Page 28 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 47, 48, 49
C o m p act i o n - P r essure­

s o l u t i o n

Sandstones which are not cemented early i n diagenesis
usually show signs of compaction. Since most of the
grains in sandstone are rigid, there is usually little
evidence for grain fracture and breakage (cf. limestone
compaction. p. 58). Thus, apart from grain repacking
during early compaction, the most important process of
compaction is pres sure-solution. This is the process
whereby a sediment under load is subject to selective
solution.
47 shows a sandstone with high intergranular porosity
(the pores. now filled with the mounting medium, are the
grey areas). Most of the quartz grains are coated with a
thin brown rim of hematite cement. At many of the
contacts between grains, one quartz grain has undergone
solution leading to the penetration of one grain by
another (concavo-convex contacts). Good examples can
be seen in the upper left part of the photograph. This is the
first stage of pressure-solution.
Where pressure-solution is more intense, the contacts
between grains become sutured. 48 and 49 show a
sandstone in which grain contacts are irregular and wavy
because of pressure-solution. Silica dissolved during the
process may be precipitated as cement away from grain
contacts, leading to the destruction (occlusion) of poros­
ity. As can be seen, the result is a texture in which the
original grain boundaries can no longer be identified. The
sample illustrated is particularly unusual in that a thin
1.0nc of clay or mica separates the sutured quartz grains. It
has a higher relief than the quartz and is clearly visible in
the photograph taken with PPL. This thin zone of
material together with the sutured contacts enables the
quan.r grains to move slightly relative to their neighbours.
This property impans Acxibility to the sandstone, demon­
�trablc in hand specimens. Sandstones of this type arc
known as flexible sandstones or itaco/umites and are
extremely rare.

47: Nell' Red Sand stone. Triassic. Cheshire. England:
nl(lgn!fimtion x 43. PPL.
4,y and 49: lraco!wnite. Brazil: magn{/ication x 31: 48
PPL. 49 XPL.

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