Structural and sedimentary controls on tunnel-valley evolution: Insights from high-resolution 3D seismic data

This study investigates the distribution, morphology, and infill of buried Pleistocene tunnel valleys in northern Germany and adjacent offshore areas, with implications for long-term safety assessments of radioactive waste repositories. Tunnel valleys, formed by pressurized subglacial meltwater, are prominent erosional landforms reaching depths of up to 600 meters b.s. Using the high-resolution 3D seismic dataset GeoBasis3D (2021), acquired in the German EEZ, we examine intersecting tunnel valleys, including one located above the Belinda salt dome.

The 3D seismic data enable detailed mapping of tunnel valley geometries and sedimentary facies. The lower parts of the tunnel valley are marked by steep flanks and chaotic internal reflectors, indicative of rapid, subglacial infill processes. Tunnel-valley infills reveal cut-and-fill structures, erosional unconformities, and parallel reflectors that indicate multiple depositional phases. Crestal faults above the Belinda salt dome displaced the tunnel-valley fill and locally reach the seafloor, confirming Pleistocene fault activity. However, fault activity did not impact the incision depth and only slighty affect the course of the tunnel valley.

On a regional scale, structural controls were evaluated by correlating tunnel-valley orientations with the trends of faults and salt structures. While some tunnel valleys align with (neotectonically active) faults, particularly where ice flow paralleled fault trends, no consistent correlation with salt structures was observed.

Our findings highlight the importance of integrating seismic stratigraphy and structural geology to understand tunnel-valley evolution. They also underscore the limited predictive value of structural trends for future subglacial erosion, which is essential for assessing repository site integrity over long timescales.