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.

The country between Lake Baringo and the Kerio River in the northern part of the Kenya Rift Valley is dominated by the Tugen Hills, a sector of the Kamasia Range, which passes from north to south across the western side of the area mapped. The hills are faulted and uplifted and expose about 7,000 to 8,000 feet of phonolites and tuffaceous sediments with subordinate basanites (the Tugen Hills Group of Miocene to,mid-Pliocene age) resting on an older sedimentary formation and metamorphic basement. Widespread flood trachytes follow the Tugen Hills Group, and these are over lain, mainly to the east of the Tugen Hills, by the Kaparaina Basalts whose lavas formed a shield volcano towards the north of the area. They are of upper Pliocene age. Thick fossiliferous sediments of Plio-Pleistocene age follow the basalts, and they accumulated in tectonic basins to the east of the Tugen Hills. Eruption of later phonolites, trachytes, basalts and mugearites occurred in the east of the area. The area is shown to reveal a wide variety of structures consistent with tensional tectonics. These include tilted blocks, collapsed and extended upwarps, and recent tension gashes. A tectonic history is described consisting of both major and minor faulting episodes, occurring more frequently than has been established elsewhere in the Kenya Rift. They are interspersed with periods of subsidence involving sedimentation. Uplifts are shown to be reflected in erosional rejuvenations. The petrography of the thick and varied sequence of lavas is described in an appendix at the end of the thesis.

Tunnel-valleys are morpho-stratigraphical entities within glaciated sedimentary lowlands, which record large-scale channelised drainage beneath ice sheets. The southern North Sea Basin contains large examples, locally over 400 m in relief, cut and filled during the maximum extent of the mid-Pleistocene Elsterian glaciation. These are examined over a 100 x 150 km area of the UK/Dutch sector using seismic reflection and downhole data, collected mainly in the course of hydrocarbon exploration. The reflection dataset comprises over 12,000 line-km of profiles (≤1 km grids), and a 3D-seismic volume over a 39 x 22 km area. New information on basal morphology and fill stratigraphy is used to support a model of time-transgressive formation by en- and sub-glacial drainage beneath the ice margin during its deglacial recession. Evidence for basal erosion to the south, erosional overlap to the north, and glaciofluvial fill progradation to the north are reconciled in a model of contemporaneous headward excavation and backfilling or elongate basins during ice margin recession. This resulted in an axially diachronous valley base and fill sequence, younger to the north. Clinoform surfaces (sequence I) are identified as sub-marginal <I>backsets</I>, not previously observed at such a scale. They are explicable as subaqueous outwash from a distributed system of subglacial streams feeding grounding line fans, within the outer 5-20 km of ice-filled basins. Overlying surfaces (sequence II) record proglacial outwash of sands, and increasingly distal accumulation of muds, in lake basins which elongated with ice recession and persisted until the interglacial marine transgression. The physical characteristics of the tunnel-valleys indicate reworking of Cenozoic sedimentary materials (most of which remain within them) by drainage beneath the outer 50 km of the former ice sheet margin during its recession. Their size and spacing in relation to substrate thickness, and their arborescent geometry, are consistent with non-catastrophic models of tunnel development as a form of stable subglacial drainage over and through deformable aquifers. Sub-marginal drainage is dominated by surface supply, which exceeds basal melting by two orders of magnitude or more during deglaciation. Tunnel-valleys are therefore argued to form in response to both substrate characteristics and englacial hydrology during deglaciation, in a marginal zone which migrates with the ice sheet as it melts and passes through its own margin.

No description available.

Carbon (C) and nitrogen (N) burial within new intertidal and saltmarsh sediments from the Blackwater estuary, Essex were investigated. These sediments were created as part of a 'managed realignment' of coastal sea defences in the East of England to address issues such as loss of intertidal habitat, the effects of relative sea level rise and the unfeasibility of maintaining aging sea defences. The fluxes of greenhouse gases (CH4 & N20) from the sediments were quantified using non-steady state chambers and their ability to offset a portion of the C burial to give net carbon sequestration was investigated. C and N contents in natural intertidal sediments were higher than in managed realignment (MR) sediments and comparable to saltmarsh sediments from around the East coast of England. Mature MR sites possessed C and N burial rates at least as great as natural marshes and if increased sedimentation in these predominantly low lying intertidal areas is accounted for, the mature MR sites far outstrip natural marsh C burial rates. Less mature, MR midmarsh areas had lower C and N burial rates more inline with those found in intertidal mudflats. Both natural and MR intertidal areas were small sources of the powerful greenhouse gases CH4 (0.10-0.40 g m'2 y(l) and N20 (0.03-0.37 g M-2 y('). These gas fluxes reduced net C sequestration within the MR marshes by as much as 49%, but by only 2% from natural saltmarshes. The current C sequestration of Blackwater estuary managed realignment sites is -690 tonnes of CO2eq yr' (carbon dioxide equivalents). If the total area identified as potentially suitable for MR to take place is reverted back to intertidal area this will sequester -10200 tonnes of CO2eq yr' and -440 tonnes of N per year. Another -480 tonnes N will be removed through denitrification and -80-320 tonnes P yr"' will be buried within the new intertidal areas.

The sediment sources and yield of the Jinshajiang basin, the largest tributary in the upper Yangtze basin are of major significance for the exploitation of the water resources of China's largest river. At Pinshan, the outlet gauging station, the area of the Jinshajiang catchment is 485,500 km2 and the mean annual discharge is 4430 m3 s- I or 32.1 % of that at Yichang, just below the Three Gorges Dam site. The mean annual sediment load at Pinshan of 247 million tonnes is 47.1 % of that at Yichang. The sources and delivery of sediment in the Jinshajiang are assessed in the framework of a catchment system which looks at the interplay of hydrometeorological, terrain, vegetation, and human land use factors. The key factors in the present pattern of soil erosion and sediment delivery are: rainfall, topography, slope characteristics, vegetation cover, land use, soil properties and the duration and areas of storm cover. The main sediment sources are in the lower sector of the Jinshajiang, particularity - the reach from Dukou to Pinshan, with a local catchment area of 61,897 km2 : L" , an average annual runoff of 25.88xlO9 m3 and an average annual sediment load of 54.6 million tonnes. These quantities mean that this reach representing only 12.8 % of the area above Pinshan, supplies 18.1 % of the runoff and 62.6 % of the sediment load. The high sediment load comes from both the steepness of the terrain and the intensity of agricultural land use. No clear time trend in the sediment yield of the Jinshajiang emerges. Some tributaries have decreasing yields, perhaps due to sediment deposition in reservoirs, while others exhibit an increasing trend, in some cases due to increased agricultural pressure on the land, and also to construction and engineering activities which cause high sediment yields for short periods of time at individual localities. The pattern of change sediment discharge has been modelled using a distributed rainfall: runoff and runoff: sediment model using data on precipitation, evaporation and proportion of forest cover (land use). The model has been tested against the daily discharge and sediment yield of the 3074 km2 Heishuihe catchment and the 1607 kM2 Meigu catchment.

The Upper Old Red Sandstone in the south west Scottish Borders was deposited as a fluviatile sequence in an interior alluvial basin named the Border Basin. High and low sinuosity streamu deposited sandstones and pebbly sandstones as point bars and channel bars. Thick flood deposits accumulated in persistent floodbasins. The streams flowed from the south west and brought in detritus from sourcelands in Galloway. A coarsening-up trend within the Upper O. R. S. may have resulted from increased stream gradients caused by gradual isostatic uplift in the hinterlands. The overlying Tournaisian sediments in the Langholm and Newcastle areas were deposited in the newly formed and subsiding Northumberland Basin. The Whita fluvio-deltaic system, draining the southern Uplands to the north, dominated sedimentation during the early part of the Tournaisian. Interior and marginal (coastal plain) fluviatile facies occur in the Langholm area and deltaic facies occur in the Bewcastle area. In the upper Tournaisian the Bcwcastle deltaic system, flowing from the north east, was dominant. In both delta systems periods of delta advance, during which pro-delta, delta front and on-delta facies were deposited, were followed by delta abandonment, slow subsidence and gulf carbonate sedimentation. Carbonate sedimentation included stromatolite and serpulid growth and formation of a range of marginal and offshore marine facies. Objective carbonate facies divisions are made using Q-mode cluster analysis. Tournaisian stromatolites are composed of detrital/micrite laminae formed by filamentous blue-green algae and calcareous algal filament laminae formed by encrusting Codiacean algae. biostromes with dome, blister, polygonal, pillar and club growth forms grew in shallow subtidal through to intertidal environments. Biostromes with blister, cusp and crinkle growth forms grew in high intertidal and supratidal environments with gypsum crystals and non-ferroan dolomites forming in the shallow subsurface. Bioherms and oncolitcs grew in sheltered subtidal and agitated shoal water environments respectively. Serpulid biofacies occur as autochthonous biostromes, bioherms and a single reef. Thu Birrenswark Lavas, which separate the radically different Upper O. R. S. and Tournaisian palaeogeographies, provide the key to the initiation and origin of the Northumberland Basin. These bazalts show evidence of fissure eruption and probably formed by partial melting of the upper mantle along the northern basin margin. Partial melting caused upwarp, tensional stress and basin subsidence by normal faulting. Substidence established a southerly in the central Southern Upland: and enabled the plantaion of the Southern Uplands by the Lower Visean.

Three sandstones of varying complexity have been examined with the aim of quantifying the effects of compaction and quartz cementation upon their diagenetic evolution. Emphasis has been placed on the scales of heterogeneity, determination of the physical controls on cement distribution, porosity and permeability. The shallow marine, Fontainebleau Sandstone provides a relatively simple system in which to examine progressive porosity and permeability reduction as a result of compaction and quartz cementation. Cement was precipitated as lenses in response to ground water outflow during the recent morphological development of the Paris Basin. Conventional modal and textural analyses using transmitted light proved difficult, because grain/overgrowth boundaries are poorly defined. However, combined SEM BSE CL and image analysis enabled accurate quantification of detrital and authigenic quartz, minus-cement porosity and intergranular porosity. Analyses indicate an extremely mature sand in terms of both mineralogy (100% quartz) and texture (Fine Upper, Well Sorted). Minus-cement porosity values are high (-33%bv) indicating that cementation occurred at a shallow depth. The degree of silicification is the dominant variable control on permeability. Helium porosity and liquid permeability measurements are in the range of 8.6% and 114mO to 26% and 60, the data agree with predicted values. The distribution of quartz cement, in relation to the Eden Valley Basin structure and stratigraphy, for the Penrith Sandstone has been examined via literature review, fieldwork and aerial photographs. Quartz cement is found dominantly north of Cliburn and towards the top of the formation; the base of the formation lacks quartz cement and has undergone greater pressure solution. Models for convective fluid flow are proposed to account for the distribution of cement observed. The Bowscar dune study examines how quartz cement influences dune-scale diagenetic evolution within a transverse aeolian dune. The Locharbriggs dune is used to compare porosity and permeability characteristics. SystematiC sampling of cores cut parallel to lamination and collected horizontally and vertically over the preserved dunes ensured adequate and unbiased sampling. Modal and textural analysis of the Bowscar dune indicate that dune-scale controls on quartz cement distribution include effects of primary depOSitional fabric, detrital mineralogy and compaction. Porosity and permeability measured from the cores give a three-dimensional (3-~) analysis and demonstrates lateral and vertical variations associated with the distribution of lamination types within the dunes. At the lamina scale, 3-D geometrical quantification of detrital framework, quartz cement, minus-cement and intergranular porosity networks has been achieved using preciSion serial grinding/polishing, coupled with SEM BSE CL imaging and 3-~ reconstruction techniques.

The Lower Cutler Beds (Cutler Group) is a mixed siliciclastic-carbonate Pennsylvanian-Permian age succession deposited in the Paradox foreland basin, southeast Utah. This research performed a detailed and high resolution sedimentological and stratigraphical analysis in order to answer a series of currently unresolved questions regarding the response of coastal desert systems to changes in stratigraphic controls and how they are reflected in the expression of a mixed aeolian, fluvial and shallow marine sedimentary succession. The Lower Cutler Beds is an ancient coastal desert system which underwent periodic shutdown, deflation and inundation from the northeast of the basin by fluvial systems derived from the uplifting Uncompahgre Hinge, and was periodically transgressed by a shallow sea to the west. The development of a new facies scheme has been used to document the transition of various sub-environments from the continentally-dominated deposition in the most northern study regions (Lockhart Basin and Shafer Basin) to marine-dominated deposition in the most southern study regions (Canyonlands District and Grabens District). Complex architectural interactions between aeolian, fluvial, shallow marine and nearshore facies associations have been related to autocyclic and allocyclic control mechanisms, and have allowed the development of depositional models accounting for the architectural expression. Field data have been used in the production of both a regional stratigraphic computer model and a detailed reservoir model for a mixed shallow-marine, aeolian and fluvial succession, and accurately portrays the system in three-dimensions. Various different applications of computer modelling techniques are presented and examined. Sedimentaryd epositsf rom both marinea ndn on-marines uccessionas rer ecognised to be the result of complex interactions and feedbacks between climatic, eustatic and tectonic controls. Stratigraphic analysis has enabled units of genetically-related packages of sediment to be correlated from continental facies types to their contemporaneous marine counterparts. A series of conceptual models that account for system accumulation and preservation have been developed with each model being related to specific climatic and eustatic episodes. Sea level lowstands equate with more and periods which encouraged aeolian accumulation and preservation. Rising sea levels resulted in erg shut down and the generation of a deflation surface. Deposition of the Lower Cutler Beds is thought to be the expression of high frequency sea level variations driven by glacio-eustatic cycles superimposed on a longer-term tectono-stratigraphic cycle.

No description available.