SEDIMENTARY ROCKS

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5. SEDIMENTARY ROCKS - examples of types - formation and properties

A sedimentary rock is defined and what sedimentary rock formations look like. The formation of the sedimentary rocks sandstone, limestone, chalk, shale, mudstone, coal, salt deposits. The formation of fossils is described and why they mainly occur in sedimentary rocks and their usefulness to geological scientists studying the age of rocks and to biologists and zoologists studying the evolution of plant and animal species.

5. Sedimentary Rocks slowest to form, and weather the fastest!

5(a) A sedimentary rock bed is formed from plant/animal remains or weathered and eroded particles from pre-existing rocks. The material is deposited in layers that will eventually form the sedimentary rock. These may be transported, usually by water (or wind in the case of sand) and deposited to form sediments. These become buried under later forming sediments and water or by major tectonic activity, and then become subjected to compression as enormous pressures are created deep in the crust from the weight of rocks or sediments above them. Over millions of years from the fragments of eroded pre-existing rocks, water is squeezed out and the particles cement together with the help of dissolved salts and silica crystallising out.  Other changes come about depending on the type of material from which the sedimentary rock is formed.

 5(b) Types of sedimentary rock

• Shale and Mudstone is formed from relatively fine grained weathered rock material transported into seas and lakes before settling out as clay or  mud sediment. It then becomes compressed under the weight of water and other sediments and the water is squeezed out and the particles cement together. These rocks are clearly layered and crumble easily. Shale can contain significant amounts of oil-like organic material.
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• Above is the pebble beach at Charmouth in southern England and the Jurassic fossil bearing shale cliffs in the distance (180-195 million years old). Fossils found include ammonites, nautilus, belemnites, crinoids, shells and bones etc. Shale cliffs are structurally weak and collapses of the cliff faces are regular, but they often reveal lots of good fossil specimens!
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• Limestone (above) is formed from the deposition of hard mineral remains of sea creatures and chemically is mainly calcium carbonate CaCO₃. This sedimentary rock mineral contains the 'shelly' remains of marine organisms, including coral, that once lived in warm shallow fertile seas. Limestone is grey-white in colour and contains fossils and sand grains. The 'shelly' remains, including coral, get buried and compressed and cemented together by the weight of water and other sediments. Limestone tends to form beneath warm shallow seas rich in plant and animal life.
  • The picture above shows Gordale Scar in the Yorkshire Dales in the Pennines of the North of England. The scar was gouged out of the easily eroded limestone rock. Because limestone has not been subjected to high temperatures it often contains many good fossils. Limestone is relatively easily weathered and dissolves in acid rain so after a few thousand years deep valleys are formed like the one illustrated in the photograph. Medieval limestone buildings have suffered grievously at the hands of acid rain from the fossil fuel combustion smoke of the industrial revolution.
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• Chalk cliffs (above) are formed from the mineral remains of tiny marine organisms in the sea and is chemically relatively pure calcium carbonate and it contains microscopic fossils readily seen under a microscope. The picture above shows the chalk White Cliffs of Dover. Like limestone, chalk is quite easily eroded over thousands of years by wind rain, and waves if the cliffs are by the sea.
• Sandstone is formed from weathered particles of igneous rock and these particles mainly consist of colourless silica (silicon dioxide, SiO₂).
  • The rock particles are laid down in lakes, estuaries or seas from water transportation or wind blown to form sand dunes.
  • The layers of sand get buried and compressed and the particles get cemented together by other minerals including iron oxides which give sandstone its distinctive orange or red colour.
• Coal is formed from the decayed (without oxygen) remains of plant materials e.g. giant ferns and trees from hot swampy forests.
  • The organic materials are buried, heated and compressed and form clear sedimentary layers often showing well preserved fossils of leaves or tree trunks.
  • You can think of coal as fossilised layers of plant material with high percentage of carbon content formed under high temperature and high pressure conditions.
  • The deeper and older the layer, the more carbonised is the coal (anthracite is almost completely the element carbon).
• Salt deposits : These are formed from the evaporation of ancient seas or lakes leaving huge salt deposits which become buried and compressed underground by later sedimentation above them e.g.
  • Rock salt is mainly sodium chloride and occurs in large deposits a few hundred metres underground in the UK (e.g. Cheshire and Teesside Co. Durham) and other countries like Germany.
    • It can be mined as a solid or extracted as a concentrated solution.
    • It is used for food preparation, de-icing  roads or to make chlorine etc. via the process of electrolysis.
  • See the Group 7 Halogens and Use of Salt
  • Potash contains potassium chloride, sodium chloride and magnesium sulphate and used in fertiliser manufacture.
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5(c) Since limestone is mainly calcium carbonate CaCO₃, and a simple test is to add acid - should giving fizzing of a colourless gas that turns limewater 'milky' i.e. carbon dioxide CO₂ is formed. Heating limestone to a high temperature in a limekiln produces calcium oxide (quicklime, a strong alkali). Lime is used in agriculture to treat fields which are too acidic for healthy crop growth. Limestone is used as building stone and in the manufacture of glass and concrete.

5(d) Any rocks which are not eroded away, are eventually returned to the mantle when plates descend in tectonic activity - see later. 

5(e) A potted history of fossils ...

• Fossils are formed by plants and animals becoming trapped in deposits or sediments and then buried by other layers and compressed as the sedimentary rock forms.
  • This is why the vast majority of fossils are found in sedimentary rock layers.
  • Fossils are the remains of or 'imprints' of dead organisms (plants or animals).
  • In most cases the original organic material is replaced by other minerals but this leaves the trace and structure of the original plant or animal.
  • Fossils can give information on the age of the sedimentary rock they were formed in and the ambient conditions under which the rock was formed e.g. was it formed under water with 'tell-tail' fossil fish, shells.
  • Even ripples in the water from rivers or seas can be detected if the sand becomes covered with other material and then the layers harden.
  • The sediment that forms rocks like sandstone will be formed from wind-blown sand grains (forming surface layers initially, like great sand dunes) or water-borne sand grains (underwater deposition) will form sandstone underwater.
    • By examining the shapes of sand grains in the sandstone you can determine whether it was formed under water or out of water.
• In undisturbed sedimentary layers the lower the layer the older the layer, so the geological sequence of formation can be worked out.
• Why are fossils so useful as well as interesting to scientific studies?
  • Fossils are useful to geological scientists studying the age of rocks and to biologists and zoologists studying the evolution of plant and animal species.
  • Whole sequences of fossils can tell you about the evolution of the same species, new species, extinctions etc.
• Fossils allow us to date the age of the rocks from the species present and also the sort of 'environment' present at the time of fossil formation e.g. the climate and the nature of the land. The older the fossil, the older the rock!
  • Note: Fossil dating is NOT absolute and accurate dating can only be obtained from radioisotope studies.
• The fossil record provides powerful evidence for species evolution as the development of individual species can be followed and their divergence into other later species.
• Fossils 'emerge' when the sedimentary rocks in which they lie in are eroded away. The original harder parts of the organism tend to be better preserved e.g. shell, bone, coral or bark etc. They then require careful extraction from the surrounding rock or mud material.

5(f)  Why are there no fossils in igneous rocks?

• You would not expect fossils in igneous rocks because they are formed from molten mixed up magma.
  • Any organism, plant or animal falling into magma, would be totally destroyed.
  • Even if a pre-existing sedimentary rock had fossils in it, they would be destroyed if the rock was re-melted e.g. in a subduction zone - see plate tectonics later.
• Fossils are rare in metamorphic rock but their trace can sometimes be preserved in e.g. slate, despite the effects of heat and pressure involved in their formation (see 6.).
  • It is not impossible for the 'traces' of fossils in sedimentary rock to be preserved through the re-crystallisation process in metamorphic rock formation.
  • However the fossils are likely to be distorted or destroyed by the heat and pressure factors involved in metamorphic rock formation.
• Why are fossils so useful as well as interesting to scientific studies?
  • The formation of fossils is described and why they mainly occur in sedimentary rocks and useful to geological scientists studying the age of rocks and to biologists and zoologists studying the evolution of plant and animal species.

5(g) At the surface of the Earth younger sedimentary rocks usually lie on the top of older rocks. All sorts of features found in sedimentary rock formations allow scientists to work out their origin and what has happened to them over long time periods of time see Fig 9.2/Fig 7. e.g.

1. order of layers - the deeper the layer, the earlier the sedimentary rock was formed
2. discontinuous deposition where different layers of different rocks are successively laid down at different times.
3. a more recent (younger) rock layer might cut across an older layer.
4. ripple marks can show the layer was formed from a sea-bed or river bank from waves or currents.
5. tilting of rock formations can show very large scale movement and the angle can be followed over a large distance to show the relationship between distant rock formations.
6. folding shows the compression of layers due to plate movement, a curve down is called a syncline, a curve in an arc upwards is called an anticline.
7. fractures and fault lines provide evidence of earthquake activity.
8. inverted layers (turned upside down!) provide evidence of massive plate movement and give geologists much food for thought on deducing the 'event sequence'!
9. rock layers can be buried by these massive upheavals as well as burial by subsequent sedimentary rock formation.
10. Points 5. to 9. are evidence for the crust being unstable and subjected to tremendous forces (see Fig 9.1/Fig 2).

Fig 9.2/Fig 7 (above) and Fig 9.1/Fig 2 (below)

• NOTES Cue, QUIZ and WORKSHEET QUESTION LINKS

• INDEX OF EARTH SCIENCE PAGES

• foundation-easier multiple choice quiz on Earth Science

• higher-harder multiple choice quiz on Earth Science

•  Five Earth Science word-fill worksheets Q1 * Q2 * Q3 * Q4 * Q5

• Answers to the five GCSE Earth Science worksheets listed above