Architecture of tunnel valleys in the southeastern North Sea: new insights from high‐resolution seismic imaging

Abstract

Tunnel valleys are assumed to form near the margin of ice sheets. Hence, they can be used to reconstruct the dynamics of former ice margins. The detailed formation and infill of tunnel valleys, however, are still not well understood. Here, we present a dense grid of high-resolution 2D multi-channel reflection seismic data from the German sector of the southeastern North Sea imaging tunnel valleys in very great detail. Three tunnel valley systems were traced over distances ranging between 11 and 21 km. All tunnel valleys are completely filled and buried. They differ in incision depth, incision width and number of incisions. The tunnel valleys cut 130-380 m deep into Neogene, Palaeogene and Cretaceous sediments; they show a lower V-shaped and an upper U-shaped morphology. For individual tunnel valleys, the overall incision direction ranges from east-west to northeast-southwest. Two tunnel valleys intersect at an oblique angle without reuse of the thalweg. These valleys incise into a pre-existing glaciotectonic complex consisting of thrust sheets in the northwest of the study area. The analysis of the glaciotectonic complex and the tunnel valleys leads us to assume that we identified several marginal positions of (pre-)Elsterian ice lobes in the southeastern North Sea.

The Heligoland Glacitectonic Complex in the southeastern North Sea: indicators of a pre‐ or early‐Elsterian ice margin

“…Rather, we provide a summary of the stratigraphy of the study area (Table 1); a more detailed description is provided by Lohrberg et al . (2020).…”

“…6; Lohrberg et al . 2020). This unambiguously shows that the glacitectonic complex is older than the tunnel valleys.…”

“…2009; Lohrberg et al . 2020). Consequently, the glacitectonic complex formed during the Elsterian glaciation or earlier.…”

“…These considerations are in accordance with the regional stratigraphy of our data, which shows generally shallow Miocene and Palaeocene reflector packages close to the shore (Lohrberg et al . 2020). Thus, we suggest that our data record mainly the older glaciations of the North Sea (MIS 16 to MIS 12) with a minor influence from MIS 6–10 (up to 15 m b.s.f.)…”

Spontaneously Exsolved Free Gas During Major Storms as an Ephemeral Gas Source for Pockmark Formation

“…Furthermore, dissolved gas concentrations can vary significantly on the Helgoland Reef due to different sediment compositions. The Helgoland Reef is traversed by the organic-rich paleo-river deltas of Elbe and Eider, their side arms, and multiple glacial tunnel valleys (Figge, 1980;Lohrberg, Schwarzer, et al, 2020;Lutz et al, 2009). It is likely that dissolved gas concentrations are higher in the paleo-river beds due to a higher organic matter decomposition (Zhang et al, 2019).…”