Page 49 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 96,97,98

B i o c l asts
Echinoderms

F.chinodcrms. particularly echinoids and crinoids, arc
major contributors to the allochcmical fraction of mar . inc
limestones. They arc easy to identify because they break
down into plates which, although they may exhibit a wide
variety of shapes, arc single calcite crystals with uniform
extinction. They usually have a speckled or dusty ap­
pearance as the result of infilling of the fine pores which
permeate the plates.
96 and 97 show a crinoidal limestone in which the
�ed•mcnt is 75% crinoids. ote the speckled appearance
of the plates. most of which have uniform interference
colours and arc thus single crystals. although the ossicle in
the upper left comprises two crystals. one showing a
greenish colour and one a red colour under crossed
polars. The clear spar ·surrounding some of the crinoid
fragments is a cement. The XPL photograph shows that
the interference colour of this cement is the same as the
adjacent crinoid fragment. Hence it is probable that the
cement is in optical continuity with the crinoid. Such
cements arc common in echinoderm-bearing sediments
and arc called S)'lllaxial rim cements (p. 57). The remain­
der or the sample comprises micritic sediment and
fragments of fenestrate bryozoans (e.g. lower right hand
corner).
Echinoid spines arc widespread. particularly in
Meso;.oic and Cenozoic limestones. 98 shows one
complete transverse section of a spine (lower right of
field). together with a smaller broken fragment. Echinoid
spines arc circular or elliptical in cross-section and show a
vanety of radial structures. Like other echinoderm frag­
ments. they arc single crystals.

96 all(/ 97: Stained rhin section. Eyam Limestone. Lower
Carhon{/'erous. Once-a-u·eek Quarry. Derhysltire.
F:ng/wul: magnification x 13: 96 PPL. 97 X PL.
98: Srained tltin section. Quaremary. C a p Rltir. Morocco;
nwgni/imtion x 31. P PL.
f:'cllinoderm .fi'(lf4/III!IIIS are also shown in 73, 76. 78. 132.
133. 139. 148. 154. 178. 183 and 184.

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