Tunnel valleys in the southeastern North Sea: more data, more complexity

Abstract

Large Pleistocene ice sheets have produced glacial structures both at and below the surface in northern Europe. Some of the largest and most erosive structures are so-called tunnel valleys (TVs): large and deep channels (typically up to 5 km wide and up to 400 m deep, with lengths up to 100 km), which formed below ice sheets. Although the subject of many studies, the details of their formation and fill are still not well understood. Here, we present an update on the distribution of TVs in the southeastern North Sea between Amrum and Heligoland based on a very dense grid of high-resolution 2D multi-channel reflection seismic data (400 m line spacing). The known tunnel valleys (TV1–TV3) in that area can now be traced in greater detail and further westwards, which results in an increased resolution and coverage of their distribution. Additionally, we were able to identify an even deeper and older tunnel valley, TV0, whose orientation parallels the thrust direction of the Heligoland Glacitectonic Complex (HGC). This observation implies a formation of TV0 before the HGC during an early-Elsterian or pre-Elsterian ice advance. For the first time, we acquired high-resolution longitudinal seismic profiles following the thalweg of known TVs. These longitudinal profiles offer clear indications of an incision during high-pressure bank-full conditions. The fill indicates sedimentation in an early high-energy environment for the lower part and a subsequent low-energy environment for the upper part. Our results demonstrate that a very dense profile spacing is required to decipher the complex incisions of TVs during multiple ice advances in a specific region. We also demonstrate that the time- and cost-effective acquisition of high-resolution 2D reflection seismic data holds the potential to further our understanding of the incision and filling mechanisms as well as of the distribution, complexity and incision depths of TVs in different geological settings.