Discussion on 'A new look at the Mona Complex (Anglesey, North Wales)'

Abstract

M r M. K ohnstamm writes: Barber & Max (1979), in their 'New look at the Mona Complex', explain the geology of their 'Northern Region' (which includes the Northern and Western Regions of Greenly 1919 ) by invoking 3 tectono-sedimentary units and a mylonite belt. The truth of this model depends not only on the controversial lower contact of their middle, or New Harbour Unit, but also on its upper contact. The authors failed to reply to the point raised by Professor Wood in the discussion of their paper, that field mapping and petrographic evidence show the upper contact to be of a 'purely hallucinatory nature'. In fact, the contact does exist, but only as a gradation between the upper, or Cemlyn Unit and its tectonized equivalent. The structure of the Northern Region (in the original sense of Greenly) of the Mona Complex is explained by Barber & Max as 3 parallel belts––the Cemlyn Unit in the NW, the New Harbour Unit in the centre, and the mylonite belt along the SE margin. It requires emphasising that, while the authors make important corrections to Greenly's stratigraphy in demonstrating that the New Harbour Unit is essentially a region of phyllonites rather than a stratigraphic unit, they persist in considering it a separate sedimentary unit. This results in their need to thrust the unit to the middle of the succession. Barber & Max claim that 'all the contacts between the Cemlyn and New Harbour Units are faults' and their map shows a single dashed