle Jurassic.
l,ecklwmpton Hill, G/oucestershire, England: magnific­
ation x 12. PPL.
125: Stained thin section, Llandyf a n Limestone, Lower
Carhon({erous, Black Mountains. South Wales; magnific­
tlfion x 4 3. PPL.

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