Page 47 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 90, 91, 92
B i o c l asts

B r achiopods

The articulate brachiopods are important constituents of
Palaeozoic and Mesozoic limestones. They were origin­
ally calcite and so their shell structures are weB-preserved.
Typically, brachiopods have a thick inner layer of calcite
fibres aligned with their length at a low angle to the
shell wall. A thin outer prismatic layer may be preserved.
90 shows a broken brachiopod of which parts of both
valves arc present and surrounded by a micrite envelope
(p. 54). The fibrous structure is clearly visible, as are
tine tubes at right angles to the shell wall. filled with blue­
�taincd fer roan calcite cement. These are endopunctae and
they characterize some groups of brachiopods. The
sample also shows a good example of coarse. blue-stained
f c rroan calcite cement.
91 shows two large fragments of pseudopunctate bra-
chiopods. In these. the fibrous wa11 structure is inter­
rupted. not by open tubes but by calcite rods. The left­
hand fragment shows the pseudopunctae sectioned par­
allel to their length. Note the wavy nature of the fibres
adjacent to the pseudopunctae. The right-hand fragment
is a section of a shell showing the pseudopunctae in cross­
section.
92 illustrates a brachiopod fragment with its outer
prismatic layer preserved. The f o liated nature of the inner
part of the wall is also well shown. The shape of the
fragment suggests that it is part of a ribbed shell. It is also
illlfllllltlllte. lacking either endopunctac or pscudopunc­
tae. These factors in an Upper Jurassic brachiopod
indicate that it is part of a rhynchonellid. The fine-grained
calcite matrix contains abundant colourless fine sand­
and silt-size quartz.

90: Swi11ed tlti11 section. I n f erior Oolite. Middle Jurassic.
l.ec� hampton Hill. Glm1c�srersltire. England: magnific­
ation x 32. PPL.
91: Stained tltin section. Monsal Dale Limestone. Lmrer
Carhon(f' erous. Cressbrook Dale. Derbyshire. England:
lllliKII(/ication x 16. PPL.
92: Stained tltin section. Upper Jurassic. Jehel Amsilf<'ll.
Morocco: magnification x 40. PPL.

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