Dromore Big
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Geology & landscape
Geological Feature: Rock Outcrop · Mining / Industrial: Quarry · Geological Period: Carboniferous · Rock Type: Sandstone, Shale
Description
Dromore Big Carboniferous stratigraphy The quarry in Dromore Big is important because of its geology. In particular the quarry exposes sandstones that are typical of the Carboniferous Dromore Sandstone Member, a geological unit of the Mullaghmore Sandstone Formation. This is also a key locality for understanding how the environment was changing at this period of geological history. At the northern edge of its outcrop in the Kesh area, the Dromore Sandstone Member rests directly on soft grey shales of the Bundoran Shale Formation. Being composed of more resistant sandstones the Dromore Sandstone Member forms a prominent ridge with a steep north scarp face and a gently inclined southern dip surface. The stratotype quarry is excavated into the steep north face of this ridge and exposes 9.7m of bedded sandstones. Bed thickness and grain size increase upwards and the top sandstone bed consists of 0.85m of very pale grey to almost white, coarse-grained sandstone. Sedimentary structures are somewhat limited but include parallel laminations and, less commonly, large scale planar cross laminations. No ripple marks were found. Indeterminate bioturbation structures occur on some bedding surfaces along with well defined Arenicolites in one example. Fragments of carbonised wood 0.03 cm wide and up to 0.2m long occur on one bedding surface. During the period of formation of the marine shales of the underlying Bundoran Formation, conditions gradually changed as the influx of silt and fine sand sediment increased. In the upper part of the Bundoran Formation these sediments maintained the overall grey colour of the underlying shales but the fossil content almost totally disappeared. The culmination of this influx of coarser sediment is reflected in the thick, clean sandstones of the Dromore Sandstone Member which were deposited universally across the basin and were accompanied by a marine regression. The sandstones were probably deposited in shallow water, a high energy sandflat type environment. The nature of this environment of deposition is supported by the occurrence of ooliths and oolitic sandstones elsewhere in the area. Sandstones at Dromore Big were therefore deposited in a marginal marine location that received increasing amounts of detritus in a location close to the sediment source. https://www.daera-ni.gov.uk/publications/dromore-big-assi