Geoscience Reference
In-Depth Information
Table 6.1
Continued
Sedimentary
rock type
Depositional features and
sedimentary structures
Post-depositional features
and sedimentary
structures
Commonly associated
features
As carbonate (non-organically
bound above)
Fossils
: by defi nition these
are
in situ
and can
provide much
information about the
environment and
palaeoecology
Carbonates
(organically
bound)
Sedimentary structures
: often
massive with distinct vertical
relief. Usually closely
associated with horizontally
bedded units representing
the lagoon and steeply
inclined chaotic beds
representing the bioherm
debris
6
Conglomerates
and breccias
Clast types
: identify the rock
types of the clasts, which
gives a good indication of
the source and sometimes
the depositional processes
Sedimentary structures
:
cross-stratifi cation, bedding,
etc. (see Section 6.2.2)
Compaction of matrix around
clasts and cementation
Fossils
: only the robust
large megafossils such
as vertebrate remains
and large pieces of
fossilized wood tend to
survive intact
Coal
Colour and shininess
: indicates
type of organic matter and/
or clay content (see also
Appendix A6, Table A6.3)
Check for an underlying
palaeosol that would
indicate the coal is
in situ
and not derived. The coal
itself may be interbedded
with other sedimentary
deposits
Pyrite
: associated with
high sulfur content
Carbonate nodules in
coals can preserve plant
remains
Evaporites
Note distribution within
sedimentary deposits and
crystal shape. Large crystals
and fi brous ones are
recrystallized
Dissolution and reprecipitation
common. Complete
dissolution is quite common
Faults and folds due to the
ductile nature of
evaporites and their
high solubility
Associated with shallow-
water deposits,
particularly carbonates
Ironstones,
cherts and
phosphates
Colour
: can refl ect
compositional variation and
in the case of phosphates
the amount of reworking
(pink/buff when a
precipitate, grey/brown and
black when reworked)
Banding
: these deposits are
susceptible to dissolution
and reprecipitation so the
banding may be entirely
post-depositional
Phosphates are indicative
of high productivity
areas. The silica for the
chert may come from
diatoms, radiolaria or
sponge spicules. These
indicate a wide range
of depositional
conditions so
determining the origin
of the silica is important
