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Geological Unconformities.

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Introduction

Guided study 1: Unconformities. According to Collins dictionary of geology an unconformity is defined as a break in the sequence of strata in an area that represents a period of time during which no sediment was deposited. It indicates a change in the conditions prevailing in the area. An unconformity may be the result of uplift and erosion, an interruption in sedimentation, or non - depositional of sedimentary material. The absence of rocks normally present in a sequence indicates a break in the geological record. The De La Beche unconformity is situated in Frome, Somerset, and it gives an opportunity to investigate unconformities and to compare and contrast a number of different positions in the area to create a better understanding. The first place visited was Black Rock Quarry. The lower bed seemed to be a horizontal bed of carboniferous limestone, but at this point it was a possibility that it could have been a strike. There were a number of fragmented fossils visible in this section of rocks; these were small circular "polo" shaped fossils. Along the top of the beds was vegetation, a number of small trees and grass. There were a number of deep cracks and at the bottom was some debris and fallen rocks. ...read more.

Middle

The top of these rocks were then eroded. This flat surface became the base for the newer rocks to be deposited. Below is a diagram of this process:- The lower beds of rocks are carboniferous limestone. There are many fossils present, which suggests that these rocks were formed in a tropical, warm environment. The sediments would have been deposited underwater on a carboniferous limestone seabed, and there could have been tropical coral reefs present. In order to date the rocks fossil content can also be analysed. The principle of fossil succession can be used to do this. The "polo" shaped fossils found here are crinoids. These fossils are found in more than one location in the area. These fossils suggest that a possible position for the formation of the rocks could be just south of the equator, and that would correspond to formation at approximately 330 - 340ma. This process of aging is used to differentiate between the different types of rocks present in this unconformity. There is a difference of approximately 200ma between the carboniferous limestone of the lower beds and the oolitic limestone of the upper beds. The upper beds that were deposited at a much later date than the lower beds appear to be oolitic limestone. ...read more.

Conclusion

From 1840 - 1850 saw the beginning of the following of the evolution of animals to date - biostratigraphy. Lord Kelvin calculated in the 1890's that the earth was between 20 and 400ma old. He based his theory on the cooling rates of the earth, but he didn't account for a major factor - radioactivity. In 1899 John Joly estimated the earth to be 90ma old, by analysing the salt in the earth, but one of the problems with this was the fact that the sea and land were not created at the same time. In 1812 William Smith, whilst laying canals, discovered the faunal succession. This led to the eventual means of worldwide correlation. By using this we can analyse the fossils found in unconformities to discover the relative dates of the strata. James Hutton (1726 - 1797), who is often referred to as the father of geology proposed the theory of uniformitarian theory of geology, which states that all earth processes affect the surface of the earth, and had been at a constant rate for a very long time. He based a large majority of his work in Scotland nearly 200 years ago, at Siccar Point and at Newton Point, Arran. His exploration led to the theory of deep time, and he is known to have quoted that "the result, therefore of this physical enquiry is that we find no vestige of a beginning, no prospect of an end". ...read more.

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