The Dalradian Geology of the Creeslough Area, Co. Donegal

Abstract

Summary In the Creeslough area, folded and metamorphosed Dalradian sediments with sills of metadolerites are intruded by a member of the Donegal granite series. The preservation of original sedimentary features makes it possible to establish a succession of about 6000 feet of sedimentary rocks. The lower beds accumulated slowly in a restricted trough; the appearance of thick gritty quartzites marks an increase in the supply of detritus; the overlying sediments show evidence of emergence and the highest beds indicate a return to slow sedimentation. The intrusion of the dolerite sills produced a thermal alteration of the sediments which has resisted the later orogenic metamorphism. The major folds run NE.-SW., are overturned towards the north-west and are associated with slides. A phase of orogenic metamorphism, of no high grade, was followed by the uprise of the local phase of the Donegal granite and the development of sillimanite, staurolite, kyanite and garnet along the granite margin. The granodiorite and its aureole rocks were then strongly sheared, probably as a result of pressure from the deeper part of the pluton. A suite of lamprophyre dykes was intruded before the end of the shearing. I. Introduction A knowledge of the Creeslough area is important for the study of Donegal geology in that there is here provided a lengthy succession of Dalradian rocks which retain unchanged many of their original sedimentary characters. This circumstance enables their tectonics to be interpreted with confidence. Moreover, the sedimentary rocks and the associated metadolerite intrusions present many problems in metamorphism—the