Re-mapping the Moffat Shales using Tellus geophysics
The Sandbian to Llandovery Moffat Shale Group is a deep marine mudstone sequence that forms the base of successive thrust slices of metasedimentary rocks in the accretionary Longford – Down terrane. Mapping the distribution of the shales, therefore, indicates the regional structure of the terrane, with application to mineral prospectivity and understanding the closure history of Iapetus at the Laurentian margin. Moffat shale horizons produce a strong conductivity signal in the Tellus and Tellus Border airborne geophysical survey data (Floyd, 2013; Beamish et al., 2010). We have combined interpretation of the EM data with re-examination of GSI 19th century 6″ field sheets and core drilling to re-map the Moffat shales in Co. Monaghan. Drilling of a blind anomaly proved the efficacy of the method. The re-mapping has identified a revised distribution of Moffat shales and therefore of the tract-bounding faults, including identifying a new structural panel.
Highly conductive horizons in drill core, identified with a simple multi-meter, are being examined to identify the lithological control on conductivity. A reasonable correlation between very dark coloured shale and a lower resistance exists, suggesting that the Tellus conductivity survey may be responding to the presence of graphite
The image displays the new distribution of the Moffat shale horizons and test drill holes, overlain on the Tellus EM data.
Beamish, D., Kimbell, G.S., Stone, P. & Anderson, T.B. 2010. Regional conductivity data used to reassess Lower Palaeozoic structure in the Northern Ireland sector of the Southern Uplands–Down–Longford terrane. Journal of the Geological Society, London, 167, 649–657.
Floyd, J.D. 2013. A comparison and appraisal of the geological correlation between the Southern Uplands and Down-Longford sub-terranes.