Abstract

Dr W. S. McKerrow asked the author if they thought the upward decrease in size of the vesicles in the lavas of the Lough Nafooey Group was related to an increase in depth of water up through the sequence. If so, how did this relate to the overlying clastic sediments?

Dr McKerrow later wrote: Could the authors elaborate on the relation of the Lough Nafooey Group to the Grampian Orogeny? If it was entirely postorogenic, why were the lavas not interbedded with coarse clastic detritus from the adjacent Connemara highlands?

The Authors reply that they believe the decrease in vesicle size is at least partly related to increasing water depth. It is reasonable to suggest that the Bencorragh Formation developed in shallow water, especially as the vesicles penetrate the glassy selvages. A deeper water origin for the pillow lavas in the Finny and Knock Kilbride formations is supported by their association with cherts and black shales. Furthermore, considerable subsidence would have been required for the build up of this submaride lava pile and this was probably associated with activity on the Doon Rock Fault. The coarse clastic sediments of the Derry Bay Formation record the establishment of a nearby source area and are thought to represent the emergence of the Connemara Cordillera. The Lough Nafooey Group can therefore be interpreted as recording the initial subsidence of the South Mayo Trough floor and the emergence of an adjacent source area.

In reply to the second part of Dr McKerrow's question we

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