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
1cm
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
1cm
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
1cm
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.
2cm
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
1cm
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 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
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
1cm
Only burned where
other fuels not available
Rural areas-Southern
Ireland and Northern
Scotland
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 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
1cm
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 30cm to 2m
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 1mm 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
1cm
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
1mm
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
2cm
2cm
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 CO3 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
1m
Kango Caves, South Africa
Micrite – Carbonate Mud
Microscopic CaCO3
crystals are precipitated
to form a fine white mud
1cm
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- 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 – 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)
3cm
Rock Gypsum
On Teesside significant
deposits of Triassic age
are found (248-213 Ma)
1cm
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
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!
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
Ironstone ‘Doggers’ on the beach at Hengitsbury Head
Nodular lumps of ironstone of
middle Jurassic age (188-163 Ma)
1m
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
The End