Page 32 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks 56, 57,58
L i t h a r e n i t es

Litharcnitcs arc sandstones with less than 95% quart?
and more rock fragments than f e ldspar. They may be
classified according to whether the rock fragments arc
predominantly sedimentary, volcanic or metamorphic
(Fig. B p. 24).
56 and 57 show a sedarcnitc, in which the fragments arc
from carbonate rocks. The fine-grained fragment just
abo\'C the centre is from dolomite rock. Examples of
hme�tone fragmentl> can be seen in the lower r ight-hand
quadrant. The sediment also contains monocrystallinc
quart/ and echinoderm plates. The latter are the speckled
grains with uniform interference colours (see p. 44). In
this example the echinoderms are reworked from an older
limestone and are not fragments of f o ssils living at the
time of linal deposition of the sediment. Thus they arc
classified as sedimentary rock fragments rather than as
fo s,il material.
58 and 59 show a mineralogically immature sediment
consisting mainly of igneous rock fragments cemented by
pale brown ch loritc. The clca r areas in the view taken with
PPL show high-order interference colours under crossed
polars and arc carbonate. A variety of grain types is
present and all show some signs of alteration. Many of the
rock Cragmenls contain partially-altered phenocrysts of
plagioclase in a groundmass of plagioclase laths and
another mineral too fine-grained to determine, but which
may he chlorite. Individual plagioclase crystals arc also
present and vary from euhedral laths to subhcdral
grains. The porphyritic texture of the igneous rocks
suggests a volcanic source rock and thus the sediment is a
volcanic arenite. Such an immature sediment would be
very clo\c to its source rock and it is, perhaps, a locally­
reworked pyroclastic rock.
60 and 61 sho'' a sediment which is more than 75%
quart/. The remaining grains are rock fragments and
hence the sediment may be classified as a sublitharcnitc.
The rock fragments arc of fine-grained sedimentary and
metasedimentary rocks.

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