The Seidfjellet Formation in NW Spitsbergen: A Window into Miocene Volcanism and Tectonics of Arctic-Atlantic Gateway

The Miocene epoch on Svalbard is characterized by volcanic activity and tectonic uplift, but the causes and relationship between these two processes remain debated. The evidence of coeval basaltic magmatism has probably affected a large area including Svalbard. The Seidfjellet Formation, a series of basaltic lava flows, represents a unique late Miocene subaerial magmatic event (5–10 Ma) in northwestern Spitsbergen. These flows, covering more than 200 km², are exposed on top of numerous mountains in Andrée Land overlying Devonian sedimentary rocks. This study investigates the structure, composition and origin of this underexplored igneous province within a tectonomagmatic context, focusing on defining the magnitude, paleoenvironment, and chronology of the volcanism and contributing to our understanding of the Miocene evolution in Svalbard and adjacent Arctic regions.

In the summer of 2023, we systematically mapped and sampled (n = 83) well-exposed outcrops along Woodfjorden, logging basaltic lava flows from an elevation of approximately 600 to over 1000 m above sea level. Additionally, we acquired photospheres and photographs using unmanned aerial vehicles (UAVs). Photographs were processed to obtain high-resolution georeferenced digital outcrop models (DOMs) for systematic mapping of the Seidfjellet Formation and its relationship with the pre-basal emplacement paleosurface. To enhance the consistency of our dataset, 13 legacy samples collected in 2014 were analyzed for standard geochemical characterization, including major and trace element concentrations, isotopic ratios, and ⁴⁰Ar/³⁹Ar age determination.

The mapped lava flow sequences have variable thicknesses, with 400 m being the observed local maximum, indicating significant magma accumulation.  A massive 50 m thick olivine-rich sheet-flow unit is present in the lower part of the formation. Locally, a distinct hyaloclastic unit documents subaqueous lava emplacement. In contrast, the upper sections provide clear evidence of subaerial emplacement, with pahoehoe lava flow features. The interpretation of DOMs, the distribution of the lava flows as well as GIS-based thickness and volume estimates suggest that the igneous province extends more widely than what is evident from the existing remnant outcrops.  The Seidfjellet Formation shows variable sediment-basalt transitions, including sharp valley infill profiles and erosion surfaces above Devonian sandstones. Thickness estimates and remnant outcrop distributions point towards an eruption center near Scott Keltiefjellet, where hyaloclastites and dolerite layers are also exposed. Geochemical analysis reveals both silica-saturated 'tholeiitic' and silica-undersaturated 'alkaline' magmas, with isotopic evidence of crustal-contaminated mantle-derived magmas, reflecting a complex geological setting. Six Ar/Ar ages document a timespan of over 1 million years between 8 and 10 Ma, whereas one sample has an age of about 5 Ma refining earlier estimates.

The Seidfjellet Fm. represents the only subaerial expression of Miocene volcanic activity in Svalbard. Linking this event to similar and coeval features in the Arctic, both in terms of geochemistry and paleoenvironmental studies, provides an opportunity to identify a significant magmatic event potentially linked to the region's vertical motion history.