The Kellwasser Events in the Upper Devonian Xom Nha Formation, Central Vietnam

Abstract

The Frasnian-Famennian (F-F) boundary occurs within the Upper Devonian Xom Nha Formation in Quang Binh Province, Central Vietnam. The stratotype section of the Xom Nha Formation (bed numbers Xo 1-39) comprises carbonates, and is divided into Unit I (Xo 1-13) mainly composed of whitish-gray bioclastic grainstone and wackestone, Unit II (Xo 14-28) dominated by bioclastic packstone and wackestone, and Unit III (Xo 29-39) characterized by weakly parallel laminated gray to dark gray lime-mudstone. Units I-III commonly contain well-preserved conodonts and ostracods. Some horizons of Unit II yield brachiopods, ammonoids, nautiloids, bivalves, tentaculitoids, crinoids and acritarchs. Age diagnostic conodont species of the Frasnian Palmatolepis linguiformis is commonly obtained from the upper part of Unit II. Unit III yields Pa. delicatula that is characteristic of the lowermost Famennian Pa. triangularis conodont Biozone. Two positive carbon isotope excursions are recognized, the lower excursion peaking in Unit I whilst the upper excursion peaks just below the local Frasnian-Famennian boundary (bed number Xo 28). Based on the biostratigraphy, these excursions equate to the Lower and Upper Kellwasser events. In the upper part of Unit II (Pa. linguiformis Biozone, bed numbers Xo 16-23), relatively diverse marine assemblages are recognized, indicating a temporal recovery of ecosystem after the Lower Kellwasser Event. By contrast, the diversity of lowermost Famennian marine organisms decreased markedly after the Upper Kellwasser event.