Ductile, possibly sub-glacial deformation of peat and altered Tertiary basalt driven by gypsum karstification at Knocknacran, Co Monaghan, Ireland.

A. P. M. Vaughan1, L. A. Dowling2, F.J.G. Mitchell3, S.-E. Lauritzen4 and P. Coxon2

1British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK.

2Department of Geography, Trinity College, Dublin 2, Ireland.

3Botany School, Trinity College, Dublin 2, Ireland.

4Department of Geology, Allégaten 41, N-5007 Bergen-Universitetet, Norway.

a.vaughan@bas.ac.uk

Organic-rich deposits, gravely till and altered Tertiary basalt overlying Permo-Triassic gypsum at Knocknacran opencast gypsum mine, Co. Monaghan show thrust faulting and recumbent folding. The two till and organic-rich deposits (one stratigraphically above the other) were emplaced onto a Tertiary basalt surface during high-energy flood events and were subsequently deformed with the underlying basalt as a result of mass-wasting through sinkholes in karstified gypsum. Deformation ranges from gentle sag folding to fully recumbent, nappe-like anticlines incorporating altered basalt, with associated synthetic thrusting and en echelon veining. Abundant fossil wood in peat layers provides evidence of structural transport directions. Overlying drumlinized cobble diamicton is not deformed. Uranium thorium disequilibrium dating suggests the organic-rich deposits in the upper section are at least 33 ka old and in the lower section are at least 78 ka old. Interpretation of the pollen content, although tentative because of the depositional history of the material, suggests the organic material originated in a warm stage possibly warmer than post-Eemian interstadials. Dating, palynology and field relations suggest that warm-stage peat deposits were remobilised during a change in hydrological conditions, possibly at the onset of the succeeding cold stage, and flushed as a slurry onto supra sinkhole depressions in the Tertiary basalt surface. Ductile deformation of the combined peat, till and basalt unit suggests relatively high confining pressure, combined with mass-wasting into the gypsum sinkhole system (which is still active today). The lack of deformation in the overlying drumlinized diamicton indicates that deformation pre-dates deposition of this unit. The requirement for relatively high confining pressure and evidence of mass-wasting into the sinkhole system presents the possibility that deformation occurred beneath a wet-based glacier.