Organic Chemical Sedimentary Rocks I G Kenyon Organic

Organic & Chemical Sedimentary Rocks I. G. Kenyon

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

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 1 cm

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 Electron microscope view of coccoliths Most of London’s water supply is extracted from the chalk aquifer

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

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

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

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

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

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

Coal Approximatey 12 metres of vegetation will produce 1 metre of anthracite, the highest grade coal with 90 -95% carbon content The vegetative material must eventually covered by sediment for coal to form be 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

Roots? Peat 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 Low density-feels very light when held in the hand 1 cm Only burned where other fuels not available Rural areas-Southern Ireland Northern Scotland

Lignite/Brown Coal Carbon content 70% 60 - 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 2 cm

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

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 1 cm No traces of original vegetation structure evident

The Composition of different Types of Coal

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

Distribution of Coal Deposits in the United States

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

Oolitic Limestone (Bath Stone) Made up of spherical ooliths 0. 5 to 1 mm in diameter 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 All parts of the rock react with dilute hydrochloric acid 1 cm

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 1 mm Oolite is forming today in the Persian Gulf and the Bahama Banks Shallow water marine deposit in a tropical or sub-tropical environment where evaporation rates are high and there is an abundance of calcium carbonate

Tufa, Travertine or Dripstone Banded, internal concentric structure Cross section through a stalactite 2 cm Stalactite shows a ridged outer surface Reacts with dilute hydrochloric acid 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

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 1 m Kango Caves, South Africa

Micrite – Carbonate Mud Microscopic Ca. CO 3 crystals are precipitated to form a fine white mud 1 cm 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 Classifies as a limestone containing over 50% calcium carbonate Reacts with dilute hydrochloric acid

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%

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 - 2 m 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 5 cm of evaporite rock Many cycles of replenishment, evaporation and subsidence are needed to form thick beds

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

Rock Salt and Rock Gypsum are the most important Evaporites Extensive deposits of Permian age occur in Cheshire (286 -248 Ma) 3 cm Rock Gypsum On Teesside significant deposits of Triassic age are found (248 -213 Ma) 1 cm Rock Salt 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

Evaporites – variety Desert Rose Gypsum 5 cm 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!

Ironstone Sandstones or limestones that contain over 15% iron Occur mainly in older rock formations >400 Ma Iron was more soluble in the past when the atmosphere had less oxygen content Main iron minerals are chamosite, siderite and limonite 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 1 cm

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

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 1 cm

The End