Maxine Akhurst

Sediment drifts and contourites on the continental margin off northwest Britain

Abstract Seismic reflection profiles and short cores from the continental margin off northwest Britain have revealed a variety of sediment-drift styles and contourite deposits preserved in the northeast Rockall Trough and Faeroe-Shetland Channel. The sediment drifts include: (1) distinctly mounded elongate drifts, both single- and multi-crested; (2) broad sheeted drift forms, varying from gently domed to flat-lying; and (3) isolated patch drifts, including moat-related drifts. Fields of sediment waves are locally developed in association with the elongate and gently domed, broad sheeted drifts. The contrasting styles of the sediment drifts most probably reflect the interaction between a variable bottom-current regime and the complex bathymetry of the continental margin. The bulk of the mounded/gently domed drifts occur in the northeast Rockall Trough, whereas the flat-lying, sheet-form deposits occur in the Faeroe-Shetland Channel, a much narrower basin which appears to have been an area more of sediment export than drift accumulation. Patch drifts are present in both basins. In the northeast Rockall Trough, the along-strike variation from single- to multi-crested elongate drifts may be a response to bottom-current changes influenced by developing drift topography. Muddy, silty muddy and sandy contourites have been recovered in sediment cores from the uppermost parts of the drift sequences. On the basis of their glaciomarine origin, these mid- to high-latitude contourites can be referred to, collectively, as glacigenic contourites. Both partial and complete contourite sequences are preserved; the former consist largely of sandy (mid-only) and top-only contourites. Sandy contourites, by their coarse-grained nature and their formation under strongest bottom-current flows, are the most likely to be preserved in the rock record. However, the very large scale of sediment drifts should be borne in mind with regard to the recognition of fossil contourites in ancient successions.

Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland

The Silurian–Ordovician Southern Uplands terrane occupies a key position in the Caledonian orogen, yet its genesis is controversial. Marginal-basin, backarc, and forearc tectonic regimes have all been invoked as operative at the Laurentian margin of the Iapetus Ocean. Fresh andesitic detritus within turbidite sandstones has, until now, been assumed to provide evidence for an Ordovician suprasubduction volcanic arc, a central feature of most models. However, high-precision thermal-ionization mass spectrometer U-Pb and laser-ablation data for detrital zircons from the sandstone prove Neoproterozoic volcanism at 557 ± 6 Ma (2σ) and probably also at 613 ± 12 Ma (2σ). The complex crystallization history recorded by the zircons shows assimilation of 1043 ± 7 Ma (2σ) Grenvillian basement into the andesite magma. The fact that no zircons have been found having ages that overlap the Caradocian depositional age of the host sedimentary rocks undermines all extant terrane models. The age profile of the detrital zircons is typical of Gondwana and Avalonia. This finding has important implications for the paleogeography of the Iapetus Ocean during the Ordovician, because the zircon data require the introduction of Avalonian detritus into a sedimentary basin marginal to Laurentia.

Sediment drifts and contourite sedimentation in the northeastern Rockall Trough and Faroe-Shetland Channel, North Atlantic Ocean

Abstract Seismic reflection profiles, shallow cores and seabed photography from the continental margin off NW Britain reveal the variety of bottom current influenced sedimentation in the northern Rockall Trough and Faroe-Shetland Channel. Types of sediment drifts identified include: (1) elongate drifts, both single, and multi-crested; (2) sheeted drift forms, varying from gently domed to flat-lying; and (3) isolated patch drifts, including moat-related drifts. Associated fields of localized sediment waves are developed with the elongate and gently domed, broad sheeted drifts. The contrasting style of sediment drift development reflects the complex interaction between bottom current regime, sediment supply and the bathymetry of the continental margin. The majority of the mounded/gently domed drifts occur in the northern Rockall Trough, with sheetform drifts commonly confined to the Faroe-Shetland Channel, a narrow basin which is an area of net sediment export rather than drift accumulation. Small patch drifts are present in both basins. Muddy, silty muddy and sandy contourites have been recognized from sediment cores sampling the uppermost parts of the drift sequences. Based on their glaciomarine character, the mid- to high-latitude contourites are referred to as glacigenic contourites. Both partial and complete con–tourite sequences are preserved; the former consist largely of sandy (mid-only) and top-only contourites. Modern sandy con–tourites have also been identified from seabed photographs on the Hebrides Slope. The contourites are recognized as a rippled mobile sand layer, reworked from a poorly sorted glaciomarine parent deposit.

Evaluation of the CO2 Storage Capacity of the Captain Sandstone Formation

The volume of CO2 that can be stored in the Captain Sandstone formation in the North Sea was investigated by building a geological model and performing numerical simulations. These simulations were also used to calculate the best position for the injection wells, and the migration and ultimate fate of the CO2. The overall migration of CO2 and the pressure response over the entire formation was studied by the calculated injection of 15 million tonnes CO2 per year. The injection rate was restricted to a maximum of 2.5 million tonnes CO2 per year for each of a possible 15 wells considered. An important objective was to predict how to avoid flow of the injected CO2 toward potential leakage points, such as the sandstone boundaries and faults. The migration of injected CO2 towards existing oil and gas fields was also a determining factor. The summary conclusions are: - The Captain Sandstone formation has significant potential CO2 storage capacity. Even with all boundaries closed to flow, the probable storage capacity is calculated to be about 358 million tonnes, giving a storage efficiency of 0.6% of pore volume, with an expected operating life-span of 15-25 years. - The possible storage capacity of the formation may be at least four times greater if the aquifer boundaries are open. This increase would be a result of displacement of salt water, and not CO2. - The storage capacity if the sandstone is closed to flow may be increase from 358 to 1668 million tonnes of CO2 by significant additional investment in 15 to 20 water production wells. - Injection of up to 2.5 million tonnes CO2 per year in one well has an impact on the pressure throughout the entire formation, and thus interference between different injection locations must be considered.

Late Quaternary glacigenic contourite, debris flow and turbidite process interaction in the Faroe-Shetland Channel, NW European continental margin

Abstract The Faroe-Shetland Channel is an important conduit or gateway for the southward flow of cold bottom waters formed in the Norwegian Sea. This Norwegian Sea Overflow Water (NSOW) finds several spillover channels across the Wyville-Thomson Ridge, eventually descending into the northern Rockall Trough and Iceland Basin. The Neogene channel floor succession predominantly displays a broad sheeted drift geometry. Bottom current scours and channels were apparently inherited from an episode of enhanced bottom current activity in late Oligocene/early Miocene. The late Quaternary channel-floor succession is dominated by distal glaciomarine sediments, derived from the shelf and slope during glacial stages and mostly transported by ice-rafting. Glacigenic debris flows and minor turbidity currents were also active across the slope region. Consequently, the principal channel-floor facies are glacigenic contourites that show extensive bioturbation, rare primary structures, mixed composition and marked grain size variation. These features indicate the important influence of cyclical fluctuations in bottom current velocity throughout both stadial and interstadial or interglacial periods. However, the concentration of sandy contourites, erosive surfaces and top-only contourites during interstadials/interglacials and during phases of marked cooling or warming testify to the significance of climate-control on contourite deposition.

A new high-resolution aeromagnetic dataset over central Ayrshire: insights into the concealed geology

Synopsis High-resolution aeromagnetic data have been acquired over central Ayrshire as part of a multi-component environmental and resource survey. The data were recorded along easterly orientated flight lines at 200 m separation and show a very marked improvement in resolution over the pre-existing aeromagnetic data for the region. The distribution of the extensive volcanic and intrusive igneous rocks in the near sub-surface is defined more accurately, many new features are recognized and features distinguished in the pre-existing aeromagnetic data are now seen to be laterally continuous. Northwesterly trending lineaments are clearly defined and most can be attributed to Palaeogene dykes, the majority of which are reversely magnetized. Depth to source solutions and modelling suggest that even though the dykes have a limited width at outcrop, their widths increase with depth. Permo-Carboniferous easterly trending dykes are not well resolved in the new aeromagnetic data. This is most likely due to a low amplitude resultant magnetic vector that, combined with the easterly-orientated flight lines, results in few large anomalies. One significant northwesterly trending lineament is interpreted as the trace of the Cleveland Dyke across the Midland Valley. These new data should be of value to the extractive and water industries and assist in the planning and sustainable management of these resources.

An unlikely evolutionary lineage: the Rhuddanian (Silurian, Llandovery) graptolites Huttagraptus? praematurus and Coronograptus cyphus re-examined

Synopsis The Rhuddanian (Llandovery, Silurian) monograptid taxa praematurus Toghill and cyphus Lapworth are reassessed by review of existing and new collections. The eponymous species of the late Rhuddanian cyphus Biozone is now widely regarded as a typical Coronograptus, with essentially simple tubular thecae. Nevertheless, Lapworth, in drawing up his original description, was clearly referring mainly to biform graptolites of the Monograptus revolutus group, prominent on the type rock slabs. Nomenclatural stability is best served by referring to cyphus in its common usage, i.e. in the interpretation of Elles & Wood, largely based upon Lapworth’s material from Dob’s Linn, Scotland. The monograptid praematurus, a mid-Rhuddanian taxon, was originally described as a variety, and evolutionary precursor, of Coronograptus cyphus. However, it possesses more complex, geniculate, thecae, with well-developed thecal hoods, and probably represents one of the earliest Huttagraptus species. Huttagraptus? praematurus has a range restricted to the lower atavus Biozone and may be a subzonal marker for that interval. The roots of Coronograptus are obscure, but, in possessing essentially simple tubular thecae, it might be a separate offshoot from an Atavograptus stock.

Discriminating faunal assemblages and their palaeoecology based on museum collections: the Carboniferous Hurlet and Index limestones of western Scotland

Synopsis Historical collections of Scottish Carboniferous macrofossils stored at the British Geological Survey (BGS), Edinburgh include the sole remaining sources of palaeontological data from numerous localities. Exploratory numerical analyses of such collections from the Hurlet and Index limestones of Ayrshire compare favourably with published qualitative assessments of faunal assemblages and palaeoenvironments; demonstrating that old collections can still be used in modern palaeoecological investigations. Macrofaunas from these formations comprise mainly brachiopods and molluscs and were collected from 67 localities that yielded 20 and 94 samples from the Hurlet and Index limestones respectively. Limitations of the presence/absence data were partly overcome by consolidation and restriction of aspects of the data set. Seriation indicates the lithological and environmental gradients of taxa. Cluster analysis reveals groups of samples linked to lithofacies. Principal Components Analysis (PCA) of diversity data derived from the data set in terms of numbers of genera in higher taxa highlights differences in gross taxonomic composition in terms of trophic structure, lithology and environment. Supplementary material: lists of localities taxa and sample lithologies used in this study are available at http://www.geolsoc.org.uk/SUP18401 .

Towards understanding the Dumfries Basin aquifer, SW Scotland

Abstract The Dumfries Basin aquifer supports groundwater abstraction for public supply, agriculture and industry. Abstraction is concentrated in the western part of the basin, where falling groundwater levels and deteriorating water quality both reflect the effects of intense pumping. There are two bedrock units: a predominantly breccia-coarse sandstone sequence in the west, interfingering with a predominantly sandstone sequence in the NE and east. The basin is bounded by weakly permeable Lower Palaeozoic rocks, and is largely concealed by variable superficial deposits. Surface water flows onto the basin from the surrounding catchment via the Nith and the Lochar Water and their respective tributaries. Direct rainfall recharge occurs via superficial sands and gravels, especially in the north, and discharge is predominantly to the rivers in the central area rather than the sea. A picture is developing of two main aquifer types within the basin: the high-transmissivity western sector underlain by a fracture-flow system with younger water and active recharge and a high nitrate content, compared with the east where groundwater residence times are longer and the storage capacity is higher.

Trophic structure and spatial distribution of macrofaunas in the Hurlet and Index limestones (Carboniferous : Upper Viséan and Lower Serpukhovian) of Ayrshire, Scotland

Synopsis Explorative multivariate numerical analysis of British Geological Survey historical Biostratigraphy collections from the Carboniferous Hurlet and Index limestones, Ayrshire, yields results capable of palaeoecological interpretation. The faunas are distributed along environmental gradients within carbonate facies that systematically extended out into other sedimentary settings. Clusters of genera in higher taxa plotted on ternary diagrams of trophic structure show both units commonly include epifaunal suspension feeders, although one cluster from the Hurlet Limestone includes epifaunal detritus-suspension feeders and another from the Index Limestone includes vagrant-epifaunal detritus-suspension feeders. All the clusters include the surficial and semi-infaunal tiers, non-motile and suspension feeding categories, but they show trends of increasing ecological complexity. The geographical distribution of the clusters shows the Hurlet Limestone palaeoenvironment was most diverse around Sorn, whereas that for the Index Limestone was diverse in all three main areas of outcrop. This variation is attributed to local fluctuations in depth, sea floor conditions and water quality. An ‘embayment’ in the palaeoenvironment of the Index Limestone, seen by draping the interpretations over a 3D computer model of the subsurface, was associated with the Kerse Loch Fault, where penecontemporaneous displacement and an inferred palaeotopographical fault scarp influenced marine water flow, environmental distribution and genus diversity.