Download Fossilisation - Geology Rocks

Fossilisation
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The specification states that you need to be able to:
Explain the exceptional preservation of body fossils in:
Amber
Tar
The Burgess Shale
Describe and explain the preservation of hard skeletal
tissues:
Replacement
Preservation by alteration of aragonite to calcite
Pyritisation
Carbonisation
Silicification
Cast and mould preservation
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Only a small number of dead organisms become fossilised,
and it is estimated that just one species out of every 5,000
will have had a chance of surviving in fossil form until the
present.
Usually the hardest part of the organism (bone, husk, shell,
etc) is fossilised, though these may be subject to physical
alteration, impact fracture, disintegration or dispersal
caused by atmospheric phenomena.
The hard parts may survive even if subject to such traumas,
but the softer parts of an organism rarely survive because
they are vulnerable to predation and decomposition.
The rapidity with which an organism becomes covered in
sediment is important in fossilisation.
In view of this consideration, it is easy to understand why
most ancient organisms known to us are either marine or
lacustrine; terrestrial organisms can only be preserved if
they die in a marine or fluvial environment.
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1.
Only a tiny fraction of the dead
animals and plants become
fossils.
There need to be special
conditions to allow fossilisation.
In order to be preserved the
following possibilities must occur:
There must be a quite rapid burial
or else the organism will be
weathered, eroded, scavenged and
broken up.
Ideally the environment
should be anaerobic as
soon as possible e.g. a
swamp or deep in the
ocean.
3. The environment should be
low energy i.e. no fast
flowing currents.
2.
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The following make an organism
more likely to be preserved:
The organism has “Hard parts”
which are often preserved whilst the
“soft parts” are usually lost.
The hard parts usually occur as
rigid structures either on the
outside of the animal protecting the
soft parts (e.g. shell) , or within the
body (e.g. skeleton), falling apart
(disarticulating) as the soft parts
decay.
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The soft parts are either eaten
or else decompose before
they can become preserved;
therefore animals consisting
solely of soft parts are rarely
preserved.
Rock Types That Have Fossils
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Fossils are usually found
in sedimentary rocks.
 Only rarely in low-grade
metamorphic rocks and
volcanic igneous rocks.
 Fossils are also mostly
found in fine-grained
sedimentary rocks:
mudstone, shale, siltstone
and limestone.
  Why?
Rock Types That Have Fossils 2
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These are the types of
environments where many
organisms live.
 They will not be crushed
by large clasts.
 They are environments
where there may be rapid
sedimentation and
therefore burying the
organism before it
decomposes.
 May be anaerobic (Deep
Ocean, swamps).
Exceptional Preservation of Body
Fossils
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These include:
Amber
Tar
The Burgess Shale
 Such exceptional preservation
can be called Lagerstätten
 Lagerstätten are deposits that
exhibit extraordinary fossil
richness and completeness.
Lagerstätten
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One example is the Tar Pits of Rancho La
Brea, in Los Angeles, where hundreds of
sabre-toothed cats and many other
recently extinct creatures are fossilized.
Another is the Burgess Shale, in the
Canadian Rockies.
Europe has several, including the
Solnhofen Limestone in which
Archaeopteryx, the famous dinosaurbird, was first found in all its feathered
glory.
And Australia has the extremely old and
strange Precambrian fossils of the
Ediacara Hills.
The Burgess Shale (Homework)
To be handed in by Friday’s lesson next week.
 You need to research the Burgess Shale and
produce a word processed or DTP hand-out or
PowerPoint Presentation that covers the following
points:
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What is the geological significance of the Burgess shale.
When was it deposited.
What kind of organisms were fossilised.
Why were the fossils so well preserved (what were the
conditions) /what was the palaeoenvironment.
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Many shells consist originally of
a mixture of aragonite and
calcite.
Both minerals are CaCO3
polymorphs.
Aragonite is highly unstable and
under higher T and P changes to
calcite which is more stable.
This slow change occurs by
recrystalisation, destroying the
internal structure of the shell
layers but not the overall shape.
As well as converting from one
polymorph to another there can
be a complete substitution of
chemicals:
Replacement/ Mineralisation:
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This involves the replacement of
the hard parts with minerals
dissolved in water present in the
sediment.
The most commonly occurring
ones are calcite, pyrite, silica,
phosphates, etc.
Pyritisation
Carbonisation
Silicification
Pyritisation
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Very simply this is where the
original material such as wood,
shell or bone is replaced molecule
by molecule with Iron Pyrite (FeS).
 Pyrite tends to form in anaerobic
conditions and so organisms that
are deposited in such
environments may be converted to
pyrite during diagenesis.
 Pyritisation is common therefore in
swamps and on the abyssal plain.
 The most common fossils to be
pyritised are wood and graptolites
(which are C rich) and ammonites.
Carbonisation
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This is slightly different.
 C rich organisms like
wood and graptolites are
buried.
 They are compressed
and heated up.
 Volatile organic
molecules are driven off
and a film of carbon
remains as a film on the
bedding plane.
Silicification
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This is where any organism has
its original mineralogy replaced
bit by bit by silicon (Si).
It will be either quartz or flint
(chalcedony/agate).
This often happens where a
fossil is preserved in a Si rich
sediment such as a quartz rich
sandstone or a volcanic ash.
Water permeating through this
sediment is Si rich and will
replace the original minerals in
the fossil.
Cast and Mould Preservation
• Casts and moulds are
types of fossils where the
physical characteristics of
organisms have been
impressed onto rocks.
• This happened when
organisms became buried
or trapped in mud, clay, or
other materials which
hardened around them.
• The bodies decayed,
leaving moulds of the
organism.
• There are two types of
moulds:
• External
• Internal
Cast and Mould Preservation 2
• The external is created
when the bodies dissolve
and leave empty cavities
imprinted with the
external details of the
organisms.
• Internal moulds form
from hollowed structures
when the interior of an
organism is filled with
inorganic material.
• When the shell
deteriorates, an
impression of the interior
is left.
Cast and Mould Preservation 2
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Internal cast is where an
internal mould is filled with
sediment or cement.
 If this is weathered out the
fossil will show the
impression of the internal
structures (e.g. muscle
scars).
 External cast is where an
external mould is filled in
by sediment or cement.
 If this is weathered out the
fossil will show the
impression of the external
features such as ribs and
growth lines.
What are fossils made of?
 As mentioned previously most shells are
made of calcite or aragonite (CaCO3).
 During diagenesis when the original shell
may be dissolved (see casts and moulds) the
void can be filled by calcite or quartz (the
same minerals that make most cements).
 If the mineral is replaced it could be by:
Silicon (silicification)
Pyrite (pyritisation)
Destruction of Fossils
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Once a fossil has formed it is still unlikely that it will
survive until it is found by a geologist at the surface.
 How may it be destroyed:
Acid groundwater may dissolve the fossil during diagenesis.
Metamorphism.
Melting of the rock.
Erosion