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At the head of Bantry Bay, southwest Ireland, the conformable transition between the nonmarine Old Red Sandstone facies and the marine Carboniferous clastic sequence occurs in the Coomhola Group. Within its 4 formations, 4 important partly repetitive facies are distinguished: (1) cyclic facies that become finer toward the top, interpreted to be stream deposits; (2) fine facies of rippled siltstone and mudstone, either separate or complexly interleaved, considered to be alluvial flood plain or interdistributary deposits; (3) burrowed facies, having variable proportions of wave-rippled siltstone interlaminated with mudstone, usually burrowed, thought to represent interdistributary bay deposits; and (4) parallel-bedded sandstone facies of parallel or cross-bedded sandstone u its, in places having scoured surfaces, interpreted to be submarine-bar deposits.
The basal Yellow Rocks Formation (450 m) consists of the cyclic facies variably interleaved with the fine facies, its base being the highest redbed. Paleocurrents indicate a northerly provenance, and the general environment is interpreted to be an alluvial plain. In contrast, the lowest part (40 m) of the overlying Ardaturrish Formation consists of the fine facies and the burrowed facies in equal proportions. The latter is interleaved with the cyclic facies in the upper part (560 m). This facies association indicates a transgression over the alluvial plain, the environment changing to a coastal plain and then to an interdistributary area partly affected by south-flowing distributaries. The cyclic facies persists into the overlying Reenagough Formation (160 m) before giving way to the arallel-bedded sandstone facies indicative of an offshore bar deposit. The burrowed facies then dominates, and within the overlying Ardnamanagh Formation (80 m) shows cycles that become coarser at the top, suggestive of an advancing shoreline, before being succeeded by fine-grained marine sediments (Tournaisian). This facies sequence implies a deltaic advance before the marine transgression was established.
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