The interpretation of shallow seismic profiles has indicated that open and buried channels are present over large areas of the North Sea. These features occur at various levels within the Quaternary sequence, and their age of formation ranges from Middle to Late Pleistocene. Although the majority of the channels have been completely filled with sediment, several have remained open and have only a partial sedimentary infill. Some of the channels are of undoubted subglacial origin, but the majority were probable formed by fluvial processes, with their profiles having been subsequently accentuated by catastrophic composition. Consequently, they may represent a hazard to structures sited above them. The variable composition of the fill is responsible for producing seismic "pull up" and "pull down" effects on the profiles, and these effects should be taken into consideration when examining geological structures at greater depth.

INTRODUCTION

Since the first shallow seismic profiles were obtained from the North Sea in the mid-1960s, features described as buried channels have been identified in many areas, at several different horizons within the Quaternary sequence. These channels, which vary from 0.5 km to 3 km wide and 50 m to 400 m deep, have been mapped during regional survey and occur on seismic records, either singularly or as composite features. They typically extend up to 50 km in length and are usually aligned north-south in an anastomozing pattern. In one example, a buried channel was traced from the Dogger Bank to the Witch Ground Basin, a distance of 240 km (Jansen, 1976). These filled channel features have been compared with unfilled or open channels which occur throughout the North Sea and have for centuries been known to fishermen by such evocative names as Devil's Hole and Silver Pit (Fig 1).

The stratigraphic setting of these features is illustrated in Fig 2. In the central and northern North Sea, the channels occur at three main horizons within the Quaternary succession and appear to correlate with the Elsterian, Saalian and Weichselian glacial periods (Stoker et al., 1985). In the southern North Sea, the channels are mainly of Elsterian and Weichselian age (Balson and Cameron, 1985).

Fig 1 Map of North Sea showing locations of areas, features and seismic sections referred to in text WGB, With Ground Basin; FA, Forth Approached, EB, East Bank; DB, Dogger bank; W, Wash; F. Frazerburgh; P, Peterhead, A, Aberdeen; L, Lincolnshire; EA, East Anglia, fd, Fladen Deeps; bf, Buchan Deeps; dh, Devil's Hole; bg, Bayman's Hole; wh, Whitby Hole, osp, Outer Silver Pit, nsh, New Sand Hole; sp, Silver Pit; doh, Dowsing Hole; ch, Codling Hole. Numbered dots refer to locations of Figs 4–7 (available in full paper)

Fig 2 Schematic cross-section showing the stratigraphic relationships between the three main channelled horizons in the central North Sea (modified after Fyfe, 1985). Horizon 1 is of Weichselian age; horizon 2 is Saalian age; horizon 3 is of Elsterian age (not to scale) (available in full paper)

Previous Work

Over the past 30 years, various hypotheses for the formation of channels in the North Sea have been put forward.

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