OCCURRENCE OF BURIED TUNNEL VALLEYS AND THEIR CONSEQUENCE FOR CONCEPTUAL GEOLOGICAL UNDERSTANDING AND 3D MODELLING

Tunnel valleys are formed by subglacial meltwater erosion close to the ice margin related to the retreat of one or a series of ice advances. Tunnel valleys were formed during the Pleistocene on the northern hemisphere often in intricate buried tunnel-valley networks consisting of multiple valley generations. The dimensions, frequency and complexity of the valleys vary significantly. The infill of a buried tunnel valley is the result of successive erosional and depositional events separated in time, and thus, the valleys often make up very complex valley systems with very heterogeneous sedimentary successions.

Shallow and solitary valleys may have a limited impact on the complexity of the geological setting of an area, but very often buried tunnel valleys are present in large and complicated systems with individual valley-erosions reaching depths of 300-500 m and widths of 1 to 2 km. In large areas of Denmark the buried valleys are so tightly spaced that a randomly placed borehole would have more than 50% chance of hitting a buried valley. This illustrates the need for a detailed mapping of the buried valleys in order to gain a conceptual understanding of the subsurface.

Therefore, the complexity of the subsurface in areas with buried tunnel valley systems needs to be mapped and conceptually understood before modelling in 3D can be successful. This requires mapping methods capable of resolving the geology but at the same time it requires very closely spaced measurements. This can be achieved today for instance by using airborne electromagnetic surveys in combination with seismic data and borehole data. But 3D-modeling the highly complex geology of an area with numerous buried tunnel valleys is very challenging because of the lithological and structural heterogeneities.

We will show examples of complex buried-valley geology and examples of how the valleys can be the key to the geological understanding. In addition to this we will show how we model the valleys in 3D.