Evolvemnt Of The North Sea During The Jurassic Period

Abstract

The North Sea underwent many changes in the Jurassic such as a large change in sea level from being arid at the end of the Triassic to being deep marine by the end of the Jurassic formation of a thermal dome due to a rising mantle plume, this lead to an unconformity. There was a period of extension of the crust which nucleated from the area of thermal weakness which later reactivated the existing faults which were Permian in age. As well as a period of large scale volcanism in the middle which occurred in 3 main areas, the volcanism lead to a vast debate as to what caused it and how it relates to the crustal extension which occurred.

Introduction

This report will be looking at how the North Sea evolved during the Jurassic period, this is broadly considered the time period when in which crustal extension in the North Sea reached its peak, it will also look at how rifting in the North Sea lead to an overall change in the sea level during the Jurassic. As well as looking at what the main causes of uplift in the North Sea were. The main rift system was the North Sea Central Graben, this rift is broadly considered to be an offshoot of the arctic north Atlantic rift system. This report will look at if a majority of the extension was caused by the reactivation of existing faults or if it was caused by the thermal lithospheric weakening from the formation of a thermal dome, which led to rifting. It will consider if the rifting was a result of the fault reactivation or if it was due to the weakened stratigraphy. As well as looking at the extent of the role; if any, that salt played in the formation of the basin. Also, I will be looking at the role which volcanics played in the tectonic history of the North Sea.

Methods

The Jurassic was a period of substantial change in the North Sea. At the start of the Jurassic it was more shallow marine in nature, whereas by the end of the period the north sea was considered to be more deep marine in nature, with seafloor spreading occurring in the cretaceous, there was subsidence occurring throughout the Jurassic which caused the area to become deeper marine, over time the thermal dome , which had formed due to a mantle plume close to the surface partially melting the over lying crust, subsided due to this subsidence it had thermally weakened the lithosphere this in turn caused rifting to occur, this rifting caused approximately 70km of extension across the North Sea basin (it has been calculated to be 100km by the end of the cretaceous). The extension is thought to have nucleated at the area of the subsided dome and spread out reactivation existing faults over time.

Results

Early Jurassic

Looking at the changes in the North Sea basin throughout the Mesozoic it suggests that the environment turned from mostly arid, with sandstones and mudstones deposited in the Triassic to becoming marine in the Jurassic, this occurred during the time when thermal subsidence was occurring, this lead to sea level rise. However, it is also thought that the thermal dome which formed in the North Sea is believed to have begun forming in the early Jurassic due to the formation of the dome it created an unconformity within the rock record. The dome prevented the deposition of the early Jurassic sediment, however the dome does allow Triassic sediments to be preserved as well as showing the gradual sea level rise which occurred during the Jurassic. The deposition of non-marine lithologies followed by an unconformable period which lasts for approximately 28ma where costal material is deposited the presence of this coastal plain material shows the sea level is beginning to rise, following this it shows the presence of shallow marine lithologies which are then followed by deep marine shales, the presence of chalk in the cretaceous implies that the climate was temperate with an average salinity.

Middle Jurassic

In the Middle Jurassic, the sediments began to accumulate on the flanks of the thermal dome following an unconformable period. The dome was irregular in shape and size, this is due to the fact that it is believed that the dome formed ‘by southward migrating dome centres’ in the Central North Sea. The origin of this is believed to be from a mantle plume head. the volcanism which was formed as a result from the plume produced basaltic lavas, which were then extruded, the volcanics are believed to ‘be the products of decompression-induced partial melting of the mantle, resulting from lithospheric attenuation during rifting.‘ Due to this process the mantle began to deviate and move upwards, hence leading to the formation of the dome. The area where the dome formed is ‘where the Iapetus Suture intersected the Tornquist-Teisseyre fault system the dome area was amidst the Viking Graben, Central Graben and Moray Firth Basin’ this is the area which rifted first when the thermal dome began to collapse as it was thermally weakened. The rifting that followed was in Scotland, and the Forties which are associated with the uplifting which occurred in this area. Later when the sea level dropped the Horda Basin and Long Forties were the volcanic centres. The presence of the erosional unconformity is the reason which the North Sea rift dome is visible. It shows a clear pause in the deposition so shows the region was up lifted, it also shows the shape and size of the dome. During the middle of the Jurassic the dome was above sea level, due to the dome rising up and being above sea level it caused a vast amount of uplift which in turn led to an increased level of erosion.

Late Jurassic

In Late Jurassic, there was a large amount of extensional faulting, this faulting along with subsidence of the thermal dome lead to approximately 70km of recorded extension during the Jurassic alone. These extensions occurred from the thermally weakened part of the crust which was where the dome was and then propagated out to the boundaries of the current graben systems in the North Sea, over time the faulting then extended back towards the centre of the thermal dome along the three armed rift system. The majority of the major rifting is believed to have occurred between 157 and155 million years ago the Jurassic sediments show a trend of thickening the closer to the syndepositional faults, the thickness of the sediment varies in a different way to how it varied during the phase of basin development when thermal dome occurred. When the thermal dome was formed there was an unconformable period but the sediments that did form, formed on the flanks of the dome as the top part was being eroded. In the early Jurassic the basin was lens shaped, this lead to the lens shaped deposits being formed, these were thickest in the middle and lensed out towards the edges. There were two main factors that controlled the style of the rifts, these in turn cause the rifts to be different in different parts of the basin. In the centre of the North Sea the rifts are more segmented, thus are more complex, the areas where the segmentation occurs are ‘North East ‘Caledonide’ and North West ‘Trans- European Fault Zone’ the basement composition is different in the northern North Sea, where it lacks the Permian salt. There is also a major detachment in the North Sea which is caused by the Zechstein evaporites. The evaporites which are present separate the basement and the overlaying cover sequences. T

he thermal subsidence also lead to further rifting to occur in the Cenozoic, the subsidence which was caused by the dome to collapse as a result of cooling. The large-scale block faulting lead to both uplift and extension. The erosion shown by the footwall relates to the erosion predicted by the domino extension model. ‘in which rigid fault- blocks rotate as extension proceeds’ in the Kimmeridgian there was a pause in the rate of subsidence, this is believed to be due to a change direction of movement exhibited in the fault, this then lead to a period of deposition the within Norwegian Danish basin in the north sea was shown to be both expanding and subsiding throughout the Late Jurassic. The extension which occurred was in a North -South trend. The extension and faulting began within the area of the lithosphere which was weakened by the thermal dome and then propagated outwards reactivating the earlier faults. There was multiphase faulting which occurred and it moved outwards towards the North Viking Graben as the strain rate increased.

Volcanics

There are three distinct types of magmas which propagates the magmas which are present are likely to have evolved from a basaltic rock which is probably alkali and olivine rich. The series which evolved are ‘an ankaramite-basaltic hawaiite-hawaiite series, analkali olivine basalt-hawaiite series, and anintrusive hawaiite-intrusive mugearite series.’ The volcanism which occurred in the middle of the Jurassic is assumed to be a direct result of tensional tectonic movements. The tension of these movements generated a high heat flow across the spreading axis, the volcanics exhibit a general trend of thickening to the south with approximations of the maximum thickness being up to 1500 meters, this thickness is assumed to be due to these areas being side vents on the flank of the dome. As the Viking graben was thickened it lead to the volcanics being eroded so the true thickness isn’t known. There are 4 known volcanic centres ‘the Fisher Bank and Glenn centres are closely juxtaposed and form part of the Forties volcanic province, and the Puffin centre occurs separately on the northern flanks of the Mid North Sea High. A fourth centre, Ivanhoe, has been tentatively located in the Outer Moray Firth.’ It is suggested that there are unconformable periods of time between each of the centres as they don’t all occur at a close time period so they were all growing at different times and points in the Jurassic. ’The onset of volcanism at the Fisher Bank centre may have occurred during the late Early Jurassic; the Puffin centre was active during the Mid-Jurassic, and the Glenn centre may be predominantly Callovian in age.’

There are three main theories in relation to the formation of the Volcanics; there is the tensional tectonic theory, which suggests that the volcanics were formed by the tensional extension of the crust and rifting caused volcanism to occur. The mantle plume hypothesis which is the theory that the mantle plume activity cause the volcanics to occur. The final theory is the embryonic spreading axis theory this theory suggests that the spreading axis between Iberia and newfoundland caused the volcanism to occur.

Discussion

There is a large grey area in the papers which I have read as to when in the Jurassic the thermal subsidence occurred, some papers state that it had begun in the late Triassic causing the sea level to rise which lead to the deposition of mudstones and shales in the North Sea Basin. However, other papers state that the subsidence began in the middle to late Jurassic, this in turn lead to the subsidence and caused rifting to begin which then lead to the crustal extension starting to occur. Although, the time frame at which subsidence began differs between papers and authors, the general consensus is that the subsidence is linked to the collapse of the mantle plume; therefore, the collapse of the dome. From reading around the subject I agree with a majority of the literature which leans towards saying that the majority of the subsidence occurred during the early to middle of Jurassic as even though it could have started before I think that it had a direct impact on the rifting and caused it to begin which started being a visible in the middle to late Jurassic.

A further contradiction in literature regarding this is that some texts state that the uplift transpired due to the dome forming due to the mantle plume, yet some texts say that the uplift was a direct product of the faulting, which caused the blocks to be tilted therefor leading to uplift throughout the basin along the rifts which were formed, also it leads to uplift of the Viking Graben, Central Graben and Moray Firth Basin. I think that a majority of the uplift occurred due to the thermal dome which formed, even though some uplift happened as a result of rifting and fault reactivation I think the main cause for uplift in the Jurassic was due to the formation of the thermal dome which is reflected in the lens shaped of sediment deposition in the early Jurassic, and the faults nucleated form this point therefore I think that the uplift form the dome was the most significant.

The literature on the extension all broadly relates; it all agrees that the main rifting event occurred in the late Jurassic and began form the thermally weakened part of the basin which is the area in which the dome had formed prior this means that although the approximate ages all differ it shows that the area where the rifting occurred from was where ‘the Iapetus suture intersected the Tornquist-Teisseyre fault system’. This shows that there is substantial evidence at this site to show that it is the area where the faulting propagated from and that there was further faulting which radiated outwards towards the existing faults this then lead to fault reactivation of the existing faults.

The main conflicts in the research are due to the fact that a majority of the research is quite old so as new technologies were developed it presented different ages and different sequence of events. That being said, a lot of the older research corresponds to the newer research. Another bias could be due to the fact that the same scientists worked in teams on the research so the same team released more than one paper at different times so there could be a bias there due to the fact that they don’t want their point to be proven wrong as it would discredit their previous work. Also, because they have their idea then they might discard certain pieces of information as it wouldn’t correspond with what their school of thought was. On the other hand, a majority of the sources which were used were from peer reviewed journals so they were other specialists in the field agreed with the outcomes.

The three-main theory’s relating to volcanism all differ quite significantly the tensional tectonics theory is shown to be broadly correct due to the volcanism relating to lithospheric extension however there is an issue with where the magmas were formed from as there was no evidence if the magma traveling from the lithosphere was active of passive, it I believed that it was via a passive nature, however because this can’t be proven it doesn’t prove the validity of the theory. With the mantle plume theory, there is evidence which suggests that the plume didn’t begin until the middle of the Jurassic when it’s widely accepted that the plume began to form before then.

The final theory which is the Embryonic spreading axis theory is the most likely out of all the theories since it explains the three main igneous events which occurred down to the timing and the position of them, although the theory doesn’t explain the volcanism which occurred in the other parts of Europe now. However, it seems like this is the most likely as it pin points not only the location but also the timing of the events so that they match the evidence which is observable and the ages which have been proven to be the ages of the rocks. There is the possible explanation for the volcanism in Europe using the embryonic theory and that is the theory that there was a large region in the lithosphere with a high heat flow leading to a wider spread amount of volcanism. This theory aligns with basaltic sills and alkali dykes which are present, as well as corresponding the potassium argon ages of the rocks to what we would expect to see.

Conclusion

In conclusion although some of the material studied contradicts each other a majority of it shows the same general trend, it shows that there was initially a thermal dome which then subsided. The subsidence was likely due to the mantle plume reducing in temperature, as the temperature decreased it lead to the gradual subsidence, the subsidence led to block faulting which lead to rifting the rifts then spread outwards from the thermally weakened lithosphere and propagated along the existing faults which were then reactivated after they were reactivated it caused large scale crustal extension which was in the region of 70km. There was also volcanism which occurred the magma which was produced was and alkali olivine basaltic magma which had 3 main types; an ankaramite-basaltic hawaiite-hawaiite series, analkali olivine basalt-hawaiite series, and anintrusive hawaiite-intrusive mugearite series these magmas are likely to have formed at the spreading axis between Iberia and Newfoundland this volcanism is related to the rifting that happened and also the tectonic spreading.