Page 63 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 134, 135, 136
C o m p ac i o n
t

Apart from cementation, the major process leading to
porosity reduction in sediments is compaction. Early
stages of compaction in uncemented sediments involve
the readjustment of loose grain f a brics to fit more tightly
together, the fracture of delicate shells, the squashing of
soft grains, and the dewatering of carbonate mud.
134 shows a peloidal limestone in which either the outer
layers of the peloids, or a very thin early generation of
cement, has flaked off during compaction. The micritic
grains must have been rigid or compaction would have
resulted in their def ormation. Compaction was followed
by the precipitation of a coarse sparitc cement which
'healed' the fractures caused by the flaking off of the rinds
of the grains.
135 shows a cross-section of a gastropod preserved as a
cast. The inner wall of the organism is marked by a micrite
envelope and a thin generation of early cement (sec f o r
example the chambers in the upper part of the photo­
graph). The wall of the shell has been fractured and some
fragments disoriented during compaction. Both micrite
envelope and early cement arc f r actured and the fractures
then healed by a coarse sparite cement. Thus after
deposition, the mollusc was micritized and then cemented
by a thin early generation of fine carbonate. Then the
aragonite wall was dissolved and fracturing occurred,
before the rock was finally cemented. The sample also
shows a vein running from top left to bottom right o f
photograph and brown-coloured replacement dolomite
crystals are scattered throughout the sediment.
136 illustrates a highly compacted bioclastic sediment,
consisting of complete two-valved ostracods as well as
single ostracod valves and long, thin bivalve fr agments.
Most f r agments are aligned parallel to the bedding but
some still show f o lding and fractur ing (e.g. upper left).
The complete ostracods have withstood considerable
pressure but most eventually fractured.

134: Stained acetate peel, Red Hill Oolite. Lower Carb­
onif erous. Cumbria, England; magnification x 31, PPL.
135: Unstained thin section. Woo Dale Limestone, Lower
Carboni f e rous. Derbyshire, England; magnification x 14,
PPL.
136: Unstained thin section, Coal Measures. U p p e r Carb­
oniferous, Cobridge Brickworks, Hanley, S t a f fo r d s h i r e ,
Englcmd; magnification x 19, PPL.

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