Expression of the Kellwasser and Rhinestreet Events (Late Devonian) on the Indochina tectonic block: New data from the Xom Nha Formation in Vietnam

Using a combination of whole rock and stable isotope geochemistry, scanning electron and cathodoluminescence microscopy, micropaleontology, and microtexture analysis, the Upper Rhinestreet Event, the Lower Kellwasser Event (LKE), and the Upper Kellwasser Event (UKE) have been documented on the Indochina tectonic block from the Xom Nha Formation in Vietnam. This is the first detailed lithologic description of the Rhinestreet Event in Asia, as well as the first detailed lithologic description of the Lower and Upper Kellwasser Events on the Indochina tectonic block. The Rhinestreet Event is an anoxic horizon at ∼375 Ma that is not associated with an extinction horizon. The Late Devonian’s Frasnian-Famennian extinction (372 Ma) is associated with the ocean anoxia of UKE. The Kellwasser Events have been studied globally in successions that generally exhibit characteristic black shales and/or black limestones. However, the sedimentological record of the Xom Nha section in central Vietnam differs from other successions as it shows a continuous lithology of nearly pure limestone without black shales or bituminous limestone layers. We also report microfossils from the section, including phosphatized mazuelloids that likely represent planktonic acritarchs, organic filaments, and arthropod cuticle, although beds during and after the UKE are barren. These are the youngest known occurrences of mazuelloids. The whole rock geochemistry patterns combined with scanning electron microscopy microtextural observations indicate that anoxia and potentially a shutdown of the carbonate factory both occurred during the UKE and Upper Rhinestreet Event. In contrast, the LKE does not show anoxia but does exhibit increased productivity. These contrasting signatures are consistent with other studies of the UKE/LKE globally and indicate that they had differing causes. We conclude that the UKE and LKE should be considered distinct events despite their temporal proximity and common nomenclature