The geological significance of electrical conductivity anomalies of the Ordovician-Silurian Moffat Shale Group , Northern Ireland

The Tellus airborne geophysical survey revealed sets of narrow, linear, north-east to southwest, mostly parallel electrical conductivity (electromagnetic – EM) anomalies in the Longford–Down area. Subsequent geological mapping and ground geophysics have demonstrated that the anomalies coincide with and match the width of bedrock outcrop of the Moffat Shale Group. Ground-based geophysical surveys show variations in conductivity with highest values corresponding to carbon-rich mudstones. These findings allow the regional airborne geophysics to be interpreted with greater confidence for incorporation into bedrock geological maps, which underpin aspects of economic and environmental decision making.

The Longford-Down area, some 6000 km 2 , stretches across counties Down and Armagh in Northern Ireland, and counties Louth, Monaghan, Cavan and Longford in the Republic. It is part of the Southern Uplands -Down-Longford terrane that extends into Scotland ( Fig. 13.1). Crushed rock aggregate is widely quarried and the area is in places mineralised with base metals and potentially economic gold deposits. There is also some potential for exploiting deep geothermal energy in granitic intrusions of late Silurian to Devonian and Palaeogene age.
Bedrock of the Longford-Down terrane comprises packages of rock known as 'tracts', which are largely composed of turbiditic greywacke sandstones, siltstones and mudstones. These rocks are divided into the Ordovician Leadhills Supergroup, the Silurian Gala and Hawick groups, and the mudstone-dominated Moffat Shale Group (MSG), which commonly crops out to the north of the north-easterly striking faults that bound the tracts (Anderson, 2004).
This chapter complements a previous regional scale interpretation of Tellus airborne EM anomalies by Beamish et al. (2010) by presenting the first results of ground investigation of selected airborne anomalies. This work includes detailed geological mapping to compare anomaly width with actual bedrock outcrop of MSG, and ground-based geophysics to examine internal variations in the MSG that might more specifically explain the EM anomalies. Th is study adds confi dence to interpretation of Tellus EM data for incorporation into geological maps that are being updated by the two Geological Surveys.

Case study area
Th e case study area selected for investigation, referred to as McCosh's Bridge, lies immediately south-west of Newtownhamilton in County Armagh ( Fig. 13.1), close to the border with County Monaghan. Th is location was chosen because of the clarity of the EM anomalies ( Fig. 13.2), the presence of available bedrock exposure, and the absence of major sources of anthropogenic interference.

Geological mapping
Detailed, 1:5000 scale, fi eld mapping of bedrock and superfi cial deposits was made across the case study area. Identifi cations of dominant lithologies and measurement of key structural data, such as strike and dip of bedding (including way-up), faults and thrusts were made for bedrock. Th e distribution and nature of superfi cial deposits was also mapped ( Fig. 13.3).

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Electrical conductivity mapping of the Moff at Shale Group

Ground-based geophysics
Five electrical resistivity tomography (ERT) profi les were surveyed across the bedding of the case study area. Th ree very low frequency electromagnetic (VLF-EM) profi les were acquired parallel to the ERT lines.

VLF-EM
Th is standard electromagnetic survey maps shallow variations of electrical conductivity. Th e primary signal source for VLF-EM prospecting systems is radio stations that radiate EM waves in the VLF bands (15-30 kHz). At distances far from the transmitters the primary fi elds resemble plane EM waves with horizontal magnetic and vertical electric fi elds. Th e primary electric fi eld generates eddy currents which produce much smaller secondary magnetic fi elds in buried electrical conductors. Th e secondary magnetic fi eld comprises a component in-phase (IP) with the primary fi eld and a quadrature (Q) component at 90°, or out-of-phase. Responses from transmissions from three diff erent stations at three VLF frequencies (19.6, 20.9 and 23.4 kHz) were recorded along each of the three profi les, using a Scintrex ENVI system. In-phase, quadrature, total fi eld, tilt, resistivity and phase components were measured from the three VLF stations. Th e IP component of 19.6 kHz (Anthorn, Cumbria, transmitter) is presented here since it gave the strongest signal and propagates  more deeply than the two higher frequencies. The VLF data were filtered according to the method of Karous and Hjelt (1983), using KHFFILT software of Pirttijärvi (2004), which also generated vertical pseudo-sections showing the density of the induced eddy currents.

ERT
This standard electrical survey technique generates a vertical cross-section of the variation of electrical resistivity beneath a linear array of electrodes laid on the ground. A Tigre resistivity system using a Wenner array setup was employed, with 5 m electrode separation. A small current (<50 mA) is passed between different electrodes and a resistance measured. A 2D section of the apparent resistivity of the subsurface was then produced using the inversion software Res2Dinv (Loke and Barker, 1996). The depth of penetration increases with the number of electrodes and length of profile. Depth penetrations of up to 50 m below ground level were achieved along the longest (315 m) profile.

Results of the ground surveys
Bedrock geology Geological mapping (Fig. 13.3) revealed bedrock exposure in streams and areas of rock near surface associated with Tellus EM anomaly 1 (Fig. 13.2). Bedrock exposure diminished to the north-west due to the presence of thick glacial till, so the case study area was restricted to this anomaly. Stream section mapping in the area of anomaly 2 did however show the presence of dark grey, slaty mudstone at outcrop. At the north-west corner of the area mapped, medium-very thick bedded (1-10 m thick) greywacke sandstone is inverted, dips steeply to the southeast and youngs to the northwest (Location 1, Figs 13.3 and 13.4a). Some 150 m downstream (Location 2, Fig. 13.3), the first significant mudstones encountered are composed of dark grey-cream, slaty mudstones and siltstones interleaved with dark grey, sometimes cherty, pyritic mudstones, all of which show much evidence of shearing and colour change or bleaching. These and other similar intervals downstream are interpreted as thrust packages. Further downstream (Locations 3, Fig. 13.3), dark grey to black laminated mudstones were found containing graptolites and bentonites. A third mudstone type is present at various places in the stream section (e.g. Locations 4 and 5, Fig. 13.3), and is composed of black-dark grey, cherty, thinly bedded-laminated, pyritic mudstone. The strike of bedding within mudstone was predominantly north-east to southwest, and dips varied from vertical to moderate to the north-west and south-east. Way-up within the mudstones was not identifiable in the field. South-west of Location 7 (Fig. 13.3) greywacke sandstone returns as the dominant lithology.
In addition to the numerous sheared thrust packages (as described above for Location 2), other strike parallel tectonic structures were observed. These include a spectacular dislocation surface (Location 4, Fig. 13.4b) that dips 15° to the north-west and displays much brecciation, gouge development, bleaching and mineralisation (quartz and pyrite) related to fluid flow. Several highly altered, mica-rich, lamprophyre dykes were also identified cross-cutting the mudstones and striking west to east. These minor intrusions most likely belong to the Late Caledonian population present in the area and associated with the Newry Igneous Complex (Reynolds, 1931;Cooper and Johnston, 2004;Cooper et al., 2013).

Superficial deposits
Glacial till is present across much of the area in the form of north-northwest to southsoutheast aligned drumlins (Greenwood and Clark, 2008). As a consequence, till tends to thicken rapidly away from the modern streams and rivers (Fig. 13.3). The composition of tills across Northern Ireland tends to reflect underlying bedrock (Dempster et al., 2013) and the till here is no exception; it is a sandy, clayey silt with locally derived clasts of greywacke, siltstone and mudstone to boulder grade (Fig. 13.4c). An area of lacustrine alluvium is present at the western margin of the mapped area and is composed of clayey silt with thin lenses of sand and gravel. Adjacent to the streams and Tullyvallan River, alluvium continues to form and is composed of clasts derived from bedrock and till that range from boulder to silt grade (Fig. 13.4d).

Geophysical surveys
The locations of the traverses are shown in Fig. 13.3 and the VLF and ERT sections in Figs 13.5 and 13.6 respectively. Fig. 13.5a. Line VLF-EM1, 435 m, lies parallel to the airborne survey line and • nearly perpendicular to the strike of the conductors. Two well-defined conductors (orange and red) dipping to the south-east occur at c.150 and 220 m. A third anomaly at c.50 m is a response from a power line. The VLF anomalies at c.150 and 220 m correlate with black-dark grey, cherty, pyritic mudstone ( Fig. 13.3). Field observation shows that the black mudstones contain graphite, which is conductive and easily mapped by VLF. In contrast, more resistive responses (blue and green) correlate with the grey slaty mudstones and the dark grey-black, laminated mudstones. Fig. 13.5b. Line VLF-EM2, 210 m, crosses elevated ground over thick glacial till. • The response over this line is not as clear as line (a). Fig. 13.5c. Line VLF-EM3, 290 m. The anomaly observed at 80 m is again associ-• ated with black-dark grey, cherty, pyritic mudstone. On the ERT profiles ( Fig. 13.6) areas of lower conductivity (green and blue) correlate closely with mapped greywacke sandstones, e.g. the end of ERT1 and most of ERT5, while zones of high conductivity (orange-red-purple) coincide with the range of mudstones identified during mapping. Unlike the VLF-EM, this method does not appear to differentiate between the mudstone types but comparison of the ERT profiles with the regional Tellus EM anomaly (Fig. 13.3) reveals a close match of anomaly width, which is important in geological mapping.

Conclusions
This brief study reveals a close match between regional airborne EM anomalies and mapped geology, which is important in that it allows the width and lateral extent of MSG packages to be mapped using the regional geophysical data set. This knowledge is being incorporated into active mapping campaigns north and south of the border (Fig. 13.1).
Ground-based geophysics has detected variations in the conductivity of the MSG packages, with the black-dark grey, cherty, pyritic mudstone type being most conductive and the grey-cream, slaty mudstones being least conductive. Field work shows that the latter mudstone type has been affected by much thrusting and shearing, which appears to have reduced its conductivity. In addition to shearing, there is much colour change of these mudstones (from black-dark grey to light grey-cream), which is interpreted to have taken place through fluid flow during regional deformation. Fluid flow appears to have removed carbon, possibly in the form of graphite, from the mudstones along anastomosing, shear-related discontinuities, which could greatly reduce the overall conductivity of the rock; petrology and scanning electron microscopy analysis would be required to confirm whether or not this is the case. Further investigation is also required to investigate the possible effects of overburden, primarily glacial till, on the characteristics of the Tellus EM anomalies.
The results presented here add confidence to the interpretation of Beamish et al. (2010), which revealed regional scale geological structures in the Longford-Down area, including the presence of a major strike-slip duplex in the º area (mapped in detail on Beamish et al., 2010, Fig. 4) and significant strike swing in the area west of Slieve Gullion, County Armagh (mapped on Beamish et al., 2010, Figs 5 and 6). These bedrock features are important in that they are associated with igneous intrusions, some of which have potential for geothermal energy and mineral deposits. Interpretation of Tellus and Tellus Border EM data is now being applied to revising and refining the geological maps on both sides of the border. This is especially important in areas where bedrock is concealed by thick glacial deposits. The new maps will provide a regional bedrock context for potentially significant gold deposits and will allow better constraints on the extent of certain greywacke sandstone tracts that have premium value as high polished road stone.

Chapter 7
Using soil geochemistry to investigate gold and base metal distribution and dispersal in the glaciated north of Ireland DOI:10.3318/978-1-908996-88-6.ch7

Chapter 15
Information for agriculture from regional geochemical surveys: the example of soil pH in the Tellus and Tellus Border data DOI:10.3318/978-1-908996-88-6.ch15

Chapter 16
An ecohydrological investigation of wetlands in the border counties of Ireland: a framework for a holistic understanding of wetland systems DOI:10.3318/978-1-908996-88-6.ch16

Chapter 25
Refining the human health risk assessment process in Northern Ireland through the use of oral bioaccessibility data DOI:10.3318/978-1-908996-88-6.ch25

Chapter 26
Combining environmental and medical data sets to explore potential associations between environmental factors and health: policy implications for human health risk assessments DOI:10.3318/978-1-908996-88-6.ch26

Chapter 30
Spatial distribution of soil geochemistry in geoforensics DOI:10.3318/978-1-908996-88-6.ch30 End matter DOI:10.3318/978-1-908996-88-6.endmatter While every effort has been made to contact and obtain permission from holders of copyright, if any involuntary infringement of copyright has occurred, sincere apologies are offered, and the owner of such copyright is requested to contact the publisher.
British Library Cataloguing-in-Publication Data. A catalogue record is available from the British Library.
Design: Alex Donald, Geological Survey of Northern Ireland. Index: Brendan O'Brien.