A Geological Report on the Permo-Triassic of Chester Essay Sample

A Geological Report on the Permo-Triassic of Chester Pages
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Hypothesis:

* The geology of Chester is dominated by sediments of desert origin and of permo-triassic age.

Aims:

* Investigate the rocks of the South Chester area;

* Determine the depositional environment in which these rocks accumulated;

* Determine whether or not the area was dominated by a desert environment in the geological past.

Background

Chester is located in the North West of England. It is not just a world-famous heritage city with 2,000 years of history dating from the Roman invasion of Britain; it’s also the gateway to some of the finest countryside in England and neighbouring north Wales.

The district of Chester stretches from the Dee estuary in the north to Shropshire’s rolling hills in the south. To the west of the city are the stunning Welsh borderlands. East of this county town are the immense flatlands of the Cheshire Plain, carved out of the landscape by Ice Age glaciers with a history older than Chester itself. Here a host of timeless towns and villages with their traditional black and white, half-timbered architecture set amid green meadows and scenery – a vision of England as it was and still is.

Introduction.

The investigation will involve the reviewing of the depositional environments of sedimentary rocks and in particular the characteristics of desert sandstones.

To determine the sedimentology of the area, relevant details will be recorded in the field notebook along with field sketches, sedimentary logs and a base map. Factors to be investigated will include the sequence and origin of the beds at each location and dip and strike of the beds. (Rock specimens can be collected and analysed further in the lab). A clinometer will be used to determine the angle of dip as this gives angle readings to a high degree of accuracy. A sedimentary log will be used because it is a reliable method on which a large amount of data can be stored.

NB throughout the trip health and safety must be considered due to the instability of the cliff faces, appropriate safety gear must therefore be used at all times.

Permian – the extent of the Permian outcrop can be appreciated throughout this important and significant area, stretching from St Helens in the west, eastwards through Manchester and as far as Stockport. This area edges on the Chester Plain and incorporates formations of Breccia (Basal Conglomerate) including phenoclasts of Carboniferous age followed by the sandy facies of Manchester marls, which represent the movement from the Permian and Triassic periods, in at least certain parts of the region.

Triassic – Deposits widely dispersed throughout the region, and comprising in the lowest format ‘Binter’ and the red coloured ‘Lower mottled sandstone’ which can be occasionally mottled with white and yellow patches. This is recognised as ‘millet seed’ and is wind worn sand grains.

The area also has a large domination of gravely aggregates intermingled by pebble beds of yellow stone, that are in certain locations bedded. These stones and formations have been an important part of the building trade where upper mottled and lower mottled sandstone have been used for their longevity and durability.

Finally above these beds are the Kauper series of sandstones which are coarser and include the Basal beds which are grey in colour and have intermediate disposal of quartz pebble layers, an ideal source of building material that is continuously quarried. This rich source of sandstone also includes thick beds of Rock salt, a commercially sought product, for the local industries of the locality. Finally, the contrasting silt beds of fine-grained keuper marl are preceded by the silty red clays of the Waterstones.

Location 1 (GR 400600)

This is the best place to begin this investigation; located south of the River Dee along Castle Walks.

Fig.1

From the restricted distance of approximately 50m, layering can clearly be seen within the low-lying cliff. At such a distance, bedding planes cannot be seen. The dip is in a South East direction suggesting a strike to the North East. The beds of the cliff were covered with vegetation suggesting the rock is porous.

Fig.2

At closer observation, bedding planes and fractures can clearly be seen. The yellow/white areas of the rock can also be seen. This suggests a saline material is being precipitated out onto the surface of the rock. A large fracture can be seen 10 from the horizontal in Fig.2

The red rock is probably Haematite which originated from the oxidation of iron under intense heat. This suggests a desert like environment must have been present. It is evident that weathering had occurred on the surface. This therefore means the rock is poorly cemented and is typical of a red desert sandstone.

Location 2. (GR 400600)

This is to the South of Quarry Close approximately 250m from the previous locality. The dip at this location appeared to be 5 south and 8 North East, however, this was the apparent dip and the true dip was actually into the rock at a relatively low angle of 8-10 in a South East direction.

Fig.3

The layering as shown in Fig.3 is contorted to give convolute bedding. This may be the result from the loading effect of a coarse material on fine sediment.

This image also shows the differential erosion of the structure in which the softer white rock (probably Limonite) is sandwiched between a much more robust red rock (probably Haematite Fe2O3) and has not been affected by erosion to the extent of the Limonite.

The red rock have probably been oxidised, and for this process to occur, high temperatures are needed suggesting that it was once a desert environment.

It seems that sediments have been blown to conform into current beds and this presence of current beds along with intermittent beds suggests desert sands.

Two bedding rocks were ‘wave-like’ and appeared to be in no fixed direction. This is evident of dune bedding. This resulted from the action of the wind.

Fig.4

Fig.4 shows blocks of sandstone on top of the bedrock, which would have been used directly to support other structures. On the face of the bed, there is evidence of current bedding.

Rocks contain Quartz and are cemented together by haematite.

Fig.5

In fig.5 many laminations with slight displacement (micro faults) can be seen which were probably caused by the settling or compression as the bed was buried. The white areas also seen here again suggest that salts from Chester’s salt field are leaching out of the sandstone. Similarly here too the beds are dominated by oxides and the colour change could be down to a change of mineral.

Very distinct layers repeat over and over can also been seen in Fig.5. This suggests a fluctuation in the environment probably change in temperature. This repeated process of deposition also shows dune bedding.

The white beds indicate carbonate material in the form of limonite. This white rock has been weathered more than the red rock which again gives the impression that this is the softer, more loosely cemented rock seen at previous locations.

Location 3 (GR 400600) is located to the west of Mill Street approximately 700m from location 2.

Fig.6

The exposed cliff face in Fig.6 showed the beds to be completely composed of a cyclic sequence as the haematite and limonite are repeated as seen at previous locations.

In Fig.6 it is apparent that areas of this have been weathered back and a major bedding plane can be seen almost horizontal. Current bedding is similar to that seen at location 2, showing the formation of sand dunes.

Haematite beds consisted of small angular pebbles whilst the limonite was thinly bedded in between the other surrounding beds.

The dip is in still in a South East direction with a large joint present measuring almost 7 metres. This joint could possibly be separate beds joined together.

The dark area to the right of Fig.6 was caused by pollution from the city which resulted in the deposition of this material onto the rock face. This causes problems as the structure of the beds are difficult to see, and cannot be analysed fully.

A sedimentary log was completed here as this location best displayed the fullest development of succession in comparison to the previous locations. The succession of the cliffs was recorded on a sedimentary log sheet and the characteristics of the rock types were noted as follows:

Specimen 1:

Specimen 2:

Location 4. (GR 400600)

This was the final location investigated, and was situated approximately 500 metres from location 3.

Fig.7

Fig.7 shows the sediments in close up, the rocks are dominated by sand and phenoclasts – which have been shaped by the wind. This shows that energy levels within the environment remained fairly constant.

Fig.8

Fig.8 also shows part of Chester wall with a much coarser rock; here the phenoclasts have dropped out leaving spaces. The phenoclasts are characteristic of desert sandstone. They are finely grained rocks >0.2mm. The honeycomb effect has been caused by the wind etching out material.

As the layers are in no fixed direction, it is probable that current bedding caused this.

The rock here was friable, and the two types of sandstone from previous locations were found here.

Evaluation.

The investigation overall has been mostly successful and all of the aims were achieved. All of the evidence collected appeared to be quite reliable as it corresponded really well with the historical and geological background, which has already been analysed. Comparisons between diagrams/photographs and the work from the field study, with the evidence already collected by a geologist, show there are certainly some similarities.

However, some problems were encountered; the main problem was distinguishing between true and apparent dip (especially at location 2) as dip values and directions varied markedly over just a few metres. It was equally difficult to obtain an accurate degree of dip due to the restricted accessibility of some locations. For example, at location 1, the dip and strike measurements were made from a distance of approximately 50m. Due to this restriction, it was difficult to determine the structure of the bluff and evidence of the bedded material was unobtainable.

Rock faces were often obscured in some places and this made it difficult to work out the sequence of the beds and their dip and strike, therefore a partial picture as to how it was formed can only be achieved.

Hand drawn diagrams have little reliability as the illustrator may leave out important or complex details.

Mistakes may have also been made in producing the sedimentary log, because bed measurements were not obtained and the accuracy may be reduced. The investigation could be improved by collecting a greater range of data; for example more sedimentary rocks could have been collected and more locations could have been investigated. An extension to the investigation could include greater exploration into the sedimentology of the area as this would give a better picture of what the environment was like at the time.

Interpretation

It was found that the cliffs consisted of repeating beds of clay shale and soft limestone. These originally overlaid a much harder layer of nodular limestone, which formed a platform; this contained an extensive amount of fossils and was an area of death assemblage. Overtime the cliffs have been cut back (15 metres) exposing the platform. There was a large amount of ammonities present in the wave cut platform; these were dominant in the Jurassic era. Brachiopods and bivalves were also found, Bivalves were dominant from the Triassic onwards so would therefore also be very numerous like the ammonities in the Jurassic. Brachiopods have existed from the lower Cambrian in many forms, but the types found were Rhynchonellids, which were dominant in the Jurassic. The presence of these fossils and their domination of the Jurassic era suggest that the cliffs must have developed in the late Jurassic era. The bivalves and brachiopods were probably preserved in-situ, as these are usually benthonic forms, liking shallow environments. The ammonities could also have existed along with the brachiopods in the shallow environment, but because they can exist at various depths it is probable that they have been transported forming a death assemblage.

It has now been determined that the cliffs were formed in the Jurassic, they will have been lain down under the sea, but as sea levels changed become exposed. The cliffs were composed of repeating bands of clay shale and soft biochemical limestone. Limestone is of organic and formed principally of the accumulation of calcareous skeletons of organisms, in this case of benthotic origin i.e. brachiopods, this type of limestone is characteristic of a shallow environment. The other cliff rock was a clay shale, this is a classic sedimentary rock, which is very fine grained, and it is finely bedded and splits into thin layers. Clays only occur in young geological formations and are consolidated into shale overtime, this again suggests the cliffs are quite recent formations. Clay sediments tend to accumulate in deep parts of lakes or seas were gentle water movements can transport the small particles, they often occur with limestone as part of a repeating sequence. This suggests that the cycle of the cliffs formation reflects marine transgression and regression, with near shore shallow environments in which the limestone’s were deposited and deep water environments in which the shale’s were deposited. This transgression worked as a cycle producing the layered cliffs. These were then exposed when sea levels changed.

The cliffs have experienced much recognition; this is due to the nature of the rocks from which it composed. Both the limestone and the shale are relatively unconsolidated and soft they are therefore easily eroded by the action of waves and rocks, these often undercut the bottom layer of the cliff causing it to become unsupported and eventually collapse. The shale layers are also a major factor leading to the cliffs collapse. They create a layer of fluidity when wet; the cliffs have a slight dip of 8-10 towards the north, when the shale layer becomes fluid it will move in the direction of the dip taking the overlaying layers with it.

The techniques used to protect the cliffs are blocks of Hornfels and Quartzite and Groynes. The blocks of hornfels and quartzite are chosen because of their characteristics. Both are metamorphic rocks, they are extremely hard and resistant rocks and therefore able to with sand the erosive effects of the sea and protect the cliffs. They are uncreative rocks and will therefore not dissolve in the way that limestone does, they do not absorb much water (low porosity) and will not become plastic like clay. The rocks were piled up along the coast in front of the cliffs to stop the seawater hitting them and causing further recession.

These arid conditions resulted in the oxidization of the sediments giving them their distinctive red coloration.

Permo – Triassic. A period of continental desert sedimentation, initially contemporaneous with waning Hercynian igneous activity, and governed both by pre-existing Armorican highlands to the west and southwest, and N/S extension resulting in E/W trending faults.

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