Depositional and structural setting of the (?)Lower Old Red Sandstone sediments of Ballymastocker, Co. Donegal
M. McSherry, J. Parnell, A. G. Leslie, T. Haggan, 2000. "Depositional and structural setting of the (?)Lower Old Red Sandstone sediments of Ballymastocker, Co. Donegal", New Perspectives on the Old Red Sandstone, P. F. Friend, B. P. J. Williams
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Approximately 250 m of unfossiliferous sediments attributed to the Old Red Sandstone outcrop onshore at Ballymastocker, Donegal in northwest Ireland. Massive sandstones and trough cross-bedded sandstones pass upwards into coarse purple–brown basal conglomerates. The majority of the clast types, which include quartzite, marble, pelite, schist, vein quartz and metadolerite, are derived from the local Dalradian basement. Acid to intermediate porphyry clasts have no local source and may indicate a source to the north-east in the vicinity of the Devonian Lorne Plateau volcanic rocks in Argyll. The rocks are believed to represent an intermontane alluvial fan environment. The sediments occur in an outcrop of 3 km length against the northern flank of Knockalla Mountain, bounded to the southeast by the NE–SW-trending Caledonian Leannan Fault and resting on fractured Argyll Group Dalradian basement. No unconformable contact is observed and all contacts may be tectonic. Coarsening-up cycles within the sediments may reflect syn-sedimentary tectonic activity. The Devonian rocks are transected by a fracture system implying dextral transpressional activity on the Leannan Fault. The Dalradian basement rocks preserve evidence of sinistral strike-slip movement in late Caledonian times. Slickenfibres observed within the sediments and Dalradian rocks provide evidence for oblique dip-slip movement to the northwest. It is considered that these structures represent the last movement of the ORS basin, dropping it below denudation level, which resulted in its preservation. Movement along the Leannan Fault played a major role in the preservation of the offshore basins in the Islay–Donegal Platform region.
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New Perspectives on the Old Red Sandstone
From the 1960s onwards, the Old Red Sandstone of both borders of the Atlantic Ocean has acted as a test-bed for the development of new ideas on the interpretation of fluvial, lacustrine and aeolian sedimentary rocks, and the investigation of tectonically-active basins. Much of the earlier reconnaissance work is now being reviewed in the light of further detailed field study, along with new developments in the understanding of the biostratigraphy, palaeobiology, geochronology, pedogenesis and tectonics.
Three general papers review recent work on the stratigraphical and chronological analysis of the Late Silurian, Devonian and Early Carboniferous strata, and summarize present understanding of the tectonics of the basins. These are then followed by twenty-seven contributions covering new work in Eastern USA, Canada, Ireland, Britain, Norway, Greenland and Spitsbergen.