SUSTAINED OCEANIC ANOXIA IN THE LATE DEVONIAN PHO HAN FORMATION, CAT BA ISLAND, VIETNAM

The Hangenberg Event at the Devonian-Carboniferous (D-C) transition is a period of mass extinction and rapid global faunal changes characterized by carbon-rich black shale deposits in many locations throughout Europe, North America, and Asia. Conodont biostratigraphy indicates that the D-C boundary is present within the Pho Han Formation on Cat Ba Island in northeastern Vietnam, which represents a basinal facies of the South China platform. The Pho Han Formation is primarily composed of whitish to dark gray carbonates punctuated with black shales. Scanning electron microscopy (SEM) imaging has been used to determine presence and size distribution of pyrite framboids, which serve as a proxy for water column redox conditions. Samples examined to date contain more than 7000 framboids/cm². Between 88.5% and 97.1% of the framboids observed were less than five microns in diameter, indicating euxinic conditions that extended throughout the basin’s history and were not constrained to the Hangenberg Event. SEM analysis also confirms the presence of kerogen within many of the beds. Whole rock geochemistry analyses indicate a Mo spike near the D-C transition, further confirming the highly anoxic conditions around the time of the Hangenberg Event. The geochemical results also indicate clastic input stratigraphically above the Mo spike, possibly due to a major regression (consistent with the Hangenberg Event in other locations). This research shows long-term, sustained anoxia/euxinia in this ocean basin and supports the location of the Hangenberg Event within the Pho Han Formation on Cat Ba Island, Vietnam.