SEISMIC INFILL FACIES IN BURIED TUNNEL VALLEYS FROM A 3D SEISMIC SURVEY IN THE CENTRAL NORTH SEA, NW EUROPE: SPATIAL CHARACTERISTICS AND ORIGIN

Introduction

Buried Quaternary valleys are widespread in the formerly glaciated NW European lowland, including most of the North Sea Basin. The valleys are tens of kilometres long, a few kilometres wide and up to 500 m deep. They have complex infill lithologies ranging from fine clays to sands, and coarse gravels comprising depositional facies ranging from glaciolacustrine to glaciofluvial and till deposits. The valleys are interpreted as subglacial meltwater channels (tunnel valleys). Onshore Europe the valleys constitute important freshwater aquifers. Recent 3D seismic data image the offshore valley morphology in unprecedented detail and along with shallow borehole data reveal important details about the infill sediments of the valleys with implications for onshore groundwater aquifers in tunnel valleys.

Observations

A 1244 km2 study area in the central North Sea contains a total of 15 buried valleys up to 39 km long, 3.8 km wide and 350 m deep. Two types of seismic infill facies are observed: a. horizontal/near horizontal – onlapping infill facies and a clinoform – downlapping infill facies. In seven of the fifteen valleys are both types of infill facies observed, in five valleys are only one of the facies found, and in three valleys the seismic response is to disturbed to make a positive identification of the infill facies. The gamma-ray logs reveal a close relation between the seismic infill facies and the lithology of infill sediments. It appears, that the clinoforms – downlapping infill is more sandy (in some places at the valley floor it is even gravely), than the horizontal/near horizontal infill, which again strongly supports a depositional origin.

In previous studies, glaciofluvial backfilling of the exposed valleys contemporaneously with the receding ice sheet margin is interpreted as the clinoform infill mechanism (Praeg, 2003). We will elaborate on this theory and present a conceptual model based on both backfilling of the valleys AND reworking of glacial sediments in a setting with a proglacial glaciofluvial meltwater channel system, which incorporates deposition of both types of seismic infill facies and explains the presence of them both in various combinations within the same valley.

References

Praeg, D. [2003] Seismic imaging of Mid-Pleistocene tunnel-valleys in the North Sea Basin. Journal of Applied Geophysics 53, 273-298.