Page 56 - Atlas of Sedimentary Rocks Under The Microscope

115, JJ6, 117 Carbonate rocks
Bioclasts

Algae (continued)

Many algae possess a central stem encased with calcium
carbonate, through which filaments pass to the outside.
liS shows numerous sections of such an alga in a brown­
stained micrite matrix. Both longiLUdinal and transverse
sections are present. The transverse sections are roughly
c1rcular or elliptical and the centres arc infilled with
micrite sediment. Around the margins of the wall can be
se e n the holes f o rmerly occupied by the filaments, now
occupied by micrite sediment. Details of the wall structure
have not been preserved so the alga was probably
aragonite. The longitudinal sections show that the centre
of the stem contains poorly-preserved casts of the algal
filaments.
The red algae arc important skeletal calcareous algae,
a n d one group, the coralline algae, are major con­
tributors to sediments, including reefs, during the
Cenozoic. 116 shows a fragment of a coralline alga, with
1ts characteristic reticulate appearance caused by thin
micrite walls separating small, more or less rectangular,
cells. The spar-filled holes within the skeleton, called
conceptaclcs, arc also characteristic. To the left of the
coralline alga can be seen part of a nummulitid foramini­
fera, with its thick radial-fibrous wall.
The blue-green algae occur typically as long narrow
filaments and only a few species become calcified. Girvan­
e/la, illustrated in 117 is widespread and occurs over a
long stratigraphic range. It is made up of bundles of
narrow tubes about a millimetre in diameter at this
magnification, with a thin micrite wall. They can be seen
in longitudinal section (e.g. upper part of the photograph)
a n d transverse section (e.g. lower right). The remainder of
the sediment comprises a few bioclasts (e.g. an ostracod,
lower left) and a mixture of carbonate mud sediment and
s p a ritc cement, the latter being partly pink-stained, non­
fcrroan calcite and partly bluish f e rroan calcite.

115: Stained thin section, U p p e r Cretaceous, Tunisia;
m a g nification x 19, PPL.
116: Stained thin section, Eocene, Greece; magnifi c­
ation x 23. PPL.
117: Stained thin section, Clzatburn Limestone, Lower
Carboniferous, Chatburn, Lancashire, England; magnific­
ation x 37. PPL.
Other algae are shown in 76, 128, 130 and 150.

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