The Lithology and Structure of a Boulder-Bed in the Dalradian of Mayo, Ireland

Abstract

Summary Exposures of a boulder-bed resembling those of Donegal have been found in North-West Mayo. The boulders are granitic in aspect, though severely affected by cataclasis, and the matrix is foliated and generally semi-pelitic. The probable succession in North-West Mayo and the position of the boulder-bed in it have been determined, but no correlation with other areas has yet been established. For the most part the boulders are about equidimensional, but locally they are intensely deformed. Many of the deformed boulders are markedly elongated and flattened. The elongation is parallel to the regional lineation, which generally plunges to the south-east. Although several analyses of pebble deformation in other areas are interpreted as indicating a main movement parallel to the elongation, it is considered that the Mayo boulders have been rolled and thus extended in b . This conclusion is strongly suggested by more than one line of evidence. Firstly, certain mullion structures (with a south-easterly plunge corresponding to the direction of boulder elongation) were found to have their peripheries determined by complex folds that have required considerable movement at right angles to the mullion trend. Secondly, mapping in North-West Mayo has revealed large-scale folding that plunges to the south-east. Thirdly, the shapes of many of the deformed boulders are not such as would be expected if they had been extended in a , though they might well have been produced by rolling and flattening. Petrofabric investigations have been carried out on some of the boulders, and the relationship between fabric orientation and deformation is briefly discussed in the paper.