Petrographic and geochemical characteristics of a possible vent-related ejecta deposit at the Paleocene-Eocene boundary in the Vøring Basin, offshore Norway

Hydrothermal vent complexes are degassing structures that form in response to rapid volatile generation and release associated with igneous sill intrusions in sedimentary basins. They are discovered in numerous basins worldwide, originating from sills and contact aureoles and terminating at the paleosurface where they form up to 10 km wide craters. Field studies and numerical models have suggested that the venting processes were explosive, releasing aureole-derived gases, sedimentary pore fluids, and fragmented sedimentary rocks to the seafloor or land surface. However, ejecta deposits originating from hydrothermal vent complexes are poorly studied and hard to identify, hampering detailed reconstructions of vent formation and evolution. Here we report the characteristics of a possible ejecta deposit from Vøring Basin Hole U1570D drilled as part of IODP Expedition 396 in 2021. During core logging, an unusual layer was identified immediately overlaying the top Paleocene strata. This layer is about 2m thick, contains Apectodinium augustum dinocysts restricted to the Paleocene Eocene Thermal Maximum (PETM), and also yields abundant reworked Paleocene and Cretaceous microfossils. Moreover, the layer is characterized by rounded fragments of claystone, angular chert and quartz fragments, dolerite fragments, fresh and devitrified volcanic tephra, and a mixed groundmass of smectite-illite with diatoms and early diagenetic pyrite. Electron microprobe analyses document a bimodal tephra geochemistry, with both basaltic and rhyolitic compositions and morphologies indicating no or minor reworking. In the presentation we will discuss two possible formation scenarios for the layer, including 1) erosion from nearby marginal highs, and 2) ejecta deposit sourced from an explosive submarine eruption from a hydrothermal vent complex, mobilizing Cretaceous and Paleocene strata from the conduit zone. In any case, the bimodal tephra composition stresses the presence of an evolved igneous system in the Vøring Basin during the PETM, with a potential genetic link to a recently discovered Paleocene granite.