Effects of the substratum on the formation of glacial tunnel valleys: an example from the Middle Pleistocene of the southern North Sea Basin

Abstract

2012 (October): Effects of the substratum on the formation of glacial tunnel valleys: an example from the Middle Tunnel valleys are elongated incisions formed by meltwater underneath ice sheets that rest on unlithified bed materials. The formation of tunnel valleys is commonly believed to be influenced by the properties of the preglacial bed; however, a detailed analysis of this relationship has not been performed to date. To determine whether tunnel-valley location and morphology are controlled by the substratum, a 3D seismic survey was combined with lithological data from the Wadden Sea area in the Dutch sector of the southern North Sea Basin. This study shows that tunnel-valley floors often coincide with seismic reflectors that mark lithological boundaries in the substratum, and that the location and depth of tunnel-valley incision vary as a function of the properties of the substratum as expressed by lithological and geophysical-log variations. Tunnel valleys are incised preferentially into fine-grained layers, while the top of coarser-grained units commonly coincide with the tunnel-valley floor. These observations indicate that the geometry and orientation of tunnel valleys in the study area are controlled by contrasts in lithological properties of the bed. An explanation for the observed lithological control might lie in large water-pressure differences over fine-grained and impermeable layers along the flow-path of subglacial meltwater flowing through the substratum, from areas with high pore-water pressure towards areas with relatively low pressures in the vicinity of meltwater channels. These pressure differences might have been sufficient for the fracturing and fluidization of these layers. The concepts presented here have implications for existing genetic models and for the prediction of tunnel-valley morphology in understudied areas.

The Glaciogenic Reservoir Analogue Studies Project (GRASP) - An Integrated Approach to Unravel Genesis, Infill and Architecture of Tunnel Valleys Reservoirs

“…In particular, a strong relationship between the contrast in permeability, porosity and cohesiveness between different layers of stratified bedrock (e.g. sand shale succession) on which the ice-sheet flows, and the morphology and depth of tunnel valley has been observed (see Janszen et al, 2012).…”

Glaciogenic reservoirs and hydrocarbon systems: an introduction

“…9;Praeg 2003;Buckley 2012;Andersen et al 2012;Kristensen & Huuse 2012;Moreau et al 2012;Müther et al 2012;Stewart et al 2012). Some studies suggest that the tunnel valleys are incised preferentially into low-permeability clays, because high-transmissivity substrates such as sand and gravel with high porosities and permeabilities would retard the incision process (Huuse & Lykke-Andersen 2000a;Janszen et al 2012b;Sandersen & Jør-gensen 2012). This mechanism may also account for observations from the Hirnantian glacial record of Gondwana, where tunnel valleys are typically incised into preglacial siltstones (Douillet et al 2012).…”

Seismic Geometry and Facies Analysis of a Quaternary Tunnel Glacial Valley Infill in the Dutch North Sea: Preliminary Results

“…The latter spaces can be subsequently filled with sediments associated with different postglacial processes and depositional environments (van der Vegt et al 2012). Time-transgressive formation of TVs has been proposed (Praeg 2003;Janszen et al 2012), occurring by a progressive headward excavation of the substrata during glacial retreat and a coeval infilling of its most distal sector where the depression is left empty by the retreat of the glacier.…”