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No description available.

No description available.

Microfossil analysis of chalk tesserae from mosaics at five sites in Roman Britain (Caerleon, Colchester, the Isle of Wight, Leicester and London) was undertaken in order to ascertain the biostratigraphical age of the chalk used and thereby to determine its lithostratigraphical position within the Chalk Group. This information was then used to determine its most likely geographical provenance. The foraminiferal evidence presented in this thesis strongly suggests that the source of the chalk used to manufacture the tesserae within the Roman province varied with time. Comparison of the results obtained with previous micropalaeontological analyses of chalk tesserae from Silchester, Norden (Dorset) and elsewhere in London suggest that Dorset may have acted as a regional source of chalk tesserae supply for mosaics dating to the first or early second century AD. This confirms previous suggestions that a ‘geomaterials complex’ was operating in the Poole-Purbeck area of south-east Dorset at this time. Chalk tesserae dating to later periods did not display this same pattern of supply and appear to have been derived from elsewhere in the province. Kent and Sussex are suggested as possible sources for chalk tesserae dating to the second and third centuries AD, whereas Baldock in Hertfordshire emerges as a possible source in the fourth. The geological evidence also shows that harder members of the Chalk Group do not seem to have been preferentially selected for use in tesserae manufacture. The results obtained confirm the value of the ‘microfossil approach’ to the problem of provenance in archaeological studies. It is suggested that the extension of this technique to chalk tesserae from other sites might enable some wider aspects of mosaic manufacture in Roman Britain to be investigated and two areas are put forward for future consideration.

The Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a PGE-Ni-Cu mineralized, layered package of mafic cumulates. This magmatic sulfide deposit is developed at the equivalent stratigraphic position to the Platreef, being overlain by Main Zone gabbronorites and in places resting unconformably on metasediments from the Transvaal Supergroup. Parental magmas to the GNPA member were of a ‘hybrid’ composition containing both B1 and B2/B3 type magma components which were strongly crustally contaminated and S saturated at the time of emplacement. At depth, the assimilation of crustal S was crucial for ore genesis. Although parental magma(s) experienced a second localised contamination event, interaction with the local footwall at the time of emplacement, did not have any control on the genesis of sulfide mineralization. A single primary sulfide liquid, enriched in PGE, Ni, Cu and semi-metals was distributed throughout the succession during multiphase emplacement of the GNPA member. The distribution and mineralogy of platinum-group and chalcophile elements results from the complex behaviour of these elements during both sulfide fractionation and hydrothermal processes. The primary assemblage is characterised by IPGE-rich pyrrhotite, IPGE-, Rh, and Pd-rich pentlandite, chalcopyrite, and associated Pt-As and Pd-Bi-Te minerals. Secondary assemblages in addition contain Pd- and Rh-rich pyrite and millerite, and discrete minerals including Pd antimonides and arsenides. Whilst correlations between the GNPA/Platreef and Upper Critical Zone remain relatively speculative, the northern limb deposits are thought to have formed from compositionally similar or related magmas, which were poorer in Mg, richer in Ca and Fe and Pd dominant relative to the magma(s) that formed the Upper Critical Zone. It is proposed that with depth the Platreef may progressively transform into a layered succession that is exposed south of the Ysterberg-Planknek Fault and represented by the GNPA member. The Platreef can therefore possible be viewed as a marginal facies of the GNPA member, and sulfide-rich magma which escaped up the margins of the northern limb chamber.

A significant challenge to the petrophysical evaluation of shale gas systems can be attributed to the conductivity behaviour of clay minerals. This is compounded by centimetre to sub-millimetre vertical and lateral heterogeneity in formation geological and therefore petrophysical properties. Despite this however, we remain reliant on Archie based methods for determining water saturation (Sw), and hence the free gas saturation (1-Sg) in shale gas systems. There is however significant uncertainty in both how resistivity methods are applied and the saturation estimates they produce, due largely as Archie parameter inputs (e.g. a, m, n, and Rw) are difficult to determine in shale gas systems, where obtaining a water sample, or carrying out laboratory experiments on recovered core is often technically impractical. This research assesses the geological implications for, and controls on, variations in pseudo Archie parameters in the Bossier and Haynesville Shale Formations in the northern Gulf of Mexico basin. Investigation has particularly focused on the numerical analysis and systematic modification of Archie parameter values to minimise the error between core SW (Dean Stark analysis) and computed Sw values. Results show that the use of optimised Archie parameters can be effective in predicting SW, particularly in the Haynesville formation, but identifies systematic bias in generated Archie parameters that precludes their accurate physical interpretation. Analysis also suggests that variability in the resistivity (Rt) log response is the principal source of error in Sw estimates in the Bossier Shale. Moreover, results suggest that where clay volume exceeds 28%, the resistivity response becomes increasingly variable and elevated, indicating an apparent clay associated ‘excess resistivity’. This is explained by a geologically consistent model that links increasing clay volume to bulk pore water freshening, supported by empirical adaptations that allow for improved Archie parameter selection and a further reduction in the error of Sw estimates.

Presented here is the shear velocity structure of the crust and upper mantle beneath the central and southern Atlantic Ocean from inversion of high resolution group velocity tomography. The path average group velocities from Rayleigh waves were picked using multi filter technique and phase match filtering for 14,000 paths. They were then combined within a tomographic inversion, to obtain the regional variations of velocity structure at a range of short to intermediate periods (14 s - 100 s). These group velocities have depth sensitivities from the surface to approximately 90 km depth, constraining the focus to velocity variations within the crust and mantle lithosphere. Tomographic results highlight short wavelength variations at periods sensitive to shallow depths, implying the possibility for a more complex velocity structure than currently expected for the oceanic region. The results show a clear relationship between increasing group velocities and increasing sea floor age. Group models are then inverted to obtain the shear velocity structure with respect to depth. The shear velocity model highlights slow velocities beneath the ridge, interpreted as the upwelling of asthenosphere between depths between 30 km and 50 km. Models of crustal and lithospheric thickness are extrapolated from the data. These models suggest the evolution of the Atlantic Ocean is more complex than the simple mathematical cooling models. It is suggested that the main control on crustal thickness is tectonic processes associated with the slow spreading rate and not controlled by to the mantle potential temperature. Additionally, results are presented which incorporate 2 azimuthal anisotropy in the tomographic inversions. At the longest periods test show that the recovered anisotropy is an artefact of the inversion process, and cannot be interpreted in terms of mantle flow. At the shortest periods there is a possible relationship between the fast direction and the stress field.

The taxonomy of benthic foraminifera recovered from 160 core samples over depth 20.59 m and representing the past ~597 Kyr (sedimentary deposit of the Tarantian and Ionian stages) at Bowers Ridge, Bering Sea IODP site U1342, includes some 52 species from 41 genera and 22 families: these species are given formal taxonomic treatment with detailed illustration. A further 16 species are discussed in open nomenclature. Foraminifer assemblages are dominated by species of Takayanagia delicata, Uvigerina bifurcata, Islandiella norcrossi and Alabaminella weddellensis accounting for more than 58 % of all specimens recovered. In addition to their taxonomic identification, this study interrogates the relationship between benthic foraminifera and interpreted changes in oxygen concentration and productivity during a series of late Quaternary glacial– interglacial cycles at site U1342. In particular, the species Bolivina spissa (Cushman) – thought to record changes in seabed oxygen level, shows no correlation between test pore density and interpreted bottom water oxygen level (BW-O2), suggesting that oxygen is not the sole driver influencing the distribution and morphology of this species. Further assessment of the total foraminiferal assemblages at site U1342 - using the sedimentological context and the proportion of deep infaunal species as a proxy for low oxygen conditions at the seabed, and shallow infaunal species as a proxy for a welloxygenated seabed, identify eight broadly defined temporally successive benthic foraminiferal intervals through the sampled core. Three of these intervals, between depths 0 - ~5.40, ~ 6.50 - 9.00 and ~16.90 - 18.67 m-CCSF, signal a well-oxygenated sea bed, whilst the other five intervals suggest increased phytodetritus flux to the seabed, coupled with variations in seabed oxygen level. In general, there is no clear connexion between these intervals and the glacial-interglacial oscillation at the site during the past ~597 ka, suggesting that ecological influences on foraminiferal distribution at Bowers are complex.

The early Turonian-early Maastrichtian planktonic and benthonic foraminiferal faunas of the Kurdistan region, NE Iraq are evaluated. Some 93 planktonic foraminifera (24 species of the family Heterohelicidae are described in detail) from the Kometan and Shiranish formations, and 115 benthonic foraminifera from the Shiranish Formation are identified and illustrated. The fauna reveals new and important data bearing on the palaeontological, biostratigraphical and palaeoenvironmental significance of the Late Cretaceous successions in the Kurdistan region, NE Iraq. Based on the planktonic foraminiferal assemblages, nine biozones and two subzones spanning the early Turonian to late early Maastrichtian were identified. Sequential changes in planktonic foraminiferal assemblages map discrete intervals within the Kometan and Shiranish formations that suggest dominantly warm, nutrient-poor marine surface and near-surface conditions during the mid-Turonian to late Coniacian, latest Santonian, and late Campanian, and cooler more nutrient-rich surface and near-surface waters in the early Turonian, early to late Santonian, early Campanian and early Maastrichtian. Species of the benthonic foraminiferal genus Bolivinoides provide a refined biostratigraphic biozonation for the late Campanian to early Maastrichtian Shiranish Formation in the Kurdistan region, NE Iraq. Three biozones and two subzones are identified for the first time in Iraq: the Bolivinoides decoratus biozone (late Campanian) subdivided into a lower B. decoratus subzone and an upper B. laevigatus subzone; the B. miliaris biozone (earliest Maastrichtian); and the B. draco biozone (late early Maastrichtian). Combined, the Bolivinoides and planktonic foraminiferal biostratigraphy enables the informal recognition of lower and upper intervals within both the Globotruncana aegyptiaca and Gansserina gansseri planktonic foraminifer biozones that may be important for a more refined inter-regional correlation in the Middle East and North Africa. The new Bolivinoides biozonation precisely locates the Campanian-Maastrichtian boundary in NE Iraq. The benthonic foraminiferal assemblages are interpreted as representing outer shelf to middle slope environments, between ca 200 m to at least 600 m water depth, with maximum water depths in the late Campanian, followed by shallowing into the early Maastrichtian. The most likely control mechanism for the shallowing of sea level is the beginning of southern Neo-Tethys Ocean closure at this time.

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