From mud to limestone: Birth and growth of a giant reef in the Eifelian (Middle Devonian) of S Belgium

Abstract

The Wancennes reef is a giant bioherm 3000 m-large and 300 m-high that developed during the early Eifelian partitus to costatus conodont zones. The basal banks are formed by an accumulation of crinoid debris stabilised by lamellar stromatoporoids forming a coverstone, with a typical pioneer fauna. A micritic core, mostly formed by coral bafflestone, forms the first relief of the reef and evolved laterally and upwards to coral-stromatoporoid bafflestone and framestone that form the bulk of the reef and corresponds to the diversification phase of building and non-building organisms. The reef-crest is dominated by massive stromatoporoids and subordinate tabulate corals whereas the back-reef accumulation of debris is very rich in organisms (climax phase). The development of the reef is strongly influenced by sea level variations. Two third-order eustatic sequences are recorded in the reef. The first one corresponds to the basal banks and micritic core (transgressive systems tract) and lower part of the skeletal framestone (highstand systems tract) ending with an emersion. The second sequence is recorded in the upper part of the framestone (transgressive systems tract) and reef-crest and associated facies (highstand systems tract). The final emersion of the reef marked its termination followed by the deposition of fine-grained siliciclastics. The bathymetry, light penetration and hydrodynamism are assessed by the facies and faunal associations. All point to shallow-water, high-energy environment at the base and top of the reef, with deeper and quieter conditions during the development of the micritic core.