Page 53 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks 107, 108, 109
B i oclasts

Foraminifera

Foraminifera arc widespread in marine limestones. Most
arc calcite but they show a variety of shapes and wall
structures. A selection of examples showing some of the
variation amongst the fo raminifera, is shown here.
The large!tt and perhaps the best-known fo raminifera
arc the nummulites of the Lower Tertiary, examples of
which arc shown in 107. Note the thick walls which have a
radml fibrou\ structure, the fibres being aligned at right
angle� to the test wall. The matrix is mainly micritic
�cdiment \ \ ith a little blue-stained fcrroan calcite cement.
108 'how., discocyclinids, a type of f o raminifer with
many shall chambers. The matrix is micrite with many
fragmented bioclasts.
109 shows a f o raminiferal limestone in which the
organisms are micrite-walled miliolids. The cement is tine
'paritc although unfilled pore-spaces remain (e.g. centre
of field of view). Partly-tilled moulds of bivalves can be
�ecn outlined by thin micrite envelopes. These arc the
elongate curved grains seen on the right-hand side of the
photograph.

107: Stained thin section, Eocene, San Salvador, Majorca;
magnification x 15. PPL.
108: Stained thin section, Eocene, Greece; magnific­
otion x 115. PPL.
109: Stained thin section, Upper Miocene, Ca/a Pi,
Major('a; magnification x 27, PPL.

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