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Title: Remagnetizations in late Palaeozoic to Early Mesozoic continental sediments of the United Kingdom
Author: Johnson, S. A.
ISNI:       0000 0004 2695 9551
Awarding Body: The University of Birmingham
Current Institution: University of Birmingham
Date of Award: 1993
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This thesis represents the results of a study concerned with the various chemical and thermal processes which produce remagnetizations in continental sediments. Information has been gathered from a number of disciplines; palaeomagnetism, petrography, diagenesis, thermal history modelling. These techniques have been combined to produce a model of the processes by which sediments in particular areas of the United Kingdom have been remagnetized. In southwest Dyfed (South Wales), the Old Red Sandstone (Silurian to Devonian in age) has been remagnetized both chemically and thermally in the Late Carboniferous by fluids precipitated during the Variscan Orogeny. This can be concluded from the palaeomagnetic data which indicate that the remagnetization occurred approximately half-way through the Variscan folding event. In Anglesey (North Wales), the Old Red Sandstone (Lower Devonian) was folded by an earlier event, the Acadian Orogeny (Middle Devonian) and was not greatly affected by the Variscan event which was terminated over 100km to the south. However, the uplift associated with the Variscan event brought the Old Red Sandstone of Anglesey into the realm of oxidising fluids and chemically-precipitated haematite produced a remagnetization which is Permian in age. Fold tests clearly show that the remagnetization post-dates folding in this area. In the southern North Sea, the Barren Red Measures (Westphalian C, Carboniferous) contain chemically-precipitated haematite which was also produced as a result of Permian uplift in the wake of the Variscan Orogeny. However, the main remagnetization is associated with Triassic haematite precipitation produced by lateral flow of fluids along permeable channel sandstone units. The fact that some of the sandstones are remagnetized at this time whilst others are not is perhaps indicative of the interconnectivity of the sandbodies. In southwest Birmingham (Central England) the Keele Formation (Westphalian D, Carboniferous) provides an onshore analogue (in terms of age) to the southern North Sea samples. In this area there is a similar Permian age of remagnetization as that seen in Anglesey, produced as a result of uplift in the Variscan foreland. However, unlike Anglesey, these rocks also contain a primary magnetization which can be isolated from thermal demagnetization experiments. Present day weathering of the Keele Formation has imposed a weak magnetization (probably held within grains of goethite) on those rocks which lie within 10m of the surface, particularly in the more permeable sandstone units. The other effect of the present day weathering is that it tends to remove the finer grains of haematite which tend to be associated with Permian remagnetization. On the Isle of Arran (West Scotland) the New Red Sandstone (Permian and Triassic) has been remagnetized by a number of igneous intrusions which were produced as a result of Lower Tertiary hot spot activity beneath the British Isles. Palaeomagnetic experiments have been compared with computer models of heat flow around a small dyke to show that the remagnetization associated with the intrusions is a product of both thermal and chemical processes. The thermal processes are a product of direct heat flow from the intrusions which thermally remagnetizes pre-existing magnetic grains. However, the chemical processes are driven by convectional flow of fluids through the permeable Permian and Triassic sandstones to precipitate new magnetite and haematite grains. Despite the high level of thermal and chemical activity in this region in the Lower Tertiary, many of the rocks still retain a primary component of magnetization associated with deposition or early post-depositional processes. In summary, the pre-Permian sediments studied in this thesis appear to be particularly prone to remagnetization as a result of Variscan movements and the resulting uplift of southern Britain. On the Isle of Arran Permian and Triassic sediments which post-date the Variscan event are affected by a combined thermal and chemical remagnetization associated with Early Tertiary hot spot activity. The results of this thesis have shown the value of using palaeomagnetic techniques to time remagnetizations in continental sediments. In addition, the results have outlined a number of key geological events since the Devonian which are likely to be responsible for a number of the remagnetizations seen in the rocks of the United Kingdom. Therefore, these results can be used in a predictive manner for future palaeomagnetic studies in the rocks of this country and perhaps even further afield. For example, the effects of the Variscan Orogeny have produced remagnetizations in the rocks of Europe, Africa and north America. It is considered essential that all future work in this field should include the study of the burial history for the sedimentary basins covered as the remagnetizations explained in this work have all been intimately related to particular burial or uplift episodes. In addition, petrographic analysis helps to distinguish the textural phases of the magnetic grains which are responsible for the remagnetizations and are thus also essential elements in the study of remagnetizations in sedimentary basins.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available