Coral-rich bioconstructions in the Viséan (late Mississippian) of Southern Wales (Gower Peninsula, UK)

Abstract

The Holkerian(?)/early Asbian succession of Blue Pool (northwestern end of Gower Peninsula, southern Wales) includes two individual, decameter-sized, low relief bioconstructions: (a) a microbe-bryozoan-coral reefmound and (b) an overlying coral reef. The bioconstructions are embedded within a grainy, bioclastic and oolitic inner shelf facies. They occur in two successive shallowing upward cycles. Especially the coral reef is unique within the metazoan-dominated shallow-water bioconstructions of the European Visean-Serpukhovian, which diversified remarkably during this time slice. Various descriptive and semi-quantitative techniques were applied during field work and microfacies analysis was used to describe the complex interaction of macro-and microenvironments in the bioconstructed facies, to evaluate potential framework building, and to demonstrate the functional position of rugose corals in Carboniferous shallow-water bioconstructions. In particular, quantitative spatial plots of the distribution of framebuilders in the field and a stratigraphic log of the quantitative distribution of allochems/carbonate textures according to abundance classes were used and critically evaluated. The microbe-bryozoan-coral reefmound is characterized by dominance of rigid non-skeletal microbial framework, enforced by diverse bryozoan morphotypes and few syringoporid corals. Colonial rugose corals occur isolated, becoming more abundant and clustered towards the top. Thus, their limited contribution to framework building is passive, providing hard substrate for microbial framework. Stabilisation and binding of loose sediment by microbes is also essential for the initiation of the coral reef developing during the next shallow-upward cycle. Primary rigidity of the reef core resulted from closely spaced, intergrowing cerioid rugose corals (Lithostrotion). Reduction of open space in the coral meshwork resulted from microbial encrusting, formation of non-skeletal framework, and bioclastic infill. Reduced coral abundance, increased number of fasciculate rugose corals (Siphonodendron), increased colony sizes of syringoporids, and a high amount of coarse bioclastic debris characterise the reef flanks. Reef growth stopped due to shallowing-upward and erosion. Locally, microbial precipitation persisted in post-constructional phases. Major factors controlling the development of both bioconstructions are changing water depth, expressed by a potential interplay of changing hydrodynamic conditions, reduced accommodation space and lateral facies shifts. The general fragility of delicate Carboniferous corals and the low fixing potential of rugosa are additional palaeobiological restraints. In a final chapter, both Blue Pool bionconstructions are compared with different Visean bioconstructions from the British Isles and with coral-rich bioconstructions elsewhere. The Welsh coral reef is most similar to Late Visean microbial-coral-patch reefs from Queensland, Australia.