Geophysical surveys near Strontian, Highland Region

Reconnaissance VLF-EM and magnetic surveys have bean carried out over Ba- Pb-Zn prospects in an area near Strontian in the Highland Region of Scotland. Rather than attempting to detect the economic minerals directly, which is unlikely to be practicable by geophysical methods, the trials concentrated on exploration for the crush zones and associated Permo-Carboniferous basic dykes which act as hosts to mineralisation. The results are encouraging, with the VLF-EM method proving effective in delineating crush zones while magnetic traverses detected the basic dykes. To the east of Bellsgrove mine a crush zone and dyke extend eastwards along the strike of the Strontian Main Vein; however, to the west of the Whitesmith mine the evidence of a westward extension of the Main Vein is insubstantial. A number of crush zones and associated dykes have been identified in the Corrantee-Whitesmith area. Probable ext_ensions are indicated to a number of known veins in the vicinity of the Fee Donald mine. The rss~llts merit geophysical, geological and possibly geochemical follow-up

Three-dimensional gravity modelling of the Labrador Sea : Baffin Bay region

The original Passive Margins Modelling Project (PmmP) undertook a programme of research on the north-east Atlantic continental margin during 1999-2002, and was financed by a consortium comprising the British Geological Survey and a group of industry sponsors. Since then, further PmmP projects have been undertaken when additional funds have become available as a result of new companies joining the sponsoring group. The first such project resulted in an update to the project GIS (Kimbell, 2008). The second project is described in this report and involved an update to the modelling methodology and its application to a new investigation of the Labrador Sea – Davis Strait – Baffin Bay region

A gravity interpretation of the Central North Sea

A gravity investigation of the Central North Sea has been undertaken with the aim of supplementing a parallel seismic investigation (Arsenikos et al., 2015) by targeting those areas where the seismic information was sparse or of poor quality. By stripping the gravity effect of the Zechstein and younger sequence it was hoped that concealed Upper Palaeozoic basins could be identified in the residual gravity signatures and distinguished from anomalies associated with Late Caledonian granitic plutons. Density logs from a set of wells across the region were compiled and used to calibrate a density model for the cover sequence. This model employed a combination of compaction trends and burial anomalies in the post-Zechstein units and a relationship between overall thickness and average density in the Zechstein unit. It was used, together with a depth-converted structural model from the seismic interpretation, to calculate the gravity effect down to base Zechstein. This, along with a long-wavelength background field, was subtracted from the observations to leave a residual gravity anomaly that was inverted to produce a 3D model of variations in the thickness of a pre-Zechstein layer that incorporated the effects of both basins and granites. The modelling results were analysed in combination with magnetic imaging and available mapping of intra-Upper Palaeozoic seismic reflectors. Granites were often easy to identify on the basis of a low in the gravity inversion surface that coincided with a structural high defined seismically and, in some cases, a magnetic signature. There are, however, some more ambiguous features that cannot be confidently classified without further information. Relatively low density rocks within the Lower Palaeozoic basement and zones of high density basement or pervasive high density intrusive rocks introduce distortion into the model, and the identification and separation of these influences requires more detailed combined seismic, gravity and magnetic modelling. Potential targets (areas of pre-Zechstein sedimentary thickening) were identified in Quads 19-20, Quads 26-28, and just to the north of an 150 km offshore extrapolation of the line which forms the southern margin of the Tweed Basin in the onshore area (the Pressen-Flodden-Ford faults). Geophysical anomalies in the Q36-37 area suggest a complex interplay between sedimentary and igneous features and would also benefit from further investigation. A ‘ramp’ in the gravity inversion surface appears to be linked, at least in part, to lateral density variations associated with overcompaction along the Sole Pit axis. The geophysical feature extends beyond previous mapping of that axis and is overlain by the Breagh gas field, so is an appropriate target for more detailed study (which could address the possibility of a basement influence on the observed anomalies). The results obtained indicate that gravity/magnetic interpretation provides a useful supplement to seismic reflection surveys, even where the latter form the primary exploration method. There are, for example, features at the southern margin of the Forth Approaches Basin and possible intra-basinal structures within the North Dogger Basin that could add to our understanding of those areas. The new government-funded seismic/gravity/magnetic surveys over the Central North Sea, which were conducted in 2015 and will be released in 2016, will provide the ideal resource with which to follow up the results of this investigation

A gravity interpretation of the Orcadian Basin area

A gravity investigation of the Orcadian Basin area has been conducted which involved the following stages: - compilation, imaging and qualitative interpretation of BGS gravity and magnetic data from the region; - compilation of rock densities from geophysical well logs and modelling of density variations within the sedimentary sequence; - construction of a structural model of the cover sequence down to the top of the Permian, based on depth-converted seismic interpretation; - calculation of the gravity effect of the sequence to top Permian using the structural and density models; - removal of this calculated gravity effect and a regional background field from the observations to leave a residual stripped gravity anomaly; - analysis of the signatures within the residual stripped gravity anomaly map, integrated with seismic evidence of Upper Palaeozoic structure and magnetic imaging. The residual stripped gravity anomaly map reveals features that can be correlated with the West Bank Basin and the eastern end of the Caithness Graben of Arsenikos et al. (2016), and with the thickened Upper Palaeozoic sequence in the East Orkney Basin inferred by those authors. Gravity signatures indicative of a thickening of the Upper Palaeozoic sedimentary rocks are also identified in the Dutch Bank Basin and the South Buchan Basin, areas in which seismic interpretation of Palaeozoic structure was difficult because of problems with data quality and line spacing. The influence of granitic intrusions is seen in a belt that extends in a north-north-east direction from Quadrant 19 into Quadrant 13, although the magnetic characteristics of the bodies might indicate separate post-tectonic and late-tectonic suites of Caledonian intrusions. Further granites are inferred in the Inner Moray Firth, in Quadrants 12 and 17. A Caledonian age is possible for these but an alternative interpretation invokes Palaeoproterozoic calc-alkaline basement, at least for the more magnetic component. Gravity signatures in the Inner Moray Firth are also influenced by low density Dalradian (Grampian Group) basement and Devonian sedimentary rocks, making it difficult to partition the response accurately between the different sources. Positive gravity signatures are associated with the Buchan Block and its offshore extension, and with Jurassic intrusives beneath the Forties Volcanic Province. Dense/shallow basement extends in a west-north-west direction from the Forties area in a broad axis, and this is an important component of the long-lived structural configuration of the region that may be linked to an early transform offset in the Laurentian margin. Recommendations for further work include more detailed and extensive gravity modelling, quantitative magnetic modelling and a geochemical/isotopic study of the samples available from offshore granite well penetrations

Palaeozoic petroleum systems of the Irish Sea

This report synthesises the results of the 21CXRM Palaeozoic project in the Irish Sea to describe the Palaeozoic petroleum systems of that area. One hydrocarbon play system dominates the basin system: Namurian organic-rich marine shales (Bowland Shale Formation) generated oil and gas with a peak during maximum burial of the system in late Jurassic/early Cretaceous time. These hydrocarbons passed to reservoirs in the Triassic Ormskirk Sandstone (Sherwood Sandstone Group) by way of structures generated during the Variscan Orogeny and Cenozoic inversion, resulting in the Morecambe, Hamilton and other gas and oil fields The Palaeozoic study of the wider Irish Sea area has assessed the potential for more widespread petroleum systems situated outside the well-known play, particularly within the Carboniferous. Within the Main Graben system of the East Irish Sea Basin, Coal Measures strata were partially removed following Variscan inversion and early Permian uplift. They are not rich in coals, and not inferred to be a significant source rock. There is some potential in the Millstone Grit and Yoredale sequences, as some shales (particularly those associated with marine bands) are known to have high Total Organic Contents. The source rock potential of shales within the Carboniferous Limestone sequence is poorly constrained by data. A Devonian source rock is unproven and considered unlikely. Potential Namurian source rocks, such as the Yoredale Group, have been largely eroded in the Peel and North Channel basins, considerably reducing their prospectivity, although terrestrial sequences of equivalent age in the Solway Basin may offer better potential. The variable seismic data quality at Carboniferous levels and sparsity of deep well control have led to challenges in interpretation, particularly of the deeper picks. The interpretation of the surfaces contains a strong model-driven element, evidenced by the onshore relationships and areas where seismic picks can be made with the greatest confidence. Based upon the integration of regional seismic mapping with a limited well, source rock and reservoir property dataset, the most prospective parts of the region, outside the Ormskirk conventional gas play, are considered to be: The thick Westphalian sequences preserved in the Eubonia Tilt-Block in Quadrant 109, outside the main Permian-Mesozoic graben system and unaffected by Cenozoic inversion. The presence and quality of seals form a major risk as the Cumbrian Coast Group seal is thin or absent and Carboniferous intraformational seals are required but untested. Based on the limited dataset available in adjacent basins, reservoir quality is also a significant risk. A belt of Variscan inversion structures correlated with structures on the Formby Platform, and Ribbledale Foldbelt onshore, from which hydrocarbons have leaked into the overlying, Ormskirk-hosted Hamilton fields. The biggest risk here is whether reservoirs remain unbreached at the Pre-Permian level, and retain good poroperm characteristics at depths of about 2500 m. A more speculative play lies in the extensive carbonate platform in Quadrant 109 and surrounding the Isle of Man, in reefal facies with enhanced secondary porosity. Here, source rock presence and migration pathways, reservoir properties and seal quality are major risks

The three-dimensional crustal structure of the Faroe-Shetland region

A three-dimensional model of the Faroe-Shetland region has been constructed, which incorporates new seismic mapping, rock property models derived from a large database of geophysical well logs, and deeper structure based on gravity inversion. New gravity, magnetic and topographic compilations were produced which integrate BGS and released Faroese data. A large seismic database spanning the Faroese and UK sectors of the Faroe-Shetland Basin was used to map seabed, the top of the Palaeogene volcanic sequence and, where possible, base volcanics, base Cenozoic and top basement. Velocity logs and VSPs were used to derive velocity models that were employed in the depth conversion of these data. Published sources were used to extend the model into neighbouring basins (the North-east Rockall Basin, Møre Basin and Northern North Sea). Analysis of density logs was used to define compaction trends in sedimentary sequences and departures from these relating to overcompaction and lithological variation. A density model was derived for the volcanic rocks which reflected the differences between the massive flows observed on the Faroe Islands and volcanic sequences elsewhere which include a higher content of relatively porous material (weathered layers, hyaloclastites, volcaniclastics) and do display compaction trends. A method was developed for quantifying the influence of sills on the average density of the sequence into which they were intruded. The cover sequence geometries and rock densities were incorporated into a regional gravity model and inversion of short- and long-wavelength gravity anomalies was used to refine the forms of the top basement and Moho interfaces respectively. Density anomalies associated with the intrusive components of volcanic centres were incorporated in the model. Forward modelling was used to predict the magnetic signatures generated by the volcanic rocks and crystalline basement on the basis of simple magnetic property assumptions. The modelling results were assessed by comparison with wide-angle and normal incidence seismic results. The modelled top basement depths were converted into two-way travel time and integrated into the seismic workstation environment so that it was possible to interrogate the seismic database for evidence of any indications of structures that might have been missed without the guidance provided by the gravity model. The geophysical imaging and modelling results were analysed in a GIS and loaded into a customised viewer that facilitates the detailed comparison of multiple mapped and modelled layers, and the linkage of these to descriptive text. The modelling results provide a new view of the compartmentalisation of the Faroe-Shetland Basin, resolving the forms of a complex set of sub-basins and structural highs. On the western side of the basin its components are strongly influenced by a north-north-east structural grain whereas, in the east, north-east to east-north-east trends are more evident. The gravity signatures over the Munkagrunnur Ridge indicate a requirement for low density rocks beneath the volcanic sequence proven by the Lopra borehole. There is little evidence for deep basins beneath the Fugloy Ridge but basins are indicated further to the north-east beneath the Pilot Whale Anticline and the Møre Marginal High. The relatively subdued gravity expression of the Faroe Bank Channel Basin is attributed to the presence within it of a substantial volume of dense igneous rocks, including both extrusive and intrusive components. The results provide new insights into the igneous history of the region, identifying a set of normally magnetised, low density intrusions north of the Faroe Islands and beneath the Faroe Bank, which may be analogues of the post-breakup felsic intrusions observed in East Greenland. Although not a primary target, the successful resolution of basement structure beneath the Northern North Sea provides a useful test of the metholodology. This project has established a structural framework which should form a useful foundatio

Controls on the location of compressional deformation on the NW European margin

The distribution of Cenozoic compressional structures along the NW European margin has been compared with maps of the thickness of the crystalline crust derived from a compilation of seismic refraction interpretations and gravity modelling, and with the distribution of high-velocity lower crust and/or partially serpentinized upper mantle detected by seismic experiments. Only a subset of the mapped compressional structures coincide with areas susceptible to lithospheric weakening as a result of crustal hyperextension and partial serpentinization of the upper mantle. Notably, partially serpentinized upper mantle is well documented beneath the central part of the southern Rockall Basin, but compressional features are sparse in that area. Where compressional structures have formed but the upper mantle is not serpentinized, simple rheological modelling suggests an alternative weakening mechanism involving ductile lower crust and lithospheric decoupling. The presence of pre-existing weak zones (associated with the properties of the gouge and overpressure in fault zones) and local stress magnitude and orientation are important contributing factors

Cenozoic pre- and post-breakup compression in the Faroe-Shetland area, within the context of the NE Atlantic

This report is primarily based upon the interpretation of oil industry 2D seismic data, and aims to elucidate aspects of Cenozoic tectonostratigraphic development in the Faroe–Shetland region, especially with regard to post-breakup compression. Evidence of Cenozoic and Late Cretaceous pre-breakup compression and deformation is briefly reviewed. We have utilised established seismo-stratigraphic frameworks and a recently updated scheme for the post-breakup Eocene (Stronsay Group) succession, which are largely based upon the recognition of units bounded by regional unconformities. The seismic expression, extent and thickness of the seismo-stratigraphic units are illustrated by geoseismic profiles, structure contour maps and isochore maps, which are used to analyse the spatial and temporal development of post-breakup compression and deformation within the Faroe-Shetland region. The Faroe-Shetland region records a complex spatial and temporal pattern of departures from the thermal subsidence normally associated with passive margins, including broad uplifts and accelerated basinal subsidence together with fold development up to kilometre scale. The phases of latest Eocene / earliest Oligocene ‘sagging’ (accelerated subsidence) and early Pliocene uplift and exhumation (tilting) appear to be coeval with compression. Indeed, compression appears to have been active throughout post-breakup times, although the loci of deformation have varied both spatially and temporally. Conceivably, some of the large scale sagging, tilting and uplift may be associated with lithospheric folding. Much of the intra-Eocene folding appears to be focused in the southwestern part of the Faroe-Shetland region, around the Munkagrunnur Ridge and Judd area, where phases of shelf progradation are preserved and may be associated with contemporaneous uplift. However, there also appears to be evidence of episodic intra-Eocene and younger uplift in the area around the northern Fugloy Ridge. The overall shaping of the Faroe-Shetland Channel appears to have been initiated at the end of the Eocene, associated with uplift on the Fugloy Ridge and Faroe Platform areas, and with accelerated subsidence in the Faroe-Shetland Basin; this shaping was further developed during the Neogene. A Neogene opening of the ‘Faroe Conduit’ oceanic gateway is favoured on the basis of regional evidence of faunal isolation and restricted environment of deposition together with uncertainty regarding the nature of the ‘Southeast Faroes drift’. A significant phase of Miocene folding is associated with the Intra-Miocene Unconformity (IMU), whereas the Mid Miocene Unconformity (MMU) represents a relatively minor break with a restricted distribution in the NE Faroe-Shetland region. Seabed relief on some folds and late Neogene seismic onlaps may indicate that fold development persisted into Recent times. Lateral offsets and local basin inversion associated with the folding, suggest a strong structural inheritance from the underlying rift architecture. A broad coincidence between the timing of formation of the unconformities and plate reorganisation events in the adjacent Norway Basin and wider region may suggest that these events made important contributions to the forces shaping the margin. The development of Miocene and younger folds may have been influenced by gravitational potential energy / body forces associated with the density structure of the Iceland Insular Margin and the Southern Scandes, or with modulations to ridge-push resulting from transient changes in ridge elevation associated with plume-related temperature (buoyancy) variations in the underlying asthenosphere. Far field stresses associated with, for example, collision between Eurasia and Iberia may also have exerted significant influence on deformation within the Faroe-Shetland region

Dynamic evolution of the Faroe-Shetland region

This report presents a series of twenty palaeoenvironment maps that span the interval between the Late Jurassic and the Quaternary, and which form the basis of a generalised reconstruction of the late Mesozoic–Cenozoic development of the Faroe–Shetland and adjacent region. The database behind this study is firmly grounded within the portfolio of existing FSC stratigraphic reports (Cretaceous to Eocene), though it also includes new work that has extended the stratigraphic time series back to the Kimmeridgian (Late Jurassic), and forward to the Mid-Pleistocene. A synthesis of the main structural elements – basins, highs, faults, folds – is also included as these features provide a reference framework on the maps, as well being indicators of contemporary deformation spanning the pre-, syn- and post-breakup stages of NE Atlantic development in this region. By considering the palaeoenvironment maps (our observations) we identify the following key stages in the late Mesozoic–Cenozoic ‘dynamic evolution’ of the Faroe–Shetland region: • Intermittent and localised rifting in the Late Jurassic (mid-Kimmeridgian–earliest Berriasian) and Early Cretaceous (late Berriasian–Hauterivian). • Early Cretaceous (Aptian–Albian) instigation of rifting in the Faroe-Shetland Basin with maximum extension and basin widening in the Late Cretaceous (Coniacian–Maastrichtian). Localised uplift, compression and folding in various basins, particularly in Cenomanian–Turonian. • The Paleocene onset of major extrusive volcanism initiated close to Danian/Selandian boundary; growth of major basaltic shield of Selandian–Thanetian age overlying continental crust in the vicinity of the Faroe Platform; plate breakup and associated volcanism in the earliest Eocene north and west of the Faroe Islands. • Eocene (post-breakup) episodic uplift and erosion along the southern and eastern flanks of the Faroe-Shetland Basin; this was followed by a period of major compressive structuration across the entire Faroe–Shetland region spanning the end-Eocene/Oligocene–Mid-Miocene interval; this set the template for the shape of the modern-day continental margin, including the formation of the deep-water Faroe Conduit which facilitated the transfer of intermediate- and deep-water masses across the Greenland-Scotland Ridge. By comparing the timing of these key phases of geological development of the Faroe–Shetland region with European and North Atlantic plate tectonics we identify a first-order correlation between the pattern of deformation that we observe and established changes in intraplate and/or plate boundary stresses. This raises the possibility that additional forces, including those postulated to be related specifically to the internal dynamics of a mantle plume, may not be a prerequisite to the evolution of the Faroe–Shetland region

Disseminated copper-molybdenum mineralisation near Ballachulish, Highland Region

Chalcopyrite-pyrite-molybdenite mineralisation, in disseminated, veinlet and fracture-filling forms, is developed in adamellite and microadamellite in the Ballachulish igneous complex. Minor scheelite is associated with the sulphides, but is mostly confined to the adamellite. The mineralisation occurs sporadically over an area of at least 1800 x 800 m. It is best developed in and around the eastern part of the microadamellite over an area of about 250 x 450 m, where it was observed over a.vertical interval of 250 m from the highest exposure to the base of a borehole. An IP survey showed that chargeability values are slightly higher in this area. The grade is variable. In 10 ft (3 m) lengths of core, the maximum Cu content was 264 ppm and the maximum molybdenum content 501 ppm, but the average tenor over the (250 x 450 m) mineralised area is not more than 50-100 ppm Cu and lo-30 ppm MO. Selected mineralised outcrop samples gave values of up to 2386 ppm Cu, 9257 ppm MO, 2434 ppm W, 0.31 ppm Au and 8 ppm Ag. Rb-Sr isotopic studies indicate that the ore minerals were deposited shortly after emplacement of the host rocks, and it is considered that they were introduced by a hydrothermal system which, compared with those of classic porphyry models, was small in extent and weak in intensity. Sericitic alteration is generally associated with the mineralisation, but there is no potassic alteration evident and the standard zonation of porphyry copper deposits is absent. There is very little K or Rb metasomatism, the best defined chemical change being a loss of Sr in altered rocks. The hydrothermal fluids, as seen in fluid inclusions, were of moderate salinity, unlike the high salinity fluids usually characteristic of porphyry copper deposits. Anomalously low Rb and high K/Rb values in the unaltered microadamellite are attributed to the separation of a Rb-rich aqueous fluid from the microadamellite before or at the time of consolidation of the rock. The mineralised area lies adjacent to and northwest of a NNE-trending shatter belt, which may have provided structural control at depth, although at the present level of exposure the microadamellite body appears to be the structural control