Page 71 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 151, 152, 153

Li mestone porosity
(continued)

151 and 152 show an oolitic/peloidal sediment in which
much of the depositional space between grains is unfilled
by sediment or cement. The rock is said to show primary
intergranular porosity. When deposited. such a sediment
may have had as much as 50% pore-space. This has been
red uccd by compaction and by the introduction of some
cement. Two types of cement are present - a fine spar.
f o rming coatings on most grains (about l mm thick at this
magnification and best seen in XPL) and syntaxial
overgrowths on echinoderms (lower left). Although loca­
liLed. the latter are volumetrically more significant.
A common type of secondary porosity is mouldic
porosity, usually formed by the dissolutiOn or aragomtc
bioclasts. 153 shows a sediment having primary inter­
granular and secondary mouldic porosity. Thin micrite
envelopes have supported the shell moulds, although that
supporting the fragment seen in the lower part of the
photograph has partly collapsed.
The bluish-grey interference colours seen in the inter­
granular pores and the shell moulds of 152 and 153 are
c.-used by strain in the mounting medium.

151 and 152: Stained r!tin si>crion. Porrland Sume. Upper
Jurassic. Dorser. England: magn{ficarion x 27, !51 P P L:
152 XPL.
153: Srained r!tin seuion, Porrland Stone. Upper Jurassic.
Dorser. England: magnificcuion x II. X PL.

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