Page 45 - Atlas of Sedimentary Rocks Under The Microscope

Carbonale rocks 84, 85,86
B i o c l asts

Molluscs

Bivalves and gastropods are common components of
limestones. Most were made of aragonite, so although
there arc a diversity of structures. these arc not seen in
ancient limestone:.. Most originally aragonitic molluscs
arc preserved as casts that is the aragonite dissolved out
during diagenesis leaving a mould which later became
filled with a spa rite cement. There arc, however. import·
ant molluscan groups which had a calcite shell, especially
among the bivalves. and these have well-preserved wall
\t ruct urcs.
84 shows a limestone with abundant molluscan casts.
In thi� casc shell moulds have been in filled with a f e w large
calcllc crystals. Gastropods can be seen. both in long
\ectton (lov . cr right) and transverse section (lower left).
The long straight shells arc bivalve fragments. Careful
111�pection shows that the long valves in the upper left
h<I\C a two-layer structure-a thick layer of coarse spa rite
and a t hin layer with a different structure. This Iauer layer
may have been calcite originally, indicating that the
organism had a mixed aragonite/calcite skeleton. The
rock matrix is micritic sediment.
85 shows a limestone made up almost entirely o f
rounded bivalve fragments preserved a s casts. The shape
or the fragments is shown by the thin micrite rims on the
margins of the shells. These arc micrite envelopes and
formed by micritization by endolithic algae (p. 54). The
cement infilling the bivalves and between the shells is a
line spa rite, initially pink-stained non-ferroan calcite, but
becoming fc rroan towards the centres of pore-spaces as
indicated by the bluish staining.
R6 illustrates a section through a large thick-shelled
ga�tropod. again preserved as a cast. The outer margin o f
the 'hell is picked out b y a thin calcite layer, not more
than 0.5 mm thick at this magnification. but the inner
margm is only clear where sediment has partially filled th e
1ntcrnal cavity. The sediment around the shell contains
abundant �mall peloids.

84 . . "iwined thin .I£'Ction. Eyam Limestone. Lower Carbo­
ni/erou.\, Ricf..low Quarry. Derbyshire, England: magnific­
tllion x 13. PPI-.
85: Stained 1/iin section. Upper Jurassic. D o rset, England:
ma�ni/iwtion x 14. PPL.
i
86: S111 ined ace/ale peel. Man n Limes/One, Lmrer Carbo­
niferous. Mil/om. Cumbria, England: magn(/ication x 7.
PPI .
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