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Organic & Chemical Sedimentary Rocks I.G.Kenyon. Organic sedimentary rocks are composed of the remains of once-living organisms, this includes both animal.

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Presentation on theme: "Organic & Chemical Sedimentary Rocks I.G.Kenyon. Organic sedimentary rocks are composed of the remains of once-living organisms, this includes both animal."— Presentation transcript:

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

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

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

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

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

9 Coal A carbon-rich mineral deposit formed from the remains of dead plant matter Most of the coal in Europe formed 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 1cm 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 Roots? Low density-feels very light when held in the hand

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 This is the main type of coal mined in the UK Decomposition of plant material is complete, little evidence of original vegetation structure Used in town gas and coke manufacture Breaks into cuboidal fragments and soils the fingers

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

16 The Composition of different Types of Coal

17 Main UK Coalfields Carboniferous in age ( 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) 1cm Made up of spherical ooliths 0.5 to 1mm in diameter All parts of the rock react with dilute hydrochloric acid Ooliths cemented by calcite cement Uniform texture and composition Can be carved with a chisel in any direction as ooliths are not fused together, slightly friable

21 Shallow water marine deposit in a tropical or sub-tropical environment where evaporation rates are high and there is an abundance of calcium carbonate 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

22 Tufa, Travertine or Dripstone 2cm Re-deposited calcium carbonate, often precipitated from solution in cave systems The lower carbon dioxide levels in the caves render Ca CO 3 less soluble Forms stalactites, stalagmites and pillars in the caves-a form of limestone Banded, internal concentric structure Cross section through a stalactite Reacts with dilute hydrochloric acid Stalactite shows a ridged outer surface

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

24 Micrite – Carbonate Mud 1cm Microscopic CaCO 3 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 Reacts with dilute hydrochloric acid Classifies as a limestone containing over 50% calcium carbonate

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

26 The Bar Theory of Evaporite Formation Subsidence occurs as evaporite deposits build up Arid climate with high rates of evaporation 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 Playa Lake

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 ( Ma) On Teesside significant deposits of Triassic age are found ( Ma) 3cm 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 Rock Gypsum 1cm Rock Salt

29 Evaporites – variety Desert Rose Gypsum 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! 5cm

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 1cm Main iron minerals are chamosite, siderite and limonite

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

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

33 The End


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