non clastic rocks

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The specification states
that you need to be able to
”Identify, describe and
explain the origin of nonclastic sedimentary rocks
using observation of
colour, fossil content,
mineral composition and
texture; ironstone,
evaporites (gypsum,
halite), limestones
(micritic, oolitic,
fossiliferous, chalk), coals
(lignite, bituminous and
anthracite).”


Limestones
This is the largest non-clastic group and they consist
mostly of carbonate minerals:
 1)    Calcite
CaCO3
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2)    Dolomite Ca,Mg (CO3)2

3)    Plus organic remains preserved as carbonate
skeletons.

 Originally these minerals form as carbonate mud
which then slowly turns into solid rock via diagenesis.
 During this process some of the original physical and
chemical properties can change slightly and frequently
calcite can change to dolomite.
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In the geological past
shallow seas were
widespread and limestone
could be deposited over
1000's km2.
 Organisms with carbonate
skeletons occur throughout
the world, so in theory
carbonate sediments can be
deposited anywhere e.g.
seas and oceans.
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However, they do not
occur everywhere, there
are several factors that
influence and therefore
control the deposition of
carbonates:
 T of water
 Salinity

 Water depth
 Amount of silica input
 Limestones tend to form in
warm seas.
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These conditions are proved by
the presence of index fossils
such as corals.
Therefore most limestones form
in tropical/sub-tropical belts 0 30° north or south of the
equator.
Most limestones formed since
the Cambrian have formed in
these latitudes.
The limestone forming
organisms are also affected by
salinity and depth of the water
and therefore tend to live at
depths up to 200 m (where
sunlight can penetrate).

If the sea is too saline then animals do not survive
as well so limestones tend to occur in normal
salinity ranges.
 This depth allows algae to photosynthesise and
animals to thrive in continental shelf areas in
particular.
 Occasionally carbonate deposits can be found in
environments deeper than 200 m e.g. abyssal
plains.
 However, the organisms forming these deposits
would not have lived there.
 They were floating or swimming organisms which
once dead sank into the deeper water and slowly
accumulated as oozes (very fine carbonate
sediments).
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Carbonate Compensation Depth
Limestones cannot form below 3 - 5 km depth
"Carbonate Compensation Depth" because at that
depth the P is so great that carbonates are redissolved.
 Carbonates will also not form if there is a large influx
of silica material or debris from the land.
 This affects the survival and growth of limestone
forming organisms and inhibits the growth of the
grasses that trap and fix the carbonate mud in place.
 Therefore limestones form either at a distance from

land or else close to land but not undergoing a lot of
erosion (low lying land masses).
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A wide range of
different types of
limestone exist:
a) Chemical
b) Detrital (minor)
c) Bioclastic
d) Biological
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Micrite and Sparite
A matrix can often be
present especially in
clastic or biological
limestones, helping to
hold the fragments
together.
 The matrix can be:
a) A mud that the
clasts/fossils fell into
(micrite).
b) A later infilling cement
(sparite).
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CHEMICAL FORMS OF
LIMESTONE:
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Limestone/Micrite:
This is mostly
precipitated CaCO3.
As a sediment it
resembles a pale very
fine mud.
Once it is compressed
into a rock it is very
fine grained,
grey/cream in colour.


Oolitic limestone/Oolite:
They have a very obvious
texture of almost
perfectly spherical grains.
 Individual grains are
called ooliths and are
rounded and spherical.
 They form in shallow

water marine conditions
usually in the tidal zone.
 Dissolved CaCO3 in sea
water gradually
precipitates around a
nucleus: shell or clast.
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Oolitic limestone/Oolite 2:
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As the tides roll the grains
backwards and forwards they
get an even distribution of
CaCO3 around the nucleus.

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Eventually it forms a series of
concentric layers (oolith).
The warmer the sea water the
more CaCO3 can be dissolved.
The better the chance of
ooliths forming.
When the grains reach a
certain size they become too
heavy to move and therefore
they stay in place eventually
they are cemented together.

These rocks are very well
sorted.

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Chalk:
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This is a very fine grained
limestone with a micrite
texture.
No grains can be seen and it is
even difficult with a hand lens.
They are rich in calcite usually
> 90% and sometimes as high
as 99%.
As a sediment it was a
calcareous ooze consisting of
the skeletons of planktonic
organisms:

Chalk
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Coccoliths calcareous algae
Foraminifera micro-organisms
As they died they settled out
from the water and as they
were pelagic (free swimming or
floating) creatures.
They settle out in a quiet water
environment and over time
large accumulations of these
skeletons can accumulate.
It is thought that chalk formed
in a shelf environment well
away from land where there
were no currents to disturb the
ooze/mud.


Shelly limestone:
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Broken shell
fragments from
various types of
fossils = death
assemblage
indicates a high
energy environment,
typically just below
the tidal zone.


Reef or coral limestone:
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Reefs can form from coral
or algae usually in warm
shallow clean water.
They are usually located
near to the continent.
Reef limestones are very
useful in stratigraphy
because they indicate the
edges of the continental
shelf and are therefore a
useful indicator of a past
environment = index

fossils.







Evaporite Deposits (playa
lakes)
The flash floods running
down wadis often run into
temporary (ephemeral)
lakes in the desert.
 These are called playa lakes
(salt lakes).
 The water that goes into the
lake has many ions in
solution.
 From where?
 Chemical weathering.
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Evaporite Deposits (playa lakes)
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The great heat will make the water in the
lake do what?
Evaporate.
The evaporated water is pure H2O.
This leaves behind the ions in solution
which will be deposited as minerals.
As the remaining water gets more
concentrated less soluble minerals will
form.
There is a sequence of evaporite
minerals.
gypsum begins to precipitate when the
volume is reduced to 30%
halite after reduced to 10%
and Mg and K salts after 5%


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(1)
(2)
(3)

Restriction of exchange with the
open ocean, in a semi-enclosed
basin, is necessary to drive the
salt content high enough for
precipitation to begin.
Sea water comes in (but cannot
escape) and evaporates and so
the water gets denser and sinks.
More water comes in but again
evaporates and sinks driving up
the salt concentration.
Such restricted bodies of water
are:
coastal lagoons;
salt seas on the shelves
early rift oceans in the deep
sea.

Restricted
Basins