Page 74 - Atlas of Sedimentary Rocks Under The Microscope

159, 160 Carbonate rocks
Limestone p o r o s i t y
(continued)

Pore-space in limestones may be filled with sediment as
well as cement. Sediment partially infilling cavities,
particularly in fossils or fenestrae, will indicate the
horizontal plane at the time of its deposition. Such
.
s e d imenl infills are known as geopetal infills. 159 shows
geopctal sediment within a gastropod. On deposition the
gastropod would have had a primary porosity within its
chambers (intragranular porosity). This was partially
in filled by micritic sediment and the cavity finally filled by
fe r roan calcite cement. lnclusions within the shell wall of
the gastropod and surrounding bioclasts suggest that they
inverted to calcite during neomorphism (p. 61 ), rather
than being cement-filled casts.
Some pore-spaces have hydrocarbons within them or
have evidence that hydrocarbons have passed through.
160 shows a limestone in which a few pores are filled with
black hydrocarbon and others arc lined by a thin coaling
of it. Examination of its relationship to the cement shows
that the hydrocarbon cntere,d the rock after an early
generation ofisopachous cement (marine?) and before the
final (illing of coarse blocky cement (meteoric).

159: Stained rltin section. Purbeck Marble. Upper Jurassic.
Dorser, Enxland; magnificarion x 12, P PL.
160: Unsrained thin seuion, Bee Low Limestone. Lower
Carboniferous. Windy Knoll, Derbyshire. Enxland; mag­
nification X 16, PPL.

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