Collected reprints: Abstract 3418

Collected reprints


Henriet, J.P.; D'Olier, B.; Auffret, J.P.; Andersen, H.L. (1982). Seismic tracking of geological hazards related to clay tectonics in the Southern Bight of the North Sea. Symposium Engineering in Marine Environment : 1.5-1.15

A joint seismic exploration programme, carried out in recent years by Belgian, British, French and Danish university laboratories in the southern North Sea and Schelde estuary, has revealed various styles of clay tectonics in Eocene and Oligocene deposits. In this paper particular attention is drawn on the remarkable deformation features observed in the Ieper or London clay, which forms the subsoil of the major part of the Thames estuary and the Belgian shelf sector. Those clay structures have eluded many previous investigations, mainly due to a lack of interfaces with large reflection coefficients. When however a seismic source is adequately tuned, the sum of reflection responses from the many subtle interfaces within the clay sequence may construct an interference composite, which closely moulds the structure of the clay beds. The deformations thus revealed are closely bound to the London Clay itself, fading out in overlying sands are not affecting the Paleocene basement. Some main observed deformation features are: imbricated fault systems with titled blocks and inclined fault planes, throwing up to 5-10 m, collapse structures and festoonlike sequences of cuspate anticlines, often developing into diapirlike escape pipes which locally pierce into the Quaternary cover. Some of these features have already occasionally been observed on land, both in the Thames valley and in south-west Belgium. The seismic records however present a first picture of their general structural context. Regarding their origin, different mechanisms have to be taken into consideration, including pore pressure - induced shear strength reduction and gravitational solicitation on gentle slopes at an early stage of compaction, overpressure relaxation, microseismic activation at the end of Ypresian times and glacitectonic stresses in Quaternary times. The presence of such slip planes in Tertiary clays might represent potential hazards, especially for sea-floor gravity structures, and should be truly evaluated. Anyhow, the further development of tunable seismic profiling systems might turn out a powerful tool for the investigation of clay dynamics in offshore work.

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