Page 52 - Atlas of Sedimentary Rocks Under The Microscope

104, 105, 106 Carbonate rocks
B i o c l a sts

Arthropods

Ostracods

These photographs show examples of the arthropod
microfossils, the ostracods, which are widespread parti­
cularly in sediments deposited in brackish or hypersaline
conditions. Ostracods have thin valves with a finely
prismatic or granular microstructure.
104 shows a group of complete two-valved shells, some
filled with sparite cement, some with micritic sediment
and some with both. Note the overlap of valves seen in
some sections - a characteristic f e ature of many
ostracods.
105 shows disarticulated ostracod valves (thin curved
shells) associated with longer straight lengths of shell,
which arc fragments of a calcitic non-marine bivalve.
lOti: Swi11ed thi11 sectio11, Red H i l l Oolite, Lower Carbo­
niferous, Elliscales Quarry, Dalto11-i11-Fumess, Cumbria,
England: mag11ijication x 40, PPL.
105: U11stai11ed thin section, Upper Carbo11i{e rous. Cob­
r i d g e Brickll'orks. Hanley. S t a ff ordrhire. E11gla11d; mag­
nifu·ation x 16, PPL.
Ostracods are shown also i11 1 1 7, 1 1 9 and 136.

Trilobites

Trilobite hard parts were originally calcite and a finely
granular microstructure is preserved. Each crystal is in a
similar but not identical orientation to its neighbours.
leading to sweeping extinction when the sample is rotated
with the polars crossed (not illustrated here).
106 shows a cross-section of a trilobite (centre) and part
of a brachiopod shell (base). Note the hooked shape seen
at the left-hand end of the trilobite fragment , produced by
incurving of the skeleton at its margin. A vein of blue­
stained fcrroan calcite follows the edge of the skeleton
along part of its length. Note that the trilobite is stained
mauve and hence consists of slightly fcrroan calcite. This
contrasts with the brachiopod fragment which is non­
fe rroan calcite. In some rocks it is thought that bioclasts
originally comprising high magnesium calcite may be
replaced by f e rroan calcite. whereas tho�c of low mag­
nesiUm calcite remain unaffected.

106: Srained thin section, We11lock Limestone, Silurian.
Engla11d; magnification x 21, PPL.

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