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Carbonate Buildups and Waulsortian Mud-Mounds |
Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland
Two late Viséan (Asbian-early Brigantian) buildup complexes occur in the Kingscourt Outlier in Ireland, near the top of the Mullaghfin Formation, a shallow-water, grainstone unit. These massive buildups at Ardagh and Cregg accumulated on the margins of a carbonate platform bordering a deep-water basin. Both have a buildup facies of fine-grained, peloid-rich, algal lime mudstones and wackestones, interbedded with coarser grained intraclastic, skeletal packstones and grainstones (interbuildup facies). Microbial structures are well developed in the buildup facies, principally domal stromatolites, thrombolites and oncoidal fabrics of cyanophytes (Ortonella and Girvanella) with encrusting formainifers (Aphralysia and Textrataxis). Algal structures include rhodoliths (Solenopora) and fragments of Ungdarella and stacheiids, with less abundant chlorophytes (Koninckopora); in the interbuildup facies, Koninckopora is more abundant.
Near the top of the Ardagh buildup is an unusual development of phylloid algal boundstone composed of the possible ancestral coralline red alga Archaeolithophyllum. This boundstone also contains encrusting bryozoans and foraminifers, and directly overlies abundant Brigantian in situ fasciculate rugose corals. This upper unit appears to be associated with a rapid shallowing event, which stimulated the development of a wave-resistant rigid framework.
Laterally, bedded intraclastic skeletal packstones and grainstones, with thin interbedded clays and shales, pass into the lower part of the massive Ardagh buildup, but appear to onlap and drape the upper part of the buildup. Buildup clasts occur within the interbuildup facies and proximal flank facies, indicating synsedimentary cementation.
Late Viséan buildups of the Ardagh type highlight an important period of diversification of colonial rugose corals and calcareous algae associated with the development of widespread shallow-water carbonate platforms worldwide. This allowed the emergence of new algal groups, particularly the red algae (Archaeolithophyllum and Ungdarella) and the palaeoberesellid green algae (Kamaenella), which subsequently dominated Upper Carboniferous wave-resistant bioherms.
