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