Page 58 - Atlas of Sedimentary Rocks Under The Microscope

121, 122 Carbonate rock�
N o n - s k e l e t a l a l g a e

Stroma to I ites

Stromatolites arc laminated rocks interpreted as fos­
�ilized algal mats. The mats are formed of filamentous
blue-green algae. The laminae in stromatolites arc usually
alternation� of carbonate mud and grainy. often pclletcd,
l>Cdimenl. The laminae are. at most. a few millimetres
thick and arc often more easily seen in hand specimens
than in thin sections. Laminoid fe nestrae (p. 68) arc often
associated with stromatolites.
121 is a photograph of a polished block showing a
stromatolite. Note that the layering is irregular and partly
picked out by colour differences. The irregularity of the
layering helps to differentiate laminated sediments
formed from algal mats from those formed by physical
processes. The laminae in stromatolites may form flat or
crinkly �tructures or may build up into columns or domes.
Concentric algal laminations about a nucleus give rise to
the grains known as oncoids (p. 38).
122 shows a thin section of the same specimen as that
illustrated in 121. The laminations consist of alternating
thin micritic layers and layers containing a mixture of
micrite and sparitc. In some areas the micrite has a
vaguely pcllctcd structure which is eh�tractcristic of
stromatolites. The more irregular micrite <trcas may have
been coating the algal filaments which then decayed,
le aving a mould which was later Ill led with spa rite cement.

121 and 122: Lower Carboniferous, Carrihe de Ia Vallee
fleureu\'e, Boulonnais, France; 121 hand specimen, mag­
nification x 1.8; 122 stained thin sec/ion, magnifica­
lion x 12. PPL.

53