Page 55 - Atlas of Sedimentary Rocks Under The Microscope

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Carbonate rocks 112, 113, 1 4
B i o c l asts
Algae

Those algae in which all or part of the skeleton b e c o mes
calcified arc known as the skeletal calcareous algae. They
arc important contributors to carbonate sediments
throughout the Phanerozoic and exhibit a wide diversity
of fo rms. Green algae arc one of the most important
groups and the photographs on this page illustrate three
examples. one from each of the major groups, the
Codiaceae, Dasycladaceae and Charophyceae. For de·
tails of calcareous algae see Johnson (1961), Wray (1977)
and t=l tigel ( 1982).
1 1 2 shows segments of one of the common f o r m s of
codiacean alga, Halimeda. which still occurs toda)
Living examples contain organic filaments embedded in
aragonite. The example shown is from a poorl)·
consolidated Quaternary sediment which had to be
impregnated with resin before a peel could be made. The
grey areas between the algal segments and in the holes
originally occupied by the filaments are the imprcgnat mg
medium. In this sample the Halimeda segments arc sull
aragonite, although the wall structure cannot be seen at
this magnification. Halimeda fragments are often poorly­
preserved because of the loss of microstructure during the
replacement of aragonite by calcite.
113 shows two types of algae. The large fragment with
the honeycomb structure and walls of fine-grained calcite
is the common Carboniferous dasycladacean alga
Koninckopora. Several algal segments of a different type
can be seen below the Koninckopora. At the level of
viewing here they have no discernible wall structure and
there is a slight resemblance to echinoderm fragments. In
fact they have a fine fibrous wall structure and hence arc
not !>inglc crystals. They may show branching and an
example of Y-branching can be seen in the lower-right
part of the photograph. These belong to a problematic
group. often referred to as ancestral coralline algae, but
sometimes classified with the f o raminifera or stromato·
poroids.
The third group of green algae arc the charophyte1.
although these are sometimes classified separately. ThC)
arc frc!>hwater plants. occurring in the Mesozoic and
Cenozoic. and usually only the reproductive parts (oogo­
n•a) arc calcified. These are small egg-shaped bodies 11ith
vanou!> ornaments. 1 1 4 shows three oogonia in cr�.
section.
112: Swinecl ace/ate peel. Quaternary. Mombasa. Kenya,
magniftcation x 13. PPL.
113: Stained thin section. Chee Tor Rock, Lmrer Carbo·
ni/erou.\, Tunstead Quarry, Derbyshire. England: magm(
ication x 17. PPL.
114: Swined thin section. lggui el Behar F o r marion. U p pt r
Jurassic, Wesrem High A rlas, Morocco: magn(fi(
arion x 56. PPL.