The Carboniferous System in North County Dublin

Abstract

Summary The Loughshinny Black Shales, P 2 , represented the highest known Carboniferous horizon in County Dublin until 1939. A survey since then of the upland stretching west-north-westwards from Loughshinny has shown that Arnsbergian beds, E 2 , cover about eight square miles. It is considered that the zones present are those of Cravenoceratoides nitidum and Eumorphoceras bisulcatum s.s. They consist of a lower Balrickard Sandstone series, about 100 feer, and an upper Walshestown Shale series, about 320 feet. The former are dark-grey, micaceous, calcareous sandstone interspread with black limestones and shales and yield Eu. bisulcatum . The shale series consists mainly of black shale, often with goniatites and marine lamellibranchs. There are two limestone bands and several mudstone, siltstone and clay-ironstone beds. Near the top Ct. stellarum occurs, indicating the uppermost subzome of the Ct. nitidum zone E 2b . No indication of an E 1 fauna was found. The Upper Bollandrian, P 2 , which on the coast is represented by black shales, consists inland of black argillaceous limestone with a little shale containing Paragonialites subcircularis and abundant Posidonia membranacea . No trace of Pd. becheri or P 1 gonitites was seen inland. Probascidella found at one localit probably indicates the base of P 1 . Below the black P 2 beds are at least 600 feet of grey limestones, some of them cherty, considered to represent the Cyathaxonia Beds, and the Kate, Carlyan and Holmpatrick limestones of the coast section C 2 -D 1 . The area is on the south margin of the Ballbriggan Lower Palaeozoic massif and seems to include basin and back-reef facies. Absence of E 1 and overlap of E 2 upon the Lower Palaeozoic represent the Sudetian movements. The mid-Avonian disturbances (Nassauian phase) are indicated inland by basement fragmental beds, as on the coast by the well-known Rush and Lane conglomerates. One north-east and south-west fault was observed and three others with a similar trend are inferred. There are two main anticlines in the Carboniferous rocks with north-east and south-west axes pitching north-eastwards, one of which is evidently continuous with that at Oldtown to the south-west and possibly also with the east-and-west anticline pitching east in the Lower Palaeozoic seen in the islands at Skerries. Its curved course would thus be parallel to that of the Swords-Portrane anticline to the south.