Late Devonian-Early Carboniferous deep-water coral assemblages and sedimentation on a Devonian seamount: Iberg Reef, Harz Mts., Germany

Abstract

Abstract Growth of the Middle-Late Devonian Iberg Reef, a small atoll in the Variscan Geosyncline of central Europe, ceased towards the end of the Frasnian. During the Famennian and Dinantian, the reef continuously subsided relative to sea level. In the latest Dinantian the reef was covered by clastics. Five deep-water organism assemblages colonized the Famennian-Dinantian seamount successively. They are well comparable to recent organism communities on deep-water coral banks. In the Famennian, abundant crinoids and ancillary rugose corals and brachiopods colonized the reef top. During the Late Tournaisian, a coral-crinoid assemblage established on the reef. It developed into a varied fauna of corals, crinoids, trilobites, brachiopods, bivalves, gastropods, and goniatites in the Early and Middle Visean. The Late Visean assemblage was dominated by goniatites and pseudoplanktonic bivalves. In the latest Visean, a monospecific brachiopod fauna and small microbial buildups thrived in extreme environmental conditions on the reef top. The skeletal debris of the organisms was eroded post-mortem and only accumulated in current-protected depressions of the reef surface, along with carbonate mud. Local depressions include hollows on the reef top, neptunian dikes, and pore space of a breccia that formed due to intensive shattering of the reef top during nearby volcanic activity. During most of the Famennian and Tournaisian, there is a hiatus caused by non-deposition. Only reworked Famennian and Tournaisian conodonts recovered from Upper Tournaisian-Middle Visean limestones give evidence of marine conditions on top of the reef. The sedimentary environment on the drowned atoll compares well to that on recent guyots, including hiatuses, slow and patchy sedimentation, erosion and redeposition. Both the aftermath of the Frasnian/Famennian extinction that drastically reduced metazoan reef builders and a trend towards lower temperatures in the Famennian and Dinantian are believed to have prevented the reestablishment of a shallow water reef on the drowning Devonian atoll.