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 Comprises over 95%
a high porosity and calcium carbonate content
permeability
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
Electron microscope the chalk aquifer
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
The rock reacts with and iron oxides
dilute hydrochloric acid comprise the matrix

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
The structures dome hydrochloric acid
upwards towards the
sky
Algal mounds known as stromatolites
constitute the bulk of this rock.
2cm

8. Reef Limestone/Coral Limestone
Coral fossils preserved
Tropical or sub-
in life position
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
Low density-feels very other fuels not available
light when held in the
hand Rural areas-Southern
1cm
Ireland and Northern
Scotland

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

15. Anthracite
Contains
Does not soil
90-95% carbon
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
(mainly co
nc ealed) extensive folding
and faulting of
UK Exposed Coalfields coal seams

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
Uniform texture ooliths are not fused
and composition together, slightly friable
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
1mm
together by calcite
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

22. Tufa, Travertine or Dripstone
2cm
Banded, internal
concentric structure Stalactite shows a
ridged outer surface
Cross section
through a stalactite
Reacts with dilute
2cm hydrochloric acid
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
1cm
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 Classifies as a limestone containing
over 50% calcium carbonate
A typical environment
would be a flat, shallow
bank where current Reacts with dilute hydrochloric acid
action is weak

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

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
Main iron minerals are
chamosite, siderite and limonite Ironstones are not forming
at the earth’s surface today
Uniformitarianism
cannot be applied
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