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The characteristic feature of a 300-m-thick shelf sequence of alternating limestone and shale (lower Carboniferous, Northwest Ireland) is the presence of extensive solitary coral-strewn bedding planes. These shale-overlain, prone, coral death assemblages from particle-supported beds 5-15 cm thick alternating with 6-75-cm beds of skeletal carbonaceous silt, sand, or mud. The death assemblages consist of randomly oriented adult corals averaging 50-100 cm in length in concentrations of 4-11/sq m; they are laterally extensive (>30 m) with rare discontinuities. Locally upward-facing surfaces of prone individuals are breached and infested with boring bryozoans and sponges, or encrusted with auloporid corals. Coral growth forms and internal structures in the death assemblage ommonly are aberrant; the cylindrical form is irregularly coiled and constricted; the intertabular space is highly variable (0.50-4.0 mm); the tabulae are thin in places and accompanied by a suppressed dissepimentarium. In the malformed parts of the corals the intertabular space commonly is infilled completely with silty biomicrite, whereas normally developed axial structures are spar filled. A few free-living upward-growing corals are present in life position in intervening units.
The coincidence of silty biomicrite axial infill and aberration in growth form and internal structures suggests that rapid local accumulation of fine sediment about the calyx forced upward growth and concomitant skeletal reduction. The coral death assemblages are lag deposits probably produced by unusually intense storm wave action which alternated with prolonged periods of quiet-water accumulation. The thickness of the quiet-water deposits (>75 cm) implies a long duration between storms, e.g., "once-in-a-hundred-years."
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