Page 42 - Atlas of Sedimentary Rocks Under The Microscope

78,79,80 Carbonate rocks
A g g regate g r a i n s a n d

l i t h o c l asts

78 and 79 show aggregate gmim. These are made up of
irregular aggregates of a small number of recognizable
particles cemented together by micrite or fine sparite. 78
shows the botryoidal form typical of these aggregates.
The component particles include ooids (the grain right of
centre) a'> well as peloids and a few bioclasts. These
aggregates arc similar to the grapestones of modern
sedimentary environments, where particles become
cemented on the sea-floor in areas of low sedimentation
ra te. The opaque material in the top centre is bitumen (sec
160).
79 shows large aggregate grains with a smooth. rather
than botryoidal, external f o rm. The micritic material
binding the part1clc together completely envelopes them,
and is more important volumetrically than the cementing
material of the grains shown in 78. It is unlikely that the
aggregation occurred by cementation on the sca-Aoor,
but the particles arc probably reworked grains and thus
could also be described as intraclasts. The matrix is
micrite with a lillie sparitc and some bioclasts.
Uthoclasts or extraclasls are eroded fragments of
lithificd sediment which have been transported and
redeposited. 80 shows lithoclasts which are made up of
ooids and bioclasts cemented by very pale pink-stained
non-ferroan sparry calcite. Note the truncation of both
particles and cement at the lithoclast margins. indicating
reworking of lithified sediment. The equant sparite ce­
ment within the lithoclasts is typical of precipitation from
meteoric waters (p. 55), so these fragments arc or a
limestone which was not cemented in either the original
environment of depo�ition of the component particles in
the lithoclasts, nor in the final environment of deposition
of the cla�ts themselves. They arc. in f a ct. fragments of a
Carboniferous Lime�tonc reworked during the Jurassic.
The final cement is lilac-\tamcd. coarse f c rroan calcite.

78. Umtained thin veuion. Bee Low Limestone. Lo11·er
Carboniferous. Windy Knoll, Derhyshire. England: mag­
nifimtiml x 27, PPL.
79: Swined r!tin section, 011anamane F o rmation. Middle
Jurmsic. Air Chehrid. Wesrem High Atlas, Morocco:
i
1 w g n i{ ication x 14, PP! ..
80: Stained thin section, S11flon Stone. Lower Jurassic,
Oxmore-hy-Sea. South Wales: magnification x 28. P PL.

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