Organic & Chemical Sedimentary Rocks

1. Organic & Chemical Sedimentary Rocks I.G.Kenyon

2. Organic sedimentary rocks are composed of the remains of once-living organisms, this includes both animal and plants

3. Chalk – a type of Bio-clastic limestone Very friable and has a high porosity and permeability Comprises over 95% calcium carbonate content Deep sea deposit Reacts violently with dilute hydrochloric acid Fossil belemnite replaced by flint Made up of microscopic marine phytoplankton shells called coccoliths 1cm

4. Chalk A white and very pure form of limestone Made up of microscopic calcite discs called coccoliths High porosity and permeability Forms the White Cliffs of Dover, the back of Lulworth Cove, the stacks Old Harry and His Wife and The Needles off the coast of the Isle of Wight Most of London’s water supply is extracted from the chalk aquifer Electron microscope view of coccoliths

5. Shelly Limestone/Bio-clastic Limestone Cement is calcium carbonate Comprises mainly broken bivalve shells 1cm Shallow water marine environment with high energy conditions such as the inter-tidal or littoral zone Some silty material and iron oxides comprise the matrix The rock reacts with dilute hydrochloric acid

6. Bio-clastic Limestone/Crinoidal Limestone Over 75% of the rock is made up of broken crinoid stems 1cm Organic remains cemented together by calcium carbonate All of the rock reacts with dilute hydrochloric acid

7. Algal Limestone All parts of the rock reacts with dilute hydrochloric acid The structures dome upwards towards the sky Algal mounds known as stromatolites constitute the bulk of this rock. 2cm

8. Reef Limestone/Coral Limestone Coral fossils preserved in life position Tropical or sub-tropical shallow water marine deposit Corals formed the living upper part of a reef complex All of the rock reacts with dilute hydrochloric acid 1cm

9. Coal A carbon-rich mineral deposit formed from the remains of dead plant matter Most of the coal in Europe formed 280-300 Ma during the Carboniferous Period Hot, wet, tropical climates with stagnant anaerobic swamps are the most favourable coal-forming environments Modern day coal forming environments occur in the Everglades of Florida and the Okefenokee Swamp in South Carolina, USA

10. Artist’s impression of coal forming swamps during the Carboniferous Period (360 to 286 Ma) in the UK

11. Coal Approximatey 12 metres of vegetation will produce 1metre of anthracite, the highest grade coal with 90-95% carbon content The vegetative material must eventually be covered by sediment for coal to form With burial and increasing compaction, volatiles such as water and carbon dioxide are expelled, leading to a relative increase in carbon The percentage of carbon is used to identify the rank of coal and its position in the coal series Coal series: Peat-Lignite-Bituminous Coal-Anthracite

12. Peat Roots? Semi-decomposed plant material Original vegetation structure still clearly recognisable Carbon content 50% Burns poorly, gives off a lot of smoke Leaves behind a lot of ash Only burned where other fuels not available Rural areas-Southern Ireland and Northern Scotland Low density-feels very light when held in the hand 1cm

13. Lignite/Brown Coal Carbon content 60-70% Darker brown colour than peat Often has a woody look to it and ‘ring’ when tapped with the fingers Generates much smoke and ash when burned 2cm

14. Bituminous Coal Carbon Content 80-85% results in black colour Breaks into cuboidal fragments and soils the fingers Used in town gas and coke manufacture Decomposition of plant material is complete, little evidence of original vegetation structure This is the main type of coal mined in the UK

15. Anthracite Contains 90-95% carbon Does not soil the fingers when handled Burns slowly with a hot, bright flame, gives off minimal smoke and leaves very little ash Shows a vitreous to metallic lustre and conchoidal fracture 1cm No traces of original vegetation structure evident

16. The Composition of different Types of Coal

17. Main UK Coalfields Carboniferous in age (360-286 Ma) Seams relatively thin 30cm to 2m Affected by the Hercynian Orogeny which resulted in extensive folding and faulting of coal seams (mainly concealed) UK Exposed Coalfields

18. Distribution of Coal Deposits in the United States

19. Chemical Sedimentary Rocks Sedimentary rocks formed by the precipitation of material from solution

20. Oolitic Limestone (Bath Stone) Made up of spherical ooliths 0.5 to 1mm in diameter Ooliths cemented by calcite cement Can be carved with a chisel in any direction as ooliths are not fused together, slightly friable Uniform texture and composition All parts of the rock react with dilute hydrochloric acid 1cm

21. Oolitic Limestone Each oolith has a nucleus of a small sand grain or shell fragment at its centre Concentric shells of calcium carbonate are precipitated around this nucleus to build up the spherical oolith Individual ooliths are surrounded and cemented together by calcite Oolite is forming today in the Persian Gulf and the Bahama Banks 1mm Shallow water marine deposit in a tropical or sub-tropical environment where evaporation rates are high and there is an abundance of calcium carbonate

22. Tufa, Travertine or Dripstone 2cm Banded, internal concentric structure Stalactite shows a ridged outer surface Cross section through a stalactite Reacts with dilute hydrochloric acid 2cm Re-deposited calcium carbonate, often precipitated from solution in cave systems The lower carbon dioxide levels in the caves render Ca CO3 less soluble Forms stalactites, stalagmites and pillars in the caves-a form of limestone

23. Tufa, Travertine or Dripstone Stalactites extending down from the cave roof Stalagmite growing up from the cave floor A pillar connecting the cave roof to the floor 1m Kango Caves, South Africa

24. Micrite – Carbonate Mud Microscopic CaCO3 crystals are precipitated to form a fine white mud Often clastic mud is also incorporated to give a darker colour Forms in warm, shallow and tranquil marine conditions where evaporation rates are very high A typical environment would be a flat, shallow bank where current action is weak 1cm Classifies as a limestone containing over 50% calcium carbonate Reacts with dilute hydrochloric acid

25. Evaporites – material precipitated from Seawater 13% 80% % water needing evaporating for minerals to precipitate K + Mg Salts >95% Halite (Rock Salt) >90% Gypsum (Rock Gypsum) >80% Calcite >60%

26. The Bar Theory of Evaporite Formation Arid climate with high rates of evaporation Playa Lake Subsidence occurs as evaporite deposits build up The lagoon is created by waves crashing over the bar during high spring tides and storms The shallow lake just 1- 2m deep covers a large area and is known as a Playa Lake The water in the lagoon evaporates to precipitate thin beds of evaporites 3 metres of sea water produces just 5cm of evaporite rock Many cycles of replenishment, evaporation and subsidence are needed to form thick beds

27. Playa Lake – The Devil’s Golf Course, Death Valley, California The floor of the playa is covered with irregular shaped salt mounds Saline waters are drawn up to the surface by capillary action here due to high rates of evaporation

28. Rock Salt and Rock Gypsum are the most important Evaporites Extensive deposits of Permian age occur in Cheshire (286-248 Ma) On Teesside significant deposits of Triassic age are found (248-213 Ma) 3cm Rock Gypsum These deposits form the basis of the petro-chemical industry in these areas using crude oil as an additional raw material Detergents, cosmetics, plastics and fertilizers are manufactured from them 1cm Rock Salt

29. Evaporites – variety Desert Rose Gypsum 5cm Sometimes evaporites are precipitated on broad coastal salt flats called sabkhas. This specimen is from Tunisia in North Africa, where locals dig them out of the salt flats to sell to tourists. This one cost just 50 pence in 1986!

30. Ironstone Sandstones or limestones that contain over 15% iron Occur mainly in older rock formations >400Ma Iron was more soluble in the past when the atmosphere had less oxygen content Today most iron released by weathering is oxidised before it can be transported to the sea Ironstones are not forming at the earth’s surface today Uniformitarianism cannot be applied Main iron minerals are chamosite, siderite and limonite 1cm

31. Ironstone ‘Doggers’ on the beach at Hengitsbury Head Nodular lumps of ironstone of middle Jurassic age (188-163 Ma) 1m

32. Chalcedony/Agate – re-precipitated quartz Sometimes occurs as stalactitic and botryoidal forms A variety of quartz that is very finely crystalline (cryptocrystalline) Iron and manganese impurities give rise to distinct colour banding 1cm

33. The End