Gamma-ray spectroscopy in structural correlations: an example from the Neoproterozoic Dalradian succession of Donegal (NW Ireland)

The Caledonian Horn Head Slide is a spectacular ductile shear zone transect ing Neoproterozoic Appin Group Dalradian metasediments in Donegal (NW Irela nd). Two conflicting stratigraphic interpretations exist for the inverted s uccession exposed in the hanging wall of the structure. These are based on correlation with two quite separate exposed pelite formations elsewhere. Th e two formations are lithologically indistinct and unfossiliferous. Here we document the novel use of assayed and logged spectral gamma-ray measuremen ts in comparing the contentious pelite in the hanging wall of the Horn Head Slide to the two possible correlative pelite formations from a wide area o f their unequivocal outcrop. The data from the contentious pelite show a cl ear statistical and stratigraphical affinity with one candidate unit only, thus providing the stratigraphical definition necessary for refining previo us cross-sections. A new model, based on our spectral gamma-ray corn-elatio n, is proposed to account for the northwestwards directed emplacement of th e Lower Falcarragh Pelite Formation along the slide. This model requires pr e-Caledonian normal faulting as a precursor to the ensuing compressional ev ent in which stratigraphically younger rocks were thrust over older, a comm on instance in fold and thrust belt geometry Our work suggests that spectra l gamma-ray measurements may provide a rapid, field-based method for differ entiating unfossiliferous pelite or mudstone units at outcrop and in geophy sical well-logs, even in structurally complex areas.