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.