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Sedimentary Rocks
When you find a sedimentary rock, it is important to able to deduce the
environment in which it formed. This is done by putting together all of the
available evidence.
1. Firstly the texture and composition of the rock gives a good indication how
the rock has formed.
2. Secondly the presence of fossils will indicate a particular past environment
and the age of the rock.
3. Thirdly the presence of sedimentary structures will show the processes
which contributed to the formation of the rock.
4. Finally the way in which the rock relates to the rocks above and below it in
the sequence is a very important consideration.
Sediment => Rock
Soft sediment is converted to hard rock by a collection of processes which occur
after the sediment has been deposited. Together these processes constitute
diagenesis.
Sedimentary rocks can be grouped according to the materials they are made
from. Clastic rocks are made of fragments (clasts) produced by weathering and
erosion. Other sedimentary rocks are produced by chemical or biological
processes so they are called biochemical rocks.
Diagenesis
The first process in diagenesis is compaction. The weight of more sediment
deposited on top pushes the clasts closer together. The water between the
grains is forced out (dewatering) and new minerals are precipitated in the pores,
this is called cementation. Diagenesis affects the porosity and permeability of
the sedimentary rock.
(a) Explain the difference between porosity and permeability.
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Sorting and maturity
A sedimentary rock can be described in terms of the mixture of grain sizes it
contains. If the rock is made of grains which are all of similar sizes the rock is
said to be well sorted (mature or equigranular). An immature rock has a mixture
of grain sizes, it is said to be poorly sorted (immature).
(b) Explain how sorting is related to the porosity of the rock.
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Clastic sedimentary rocks
Coarse grained sedimentary rocks have grains > 2mm in diameter. There are
two important types; breccia and conglomerate. A conglomerate has clasts
which are well rounded but a breccia has angular clasts.
You often find a fine grained, muddy material between the clasts, this is called
the matrix.
(c) How does the shape of the grains allow us to interpret the transport
histories of the clasts.
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(d) Suggest environments in which conglomerate and breccia may form.
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The medium grained rocks have clasts 0.5 mm – 2 mm in diameter, generally we
call these sandstones because this is the size of sand grains. If the sand grains
are composed of the same mineral – quartz, the sandstone is called an
orthoquartzite. Some sandstones have a high proportion of feldspar with the
quartz, this type of sandstone is called an arkose.
(e) What does the presence of feldspar tell us about the environment in
which the rock formed?
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(f) In what other environments can sand accumulate?
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.(g) Why are desert sandstones crumbly?
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Sandstones with a high proportion of clay / silt are called greywackes
(h) Using diagrams, explain how a greywacke can form from a turbidity
current?
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Fine grained sedimentary rocks have grains whose diameters are < 0.5 mm.
These rocks are generally made of clay or silt. A shale is a fine grained
sedimentary rock made of clay minerals. The clay minerals are shaped like flat
plates. They are all deposited approximately parallel and this is why shale can
split into thin layers (fissile).
Biochemical sedimentary rocks
The biochemical rocks are not made of clasts. Some rocks such as rock salt
form by the evaporation of salty water in hot environments. These rocks are
called evaporites. Another example of an evaporite rock is gypsum, the raw
material for plaster.
(i) Describe the environments in which evaporites may form.
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Limestones are rocks made of one main mineral – calcium carbonate.
(j) How could you identify this type of rock by a chemical test?
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Organisms have an important role in producing the calcium carbonate by
extracting it from sea water. This usually happens in clear, warm sea conditions
so limestones usually form in tropical conditions. Some rocks are made almost
entirely of the fragments of shelled animals, these are called bioclastic
limestones. Animals like corals build skeletons of calcium carbonate on the sea
floor which may build up to form a reef. The reef protects part of the sea – a
lagoon, shallow and quiet. In the lagoon, calcium carbonate mud accumulates
and tiny “balls” of this material are created (these are called ooids). This type of
limestone is called oolitic. Chalk is a very pure form of limestone, composed of
the remains of algae.
(k) How does the internal structure of the ooids allow relate to the way in
which they formed?
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Another way in which biological processes lead to the formation of sedimentary
rocks is the formation of coal. Coal forms from dead plant material accumulating
in swamps. Due to the low oxygen environment, relatively little decay occurs and
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much of the plant material, is converted by the action of heat and pressure, into
first peat and then coal.
Sedimentary structures
Sedimentary structures are very important to us in our understanding of the way
in which the rock formed.
Bedding is the most basic sedimentary structure (also called strata). Each layer
of sediment is called a bed and represents one depositional event. A break in
deposition leads to break between the layers called a bedding plane.
Flowing water or air can transport sand grains. These will form ripples (small
scale) or dunes (large scale). The position of the front of the ripple or dune can
be preserved in the sediment as the ripple moves forward. This is called crossbedding.
(l) Sketch a ripple and show how the symmetry of the ripple is related to
the direction of the current. (See Geoscience page 94)
Graded beds form as the current slows – the largest fragments are deposited
first, followed by the smaller fragments in sequence. This sedimentary structure
is typical in turbidites – the rocks formed from a turbidity current.
(m) Sketch the appearance of a graded bed. (See Geoscience photo C5.1).
Desiccation cracks are formed in muddy sediment. As the wet mud is exposed
to the sun it will dry out (desiccate), cracking as it does so. These cracks are
polygonal and taper vertically downwards.
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(n) Sketch the appearance of desiccation cracks. (see Geoscience page
92).
Sole structures are sedimentary structures found on the base of a bed. There
are several types of sole structures including flute casts.
(o) Sketch the formation of flute casts.
ESTA GEOTREX The Geology Teachers Resource Exchange
Contributor: Owain Thomas Establishment: Amman Valley School Date: 22:04:05
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