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Show How A Stratigraphical Sequence Can Be Show How A Stratigraphical Sequence Can Be Deduced. How Can Fossils Be Used To Tell The Relative Age? 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 un layered. Layered rock bodies are described as stratified and un layered 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 original horizontality, the idea that layers of rock were originally deposited horizontally and finally original lateral continuity. 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 fossilized these provide clear way up indicators. Marine organisms burrow down vertically and tree stumps and other vegetation may be fossilized 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 relationships Here 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 (xenolith's) 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 unconformity Angular 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 sub aerial 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 para conformity or disaster. Here the sedimentary sequence is not exposed to erosion. All these unconformities represent time gaps in the stratigraphical record, and apart from para conformity 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 disaster and possible other unconformities.
The next level of analysis is grouped under litho stratigraphy. 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 lines Also these correlation methods do not account for dichroism. 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 litho stratigraphical 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 bio zones. 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 bio zones are a vertical range of a single fossil, usually a guide fossil. Partial range bio zones 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 biozones A relative chronology of bio zones 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 litho stratigraphic 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.
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Research essay sample on Rise In Sea Level Cross Cutting
