Show How A Stratigraphical Sequence Can Be Deduced. How Can Fossils Be Used To Tell The Relative Age Essay, Research Paper
?Stratigraphy is the key to
understanding the Earths crust and it?s materials, structure and past life.? ??????????? ?Within geology the study of time is
the study of Stratigraphy.? ??????????? The earth?s crust consists of bodies of rocks that can be
divided into two groups: layered and unlayered. Layered rock bodies are
described as stratified and unlayered are described as massive. The most common
example of stratified rocks are sedimentary rocks. These have been built up by
layer upon layer of sediments, some of which will be vastly similar and in some
cases will have changed in character rapidly. ??????????? Three basic
principles must be recognised before a stratified rock sequence can be
analysed. ??????????? Firstly we
must accept superposition that states that when a layer of rock was forming the
layer beneath it was older. Secondly we must assume originalhorizontality,the idea that layers of rock were originally deposited horizontally and
finally original lateralcontinuity. This means the layers of rock
extend laterally until physically constrained in some way; this may be a
shoreline or an upstanding relief feature. ??????????? These
principles allow a rock sequence to be seen as a record of geological events
over time, with the oldest rocks representing the most ancient events at the
bottom of the sequence. ??????????? Stratified
rock bodies and indeed rock bodies must not however be seen as static. Tectonic
activity may have influence any area at one period during geological history,
and a stratified rock sequence may have a different orientation to that of its
formation period. Different methods can be used to establish it original way
up. ??????????? Sedimentological
evidence can be used. During the formation of marine sedimentary rocks,
sediments would be deposited, with the coarser sediment particles being
deposited first. A single stratum will represent this period of sediment
deposition, and the way up of the whole sequence can then be determined through
comparison of sediment grain size within the stratum. ??????????? Igneous
rocks can also provide evidence for way up analysis. In a lava flow trapped
gasses in the form of bubbles will rise vertically. This results in a
concentration of bubbles in the upper layers of a lava flow, which are held in
that arrangement as the lava solidifies. ??????????? There is
paleontological evidence. Corals grow outwards from a point while maintaining
flat bases in contact with the seabed. When fossilised these provide clear way
up indicators. Marine organisms burrow down vertically and tree stumps and
other vegetation may be fossilised also to leave good indicators. ??????????? ??????????? Once the
way up has been determined, the stratified rock sequence can be subject to
further analysis. The method of cross-cutting analysis allows an order of
events to be found. This is where a rock sequence has undergone several events
such as been faulted, subject to igneous intrusion and metamorphosed due to
intense heat or pressure. To demonstrate this, the situation below illustrates
a cross-cutting relationship.An example of cross cutting
relationshipsHere the rock layer 6 is the
oldest, on top of which progressively younger rock layers have formed. The
igneous intrusion occurred at a later date, which can be visually identified as
an intrusion into ready formed rock. It is likely that pieces of the country
rock (xenoliths) have been ripped off as the magma was thrust into the rock,
which may aid the analysis of the order of events. The fault must therefore
have occurred last as the igneous dyke has also faulted. Unconformities must also be taken
into consideration during analysis. There are four types of unconformity: Diagrams showing the four types of unconformityAngular unconformity occurs when
first a sequence is deposited horizontally following the principle of
superposition. This is then folded and uplifted and then eroded, resulting in
it being dissected and lowered. A subsequent rise in sea level results in
deposition of horizontal sedimentary beds. Disconformity occurs where units
above and below the plane of unconformity have the same angle of dip, and where
the lower rock surface has been subject to erosion. This may be caused due to a
fall in sea level, leaving the rock (lower) exposed to subaerial erosion. Again
a rise in sea level will result in sedimentary beds being deposited on top in
horizontally layered beds. Non-conformity results from the
erosion of heavily metamorphose d and deformed rocks, most commonly the result
of continental collision or exposure to a large igneous intrusion. Subsequent
deposition on top of this due to a rise in sea level concludes the
unconformity. Unconformity can also be
recognised not only where erosion has occurred, but if the rate of deposition
and sediment removal are the same. This is described as a paraconformity or
diastem. Here the sedimentary sequence is not exposed to erosion. All these unconformities
represent time gaps in the stratigraphical record, and apart from paraconformity
they all involve the destruction of some of the stratigraphical record through
erosion. This is important as a stratigraphical sequence is unlikely to be a
continuous record and will contain a number of diastem and possible other
unconformities. The next level of analysis is
grouped under lithostratigraphy. This involves formal description of rock units
in a sequence and their comparison with others in both space and time. The
level of description here differentiates a rock sequence into a selection of
formations. A formation is a unit of largely homogeneous lithology that may be
clearly recorded on a geological map. Once a stratified rock sequence has been
described in terms of formations it can be compared, or correlated against
another sequence. This is called lithological equivalency, where tie lines
connect similar formations. Geophysical methods of correlation can be used,
such as measuring the electrical resistivity of the rock. These are not however
time lines and do not aid analysis of a sequence all too much. Diagram demonstrating tie and time linesAlso these correlation methods do
not account for diachronism. Diachronism occurs when a stratum varies in age
laterally. This may occur in the formation of a delta, where deposition
progresses out in a lateral direction, resulting in a relatively horizontal
rock stratum of laterally varying age. Therefore an independent method
of relative dating is required to achieve correlation between one sequence and
another. The aims of correlation are to establish relative chronology of
lithostratigraphical units and therefore a relative sequence of geological
events. This requires the implementation of biostratigraphy to stratified
sequences. Biostratigraphy relies on the use
of fossils. These are the remains of once living organisms, some of which have
been petrified and others that actually contain some tissue and or skeletal
matter. Fossils are so useful for correlation due to the fact they are
independent of the lithology in an area. The constant and irreversible
evolution of organisms over time provides a chronologically recognisable
sequence present in rocks. Guide fossils are most used in correlation, these
have the following properties: –
Independent of their environment. Therefore the organisms were
not restricted to a certain area due to environmental parameters. –
Rapid rate of evolution, to provide a large range of varying
species to identify different geological time periods. –
Geographically widespread to allow correlation over a wider
area. –
High abundance. –
Readily preserved –
Easily recognisable. Swimming or free floating
organisms are well suited for correlation, such as ammonites. Ammonites are
good guide fossils as they have all the above properties and are highly
widespread with a species turnover of around one to half a million years. Stratified sequences can be
broken down into biozones. These are strata organised into stratigraphical
units on the basis of their content of guide fossils. Biozones can be
classified in four main ways: –
Assemblage zones are defined as a vertical range of a number
of fossils. These are used when there is a lack of good guide fossils, i.e. Sea
bottom dwellers which is of limited use, as it requires recognition of several
species. –
Total range biozones are a vertical range of a single fossil,
usually a guide fossil. –
Partial range biozones are vertical ranges in-between the last
appearance and the first appearance of fossil. –
Acme zones are based on an abundance of a fossil group. Diagram demonstrating the classification of biozonesA relative chronology of biozones
can be established in a stratified rock sequence, which then can be correlated
against another sequence if the appropriate guide fossils are present.
Biostratigraphy however does not recognise the majority of diachronous
stratigraphical features, as it?s resolving power is insufficient to detect age
variation on such a small scale. However large scale diachronous deposits like
that of the lower to middle Jurassic of southern England can be detected. Event stratigraphy may also be
taken into account. These may be volcanic events resulting in ash fallout over
a large area, which allows lithostratigraphic correlation that is temporally
relevant. Tsunamis also produce event horizons, as found around the east coast
of Scotland where a tsunami 7000 years ago left a regionally extensive sand
layer in peat bogs and clays. Event stratigraphy produces obvious chronological
markers that are often simple to use to achieve correlation between sequences. The final analysis tool is
absolute dating through radiometric dating. This relies on the principle of
radioactive decay. Absolute dating deals with absolute dates in the past which
distinguishes it from relative dating on which I have focussed, which provides
a simple ordering of sequences.