Page 33 - Sedimentology and Stratigraphy

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Stratigraphy
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Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
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Nichols/Sedimentology
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Terrigenous Clastic Sediments: Gravel, Sand and Mud
Igneous rock fragments individual grains. Analysis of the clay content of a
matrix requires other techniques such as X-ray dif-
Fragments of fine-grained igneous rocks can occur as fraction analysis (2.4.4).
grains in a sandstone, especially in areas of deposition A cement is precipitated out of fluids as part of the
close to volcanic activity. Dark grains in hand speci- post-depositional history of the sediment. It will nor-
mens can be revealed by the microscope to contain
mally be crystalline material that fills, or partly fills,
tiny laths of pale feldspar crystals in a finer ground-
the gaps between the grains. The formation of
mass that appears dark under cross-polars and can be
cements and their varieties are considered in section
recognised as pieces of basalt. Basalt weathers readily,
18.3.1.
breaking down to clays and iron oxides, and these
particles will give a brown, rusty rim to any grains
that have been exposed for any length of time. With 2.3.9 Practical thin-section microscopy
more extensive weathering, fine-grained igneous
rocks will break down to clays (2.4) and the clast
Before putting a thin-section slide on a microscope
will appear brownish, turning dark and speckled
stage, hold it up to the light and look for features
under crossed polars.
such as evidence of lamination, usually seen as
bands of lighter or darker, or larger and smaller
Metamorphic rock fragments grains. The rock might not be uniform in other
ways, with a patchy distribution of grain sizes and
Slates and fine-grained schists may be incorporated types. Such features should be noted and compared to
into sandstones if a metamorphic terrain is eroded. the hand specimen the thin-section has been cut
These rocks have a strong fabric, and break up into from.
platy fragments that can be recognised by their shape It is always best to start by looking at the slide using
as grains. This fabric also gives a pronounced align- low magnification and under plane-polarised light.
ment to the fine crystals that make up the grain, and Lithic fragments and mineral grains can often be
this can be seen both in plane-polarised light and best distinguished from each other at this point, and
under crossed polars. Micas are common metamorphic certain distinctive, coloured minerals such as biotite
minerals (e.g. in schists), so elongate, bright birefrin- and glauconite recognised. Individual grains can then
gence colour specks within the clast may be seen. be selected for investigation, and their mineral or
lithological composition determined using the techni-
ques described above. Once a few different grain types
2.3.8 Matrix and cement have been identified it is usually possible to scan the
rest of the slide to see whether other clasts are more of
The material between the clasts will be one of, or a the same or are different. For each clast type the
mixture of, matrix and cement. A matrix to a sand- following are then recorded:
stone will be silt and/or clay-sized sediment. It can be . optical properties (shape, relief, cleavage, colour,
difficult to determine the mineralogy of individual silt pleochroism, birefringence colours, extinction angle,
particles because of their small size, but they are twinning)
commonly grains of quartz that will appear as black . mineral name
or white specks under crossed polars. Tiny flakes of . size range and mean size
mica or other phyllosilicate minerals may also be . distribution (even, concentrated, associated with
present in this size fraction, and their bright birefrin- another clast type)
gence colours may be recognisable despite the small . estimate of percentage in the thin-section (either as
size of the laths. Clay-sized grains are too small to be a proportion of the clast types present, or a percentage
identified individually with an optical microscope. of the whole rock, including cement and matrix).
Under plane-polarised light patches of clay minerals The nature and proportion of the matrix must also be
forming a matrix usually appear as amorphous determined, and also the character and proportion of
masses of brownish colour. Under crossed polars the any cement that is present. The proportions of differ-
clays turn dark, but often the area of clay material ent clast types and of the cement/matrix then need to
appears very finely speckled as light passes through be estimated which add up to 100% and with this