Page 72 - Atlas of Sedimentary Rocks Under The Microscope

lS4, ISS, IS6 Carbonate rocks
Limestone p o r o s i t y
(continued)

154 shows a limestone composed mainly of echinoderm
f ra gments in a pink-stained, non-ferroan, calcite sparite
cement. However, a number of grains comprising a small
e c hinoderm fragment nucleus, surrounded by a zone of
blue-stained fe rroan calcite cement, are also present. This
cement is interpreted as a late infilling of pore-space
f o r m e d by the dissolution of an aragonite coating to the
echinoderm fragments. Such a coating is likely to have
been oolitic and after solution the sediment would have
exhibited oomouldic porosity.
Porosity may develop as a result of the burrowing and
boring activities of organisms. ISS Shows a section
through a boring made by an organism in an oolitic
sediment. Note that grains arc truncated around the
margins of the boring, indicating that the sediment was
lithified when the organism was at work and hence the
structure is a boring rather than a burrow. The boring is
infillcd with a fe rroan calcite cement, some of which has
been lost during the making of the section.
Shelter porosity occurs below curved shell fragments
which arc preserved in a convex-up position. 156 shows
bivalve fragments in a carbonate mud sediment. Those
preserved in a convex-up position. including the large
fragment extending right across the field of view, have
areas of sparite cement below them which was pre­
cipitated during the infilling of shelter cavities. Sediment
was unable to fill the cavities because of the 'umbrella'
effect of the shell.

154: Stained acetate peel, Oolite Group, Loll'er Carbo­
niferous, Daren Cilau. Lkmgaffock, South Wales; magn(f ­
ication x 15, PPL.
155: Stained thin seer ion, I n f erior Oolite, Middle Jurassic.
Cooper's Hill, Gloucestershire, England; magnific­
lllion x 16, PPL.
156: Stained thin section, Lower Carboniferous, Arbigland,
Dumfries. Scotland; IIWf{nijicarion x 16, PPL.

67