Determining the Paleoenviroment and Tectonic History of a Small Area - geology (400 Level Course)


Geology Coursework: Determining the paleoenviroment and tectonic history of a small area (Cocklawburn Beach)

Planning - In this coursework I am going to try and find the tectonic history and paleoenviroment of an

area of exposed rocks along a short section of beach in Northumberland, Cocklawburn beach is 50 metres to the south of Berwick-upon-tweed and can be found on OS map sheet 75.

The general geology of the area is that it is mostly carboniferous limestone with layers of sandstone, shale and coal existing between the layers of limestone. I know this from previous experience in the area, as I have completed a sedimentary log for another section of rock in the same area, at Cullernose Point

For the paleoenviroment I am going to look at and analyse the rock type. I will check the rocks for structures and fossils as they can be used to determine the absolute age of the rocks and they can also be used to check if the rocks have been shifted or if they have been moved upside down. The first thing that I shall investigate with respect to paleoenviroment is the relative age of the rocks. For the tectonic history I shall investigate the Dip and Strike of the rocks, as this will show the attitude of the rocks relative to the horizon and it will show if any tectonic activity has shifted the rocks, and also if there are any major igneous structures (Bowls ect) I shall look for any displacement in the rocks as this will show if there have been any faults in the area that have affected the rocks in the past. Using all of this information together I intend to construct a sedimentary log that will definitively show the history of this small area. To help me find this information I will use my own observations, as well as secondary sources such as the Internet and geology reference books (see bibliography) I think that I am most likely to find Sedimentary rocks that will be in recurring cyclotherms containing a lot of fossils and other structures, and that have also been affected by a lot of faulting and folding.

To collect the data for the items I have outlined above, I will go to the beach and construct a sedimentary log. I have already tested this method on another beach in the same area as Cocklawburn beach, at a area called Cullernose point

This log will encompass most of the data that I will need to collect, however I will also have a Field Notebook, which I will use to sketch out the layers or rock and also to sketch any structures that are found on the beach, this will be used so I can identify which type of fossils and what special features were present in the layers of rock. To fill in the above logging sheet I will use the following equipment. To find the grain size of the rocks, I will examine the rock with a hand lens and compare what I see to a grain size index card provided by my teacher, I will do this to find the energy of the transport medium at the time of deposition, this card will also to be used to checked the sorting of the rock, which will show the rate of deposition. I will look for any fossils in the rock, as they will help to show if there are any special environmental conditions in the area during that time of disposition. I will also use evidence of any sedimentary structures to determine the environment of deposition. I will also examine the colour of the rocks to try and determine the mineral and oxygen content, as this can be used to try and determine the paleoenviroment of the area at the time that the sediment was deposited that formed the rock layers. I will look for evidence of dip and strike by using clinometers to find the dip angle and using a clinometers and a ruler to find the strike, I will do this so that I can accurately show any tectonic features in the area, and so that I can form a hypothesis about what forces have acted on the area in the past. I will look for evidence of folding and faulting, as they will show if there have been any compression or tension forces acting on the rock, and also the direction that the forces were coming from.

To test these methods I have constructed a short sedimentary log of beach called Cullernose point, which is near Cocklawburn beach, I have found that these methods work in the field and I do not intend to change any of the methods used.

The limits of my investigation if that I can not check any rocks that are underground, so I will not be able to find the thickness of some beds, if I can not see the base.

Implementing

In this section I will record the results that I have found from my fieldwork, I will use sedimentary logging, charts and graphs to shows the information that I have gathered.

The first item that I am going to show is the most important, as the sedimentary log shows most of the information that I have gathered on the trip that will be useful in a clear way

Analysing evidence and drawing conclusions

From the sedimentary logging sheet there is a obvious pattern in the rock layers, which clearly show the recurring cyclotherms, I believe that I was caused by differing sea levels at the time of deposition, for example, if the cyclotherms that is composed of bed 1-5 is examined, then the pattern is:
1: Limestone – Deep Sea where dead creatures decomposed to form limestone layers
2-3: Silt – Shallowing seas mean layers are formed by fine sediment being deposited from the closing landmasses
4: Sandstone – As the sea gets shallower then the size of the sediment that is deposited increases steadily, meaning layers 2-4 have evidence of graded bedding that goes with the increasing size.
5: Coal – as the sea becomes a swamp or maybe even low land, trees fall in and coal is deposited as a top layer of the cyclotherms.
After bed 5 the process begins again with a limestone layer at bed 6 to 10. This pattern recurs along the beach.

Throughout these layers fossils and sedimentary features are in evidence, these fossils are mostly Crinoids, with Brachiopods scattered throughout, sedimentary features in evidence throughout the beds are graded bedding, and ripples which shows evidence of sea currents as the seas became shallower. A lot of the beds also contain ironstone nodules.

Below is an analysis of each bed found on the beach, this data has been taken from the sedimentary log I produced in the field.

Bed 1: This bed has a thickness of at least 3 metres, I say at least because the bottom of the bed is below the beach and cannot be seen. It is composed of limestone with a shale layer about halfway up the visible area, however this layer is not discernable enough to count as a separate bed. The composition is fine sand, with silt in the shale layer; there is evidence of paleocurrents from the south in this bed. The colour is grey at the bottom but the top is muddy and browner. Brachiopods and crinoids were found in this layer. This evidence seems to show that this layer was in deep sea, with a lowering of sea levels causing the shale layer in the middle, and then sea levels falling at the end of the bed, causing the muddy area at the top of the bed. The marks on this bed (see to the right of the marker) These marks are from the remains of a colonial coral, which would have lived on the rock, and would have been fossilised when it died

This picture shows the solitary corals (the ring shapes) and the brachiopod fossils that were found in bed 1. As these creatures could only live in a very low energy environment this shows that the rock was formed in a deep sea where there were only low energy currents.

Bed 2: This bed has a thickness of 2.5 metres. Its composition is clay and it shows evidence of ripples, there are no paleocurrents visible in this layer. The layer is black in colour, and there were brachiopods found in the layer as well. This layer was very soft and unstable, and it could be broken off easily with hands. This layer will have been formed as the sea levels fell and fine sediment from the land will have been deposited on the sea floor.

Bed 3: This bed has a thickness of 4 metres. It is composed of siltstone interbedded with mud, the grain size is silt and there are alternating paleocurrents, with south at the bottom and north facing at the top of the bed. There is a lot of cross bedding in this layer and there are also ironstone concretions throughout it. There are load casts at the top of the bed, which is brown in colour. This bed will have been formed as the sea levels fell even more, which would let larger sediment pieces fall and be deposited on it, as there would have been less time for it to be eroded into smaller particles. This is a picture of the loads casts that can be seen in the top of bed 3. These are formed when a section of the above rock falls away into the mud beneath, when the rock below fossilises the other rock is left as a fragment, called a load cast.

Bed 4: This bed on only 1 metre thick. This bed is grey and is composed of fine sand sized particles. This bed shows evidence of fossilized roots embedded in the top of the bed. This composition of this bed is known as “sea turf” and is formed from deposition on a very shallow sea; the roots are from plants that have rooted into the land while it is still underwater.

Bed 5: Bed 5 was not thick enough to measure accurately. It is a layer of clay sized grains, and is coal. This is the end of the first cyclotherms in the beds, as can be seen from the cycle limestone -> coal. This layer would have been formed in a shallow swamp, with very little water movement. As plants and other matter died and fell into the water, it would have decayed very slowly and been formed into coal.

This picture shows bed 3, 4, 5 and 6, note how thin bed 5 is in relation to the other beds, this shows that the area did not stay as a swamp for very long

Bed 6: This bed is 1 metre thick. It is fine sand sized particles and is composed of grey limestone, this fizzes with dilute HCl, and there is evidence of crinoids and trace fossils in the bed. This bed shows a return to deep water, as there are now hardly any sediment deposition, and an increase in carbon deposition leading to the formation of limestone and the fossils.

Bed 7:This bed is 1.5 metres thick, it has clay-sized particles. This bed is composed of mudstone and is black in colour. There is a lot of ironstone nodules spread throughout this bed. It is soft and comes apart easily in the hand. There is also evidence of lamination in this bed, which will have been caused by uneven deposition rates over the years in which this bed was formed. This bed will have been formed like bed two, with a shallowing sea leading to a fine sediment deposition

Bed 8: This bed has a thickness of 1.5 metres. It is composed of silt size particles and is siltstone, it is grey. This bed is interbedded with shale and there are iron stone concretions throughout it. This bed shows numerous fossils, with Trace fossils roots and also a lot of sedimentary features, such as cross lamination and normal laminations, these will have been formed due to different current directions and also due to different amounts of deposition during the years the bed was formed. This bed will have been formed in a very shallow sea, as there is evidence of fossilised roots at the top of the bed.

This picture was taken from the top of the 6th bed, and shows bed 7 and 8 on top of it. Note how much bed 8 has eroded.

Bed 9: This bed is .5 metres thick. It is composed of fine sand particles and is sandstone. This bed is grey in colour and shows evidence of lamination and root fossils. Once again these will have been formed by plants rooting in the very shallow water of the area.

Bed 10: This is the end of the second cyclotherms, as this is a coal layer, once again it is too thin to be accurately measured. This will also have been formed in a shallow stagnant swamp.

Bed 11: This bed has a thickness of 1 metre. It has silt-sized particles and is clay. The only feature shown by this bed is lamination; there were no visible fossils or any of features in this bed.

Bed 12: This bed has a thickness of 2 metres. It is composed of medium sand grained particles and is sandstone. This bed is interspread with layers of silt. This layer is laminated, it also has asymmetrical ripples, formed when the currents affecting the bed shifted, there are also trace fossils. This bed has been formed due to shallower seas, but as the next bed is composed of clay, the cyclotherms pattern has been interrupted, so there must have been an unconformity to break this cycle.

This picture shows bed 8 from further along the beach, and also bed 9, bed 10 and bed 11

Bed 13: This layer is .5 metres thick. It is composed of silt-sized particles and is clay. It is black and shows cross lamination, formed by shifting currents and irregular deposition. As I have mentioned above, if this bed followed the normal cyclotherms cycle shown in the beds below it, this bed would be composed of sea turf or perhaps even a layer of coal, however as this bed is finer grained than the one below, which suggests a deepening of the sea, then either a unknown element changed the area at this time or a unconformity has altered the bedding pattern.

Bed 14: This bed is .5 metres thick. Its is a medium grained sandstone. It is grey and shows evidence of laminations across the bed. It is black, this layer, and the layer above it (15) shows that the sea was shallowing, and that the pattern shown in the 2 preceding cyclotherms is reasserting itself.

This picture shows bed 11, bed 12, bed 13, bed 14 and bed 15

Bed 15: This bed is near identical to the one above it, it is .5 metres thick, grey and shows lamination, however this one has shale layers interspread in between it, so I have marked it as a separate layer from the pure sandstone layer underneath it.

This photo shows the top of bed 16, Bed 17, bed 18, Bed 19 and bed 20. It also shows bed 21-23, but these beds are hard to see on this photo, so I will show them on another photo further down the list

Bed 16:This bed is 2 metres thick. It is sandstone and has medium sand particles. This bed had very poor bedding and had a lot of roots throughout the whole bed. The poor bedding could have been caused from changes in deposition over the years. There were also trace fossils and ironstone nodules found in this bed. This bed was formed once again, as sea levels got shallower

Bed 17: This bed is 1 metre thick. This bed has silt-sized particles and is siltstone. This bed has a lot of roots throughout it. This bed was probably soil that formed when the sea lowered so much that there was actually land there where the sea was.

Bed 18: Bed 18 is the third coal layer in the log. As the layer above it was formed in land, then the seas must have returned to form a stagnant swamp that allows coal to form.

Bed 19: This bed is 2.5 metres thick, it is composed of clay particles, and there are root casts and ironstone nodules in this layer. This layer could have been formed either by a return to deep-sea conditions, which I think is unlikely, or as a very thick layer of soil as the sea stayed either very shallow or went to land for a long time.

Bed 20: This is another coal layer. This layer makes it appear that the sea did not return to a true deep sea for a very long time after bed 18, but stayed as a shallow area or a swamp for a long period of time.

Bed 21: This is a very thin layer of clay sized mudstone, this forms in-between two layers of coal, which supports my theory that this was a land area for a very long time before it returned to the sea, this can be shown by the fact that there is not any fossils or sedimentary features in the area to show evidence of currents of animals.

This photo shows bed 21, bed 22, bed 23 and bed 24. I have used this photo to show bed 21-23 as it is clearer

Bed 22: This coal layer has formed very close to another coal layer, which shows that the area stayed as a swampy land for a long time.

Bed 23:This is a layer of fine-grained sandstone. It is 1.25 metres thick and has evidence of calcite deposits; these were probably formed as calcite formed round particles of mud or clay.

Bed 24: This bed is 1.25 metres thick. It has medium sand sized particles, but it is a sandy limestone. This bed shows a return to relatively deep-sea characteristics, but with a high sediment deposition, which suggest a large land mass may have formed nearby. Crinoids are also in this bed, which suggests a deep sea, as the low energy environment would not break them up as it does in a high current environment.

This photo shows bed 25, bed 26 and bed 27. Unfortunately, I do not have a photo of bed 28.

Bed 25: This is a .5 metre thin bed of medium sandstone. This bed has ironstone deposits and root casts, and seems to indicate that the sea that was in the area was shallowing.

Bed 26: This bed has an unknown thickness and an unknown rock type, it was eroded and no information could be gathered about it.

Bed 27: This is another coal layer, however no thickness information could be found because the base of the layer could not be seen.

Bed 28: This bed has no thickness information, as once again, the base could not be seen. This bed is limestone, and shows that the area has returned to deep-sea conditions. This is reinforced by the crinoids and brachiopod fossils that were found in the area

Overall: In total the layers that I logged came to about 38 metres, and spanned about 100 metres of beach across, there only appears to be 3 complete cyclotherms, although there are a few that appear to have been disrupted by tectonic activity.
The cyclotherms present in the rock show that the past history of the area was repeating, over millions of years. This means that the area has gone from sea to swamp or possibly even land, and then reverted to sea again. The most obvious cyclotherms is present in bed 1-5, as it shows the formation of sandy limestone’s, representing deep sea environments, which is further reinforced by the fossils of crinoids and brachiopods found in it, all the way through finer layers until bed 5, which is a coal layer formed by decaying plant matter in anaerobic conditions.

Tectonic History
The tectonic history of the area can be shown by checking the dip and strike of the rocks and plotting a diagram that will show the direction, I have used a rose diagram with degrees around the edge to find where the forces came from. The diagram shows that the main force in the area came from the northeast; this is confirmed by the dip, which is northeast/southwest, and the strike, which is roughly south east/northwest.
This is backed up by the asymmetrical anticline seen at location 1 on the base map; this anticline had been pushed inwards from the northeast so that it formed a hump-backed appearance facing northeast. It is also shown by the plunging folds fold at location 2 on the base map, as the dip and strike measurements shown there are in alignment, and the folds run northeast/southwest. This evidence shows that there has been a major force acting on the coast in the past, forming these features, and that it was acting in a northeast/southwest direction, with more force from the northeast. This hypothesis is backed up by the evidence that I have collected on my sedimentary log.

This is a photo of the asymmetrical fold that was at location one on the base map. Note the thrust which pushes one side of the fold over, the right side of this photo is the north-east.

This photo shows the folding that has occurred in the first bed on the log. Once again, the right side of this photo is pointing to the north east, which shows the tectonic forces to be northeast/southwest

Evaluating Evidence and Procedures

I think that the techniques used to collect this data was the most reliable ones that I could use practically in the field. However, I have noted the limitations of the techniques that I have used. The sedimentary log only shows features which can be seen on the surface on the rock, there could have been fossils or features buried in the rock, which I cannot include in my study. The dip and strike measurements that I have taken, as well as the measurements for the thickness of the beds can only be as accurate as the things I used to measure them. This means there will be an error built in to the measurements, as well as human error. The statistical analysis techniques that I have used could also contain error (the rose diagram), however, I believe that have minimised the error by making measurements as accurate I could in the field.

I believe that the conclusions I have drawn from the evidence I have collected is true, however there could have been forces at work that I was not aware of, and some of my conclusions could be wrong due to this fact.

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