Three independently measured variables (magnetic susceptibility, photoelectric factor and total gamma-ray) obtained from throughout the type Kimmeridge Clay Fm in Dorset (Southern England) were used to identify regular metre-scale, sedimentary cycles. Spectral analysis demonstrates that for long stratigraphical intervals the cycles are expressed as large-amplitude cycles of 1.87–4.05 m wavelength and smaller-amplitude cycles of around half that wavelength. These cycles are interpreted to record orbital obliquity and precession, respectively. The much larger amplitude of the inferred obliquity cycles compared with the precession cycles may indicate a high-latitude climatic forcing transferred to lower latitudes via sea-level variations. Orbital tuning indicates that the Early Kimmeridgian (
sensu anglico
) lasted at least 3.6 Ma (95 longer-wavelength cycles) and the Late Kimmeridgian at least 3.9 Ma (103 longer-wavelength cycles). The first detailed productivity estimates for the Kimmeridge Clay Fm, on a cycle-by-cycle calculation, indicate that average productivity of the type Kimmeridge Clay (220 g m
−2
a
−1
) was less than the average productivity on modern continental shelves. The high average organic carbon content of the type Kimmeridge Clay (3.8% total organic carbon) cannot be attributed to high average productivity. However, the average organic carbon content is consistent with low siliciclastic mineral dilution of organic matter and/or elevated preservation linked to reduced bottom-water oxygenation.
A sequence stratigraphic framework for the late Jurassic Kimmeridgian and Bolonian stages of the Wessex–Weald Basin, southern England is proposed, based on the integration of sedimentological, geophysical and geochemical data. The NERC-funded Rapid Global Geological Events (RGGE) boreholes of Swanworth Quarry 1 and 2, and Metherhills 1, are used as reference sections. Eleven complete depositional sequences and their component systems tracts are recognized within the Kimmeridge Clay Formation, bounded by 12 sequence boundaries (Km1–12). Seventy-four boreholes have been used in this study. During the Kimmeridgian, a major transgression, associated with tectonic movements, led to a marked change in basin geometry. Major deepening of the sea during the late Kimmeridgian (Eudoxus Zone) led to a change from unconformities of basinwide extent to those confined to the basin margins. The progressive deepening of the basin towards a highstand in the mid-Bolonian (Wheatleyensis to Pectinatus zones) was associated with a broad-scale change in sedimentary, faunal and geochemical characters. The ‘layer-cake’ basin architecture and thickening of the Kimmeridge Clay Formation towards the basin centre suggests a dominantly aggradational system. The partially enclosed nature of the Kimmeridgian and Bolonian seas generated basin dynamics more analogous to those of modern day meromictic lakes. Consequently, sequence stratigraphic approaches derived from continental shelf-margins are inappropriate in epicontinental basins of this type.
Synopsis The United Kingdom Government offered a large area on Middle Chalk near Thetford in Norfolk a possible site for a giant proton accelerator for CERN. This 300 GeV machine has very stringent requirements for the ground stability and deformation under load which are outside normal civil engineering experience and practice. The Paper describes how the suitability of the site in meeting these very exacting specifications was assessed. The load-deformation properties of the chalk mass were measured directly in the field by means of three inter-related methods rather than by means of laboratory tests on rock cores. First, the chalk was visually described and classified in situ into a series of five grades taking into account those features likely to influence its load-deformation behaviour. This was done by means of over 80 large (man-sized) auger holes spread over the site. Second, these grades were quantified in terms of load-deformation behaviour at one location by means of a full-scale loading test, using a water tank 18·3 m in diameter and weighing 4500 tonnes. Third, alongside the tank and at two other points on the site, the deformation properties of the grades were measured by means of sets of plate-loading tests (0·86 m diameter) carried out at various depths at the bottom of auger holes. These three methods together enabled the deformation properties of the whole site to be mapped from a visual description of the chalk. It is shown that geotechnically the site is acceptable for the construction of the large machine. Le Gouvemement du Royaume-Uni a offert une zone étendue sur du calcaire moyen près de Thetford dans le Norfolk comme emplacement possible pour un accélératein à protons géant pour CERN. Cet appareil de 300 GeV présente des exigences trbs rigoureuses relatives à la stabilité du sol et à la déformation sous l'effet de charges qui n'entrent pas dans l'expérience et la pratique des travaux publics normaux. L'exposé décrit comment fut évaluée la convenance de l'emplacement pour qu'il réponde a ces spécifications très exigeantes. Les qualités de déformation sous l'effet des charges de la masse calcaire furent mesurées direct-ment sur place au moyen de trois méthodes liées entre elles plutôt que par des essais de laboratoire sur des sondages de roches. Premièrement, le cal-caire fut décrit visuellement et classé sur place en tenant compte des caractéristiques qui auraient probablement une influence sur son comportement relativement à la déformation sous l'effet des charges. Ceci fut effecté au moyen de plus de 80 sondages à la tarière de grande taille (d'homme) dispersés sur I'emplacement. Deuxièmement, ces catégories furent mesurées par rapport à leur comportement relativement à la déformation sous l'effet des charges en un lieu au moyen d'un essai de charge de grandeur naturelle, en utilisant un réservoir d'eau de 18,3 m de diamètre et d'un poids de 4500 tonnes. Troisièmement, le long du réservoir et a deux autres endroits sur l'emplacement, les qualites de deforma-tion des catégories furent mesurées au moyen de séries d'essais de charges sur plaque (de 0,86 m de diamètre) exécutés a différentes profondeurs au fond des sondages à la tarère. Ces trois méthodes utilisées ensemble permirent de créer une carte des qualites de deformation de tout l'emplacement a partir d'une description visuelle du calcaire. Il apparait que du point de vue geotechnique l'emplacement est acceptable pour y construire l'appareil de grande taille.
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