organic chemical sedimentary_rockssmall
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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 and plants
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
ChalkA 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 aquiferElectron microscope view of coccoliths
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
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
Algal Limestone
2cm
Algal mounds known as stromatolites constitute the bulk of this rock.
All parts of the rock reacts with dilute hydrochloric acidThe structures dome
upwards towards the sky
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
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
CoalApproximatey 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
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
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
Bituminous CoalCarbon 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
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 evident1cm
Burns slowly with a hot, bright flame, gives off minimal smoke and leaves very little ash
The Composition of different Types of Coal
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
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)
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
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
Tufa, Travertine or Dripstone
2cm
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
Banded, internal concentric structure
Cross section through a stalactite
Reacts with dilute hydrochloric acid
Stalactite shows a ridged outer surface
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
Micrite – Carbonate Mud
1cm
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 weakReacts with dilute hydrochloric acid
Classifies as a limestone containing over 50% calcium carbonate
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%
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
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)
On Teesside significant deposits of Triassic age are found (248-213 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
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
IronstoneSandstones 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
Ironstone ‘Doggers’ on the beach at Hengitsbury Head
1m
Nodular lumps of ironstone of middle Jurassic age (188-163 Ma)
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
The End