Page 46 - Atlas of Sedimentary Rocks Under The Microscope

87,88,89 Carbonate rocks

B i o c l asts
Molluscs (continued)

The photographs on this page illustrate bivalves which
were entirely or partly calcite.
The oysters arc one of the most important groups of
calcitic bivalves. 87 shows two large pink-stained oyster
fragments, each having a f o liated internal structure.
Fragments of oysters may be difficult to distinguish from
brachiopods, although their thick shells with a rath�r
irregular foliated structure arc characteristic. Note also
how the left-hand end of the upper fragment is upturned
and splitting. The rest of the sediment comprises broken­
up bioelasts set in a blue-stained fe rroan calcite cement.
The white areas are holes in the section.
Some bivalves have a thick prismatic layer. the prisms
being elongated at right angles to the shell wall. 88 shows
a f r agment of the common Mesozoic bivalve Inoceramus
(nght). The shell is sectioned more or less parallel to its
length and hence the prisms arc seen in cross-section.
Individual prismatic crystals break away easily from the
�hell and in this example most of the sediment is
composed of these crystals. seen in various sections.
8 9 shows examples of thin bivalve shells known as
filaments. These are the valves of planktonic bivalves and
an: common in Mesozoic pelagic limestones. The micritic
s e d iment between the shells contains small circular areas
of spa rite. These are probably calcite casts of the siliceous
microfossils. radiolaria (p. 82).

87 Swim'd t!tin section. I n/erior Oolite. J\1iddle Jurassic.
l.t•ck/1{/mpton Hill. Gloucevtershire. Hnglwul: 11Wf(n[/ic­
lltion x8. PPL.
88 Swmed thin section, Upper Crellln'ou:., Stmtlwird.
Sht'. Scotland: magnification x 14. PPI ..
89 Stained thin section. Triassic. Greece: magnific­
a/loll x 16. PPL.
Other molluscs are shown in 105. 124, 135. 136. 143, 153,
156 (Ill(/ 159.

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