Page 64 - Atlas of Sedimentary Rocks Under The Microscope

137, 138, 139 Carbonate rocks
Pressur e - s o l u t i o n
a n d d e f o r m a ti o n

Pressure-solution is the process whereby a sediment.
because it is under load. is subject to selective dissolution.
In limestones it is normally calcium carbonate that is
dissolved and any less soluble material such as clay and
quartz is concentrated along seams.
137 illustrates a case of grain-to-grain pressure­
solution. Before the pores of a rock arc filled by cement,
�tress is concentrated at the points where the grains meet
and part of one or both the grains dissolves. In the
example, ooids have undergone solution. The later ce­
ment is a mauve-stained, slightly fcrroan spa rite. Note the
small rhombic areas of fine calcite spar (e.g. midway up,
half-way between centre and left-hand edge). These are
calcite pseudomorphs after dolomite (dedolomite, p. 74).
138 shows a limestone which has undergone pressure­
solution to such an extent that most grain boundaries
have been modified and the rock is pervaded by thin dark
seams. Many of these have the fine saw-tooth appearance
characteristic of stylolites. This type of pervasive
pressure-solution is known as sutured-seam solution.
139 shows a limestone which has been subjected to
some stress. Speckled echinoderm plates arc recognizable,
together with syntaxial overgrowths. Most of the calcite
crystals are twinned, a f e ature which may develop as a
rt)Uil of pressure. and the twin planes can be seen to be
slightly bent.

/J7· Stained thin section, U p p e r Jurassic. C a p Rhir,
\Iarocco: magnification x 52. PPL.
118 S ta i n e d acetate peel, Woo Dale Limestone, Loll'er
(urbonif e rolls, Long Dale, Derbyshire, England: magniftc­
urum x3/, PPL.
139. Stained thin section, Torquay Limestone, Devonian.
llt1pe's Nose. Devon. Enf(land; magnification x 31, PPL.

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