Specific Priority Subject 2.1 Summary Report - Improving flood prediction, communication and impact assessment

[摘要] This report describes and discusses a borehole drilled at the Olympic Park development site at Temple Mills, in the Lea Valley near Stratford in East London. The original borehole number is MBHCZ6A-159. It has been registered as TQ38NE 1366 in the BGS Single Onshore Borehole Index. The borehole penetrates a 70 metre-thick sequence which, from the top downwards, passes through made ground and Quaternary fluvial deposits before revealing a 43 metre-thick zone of bedrock m��lange. This m��lange includes material from the lower part of the Lambeth Group, the Thanet Formation, and the Chalk. It is interpreted as being the product of pervasive soft-sediment deformation formed during a series of short-lived elevated pore water events, based upon the presence of diagnostic geological structures characteristic of ductile deformation, probably during rapid ejection of groundwater under artesian pressure. During these events, fragments of chalk were carried to within 17 metres of the surface, some 20 metres above where the top of the Chalk was encountered in nearby undisturbed sequences. Conversely, the presence of glauconitic sand apparently derived from the adjacent Palaeogene bedrock at 69 m depth, more than 30 m below the top of the Chalk nearby, implies that there was also some downwards movement during m��lange formation. The presence of such a thick disturbed sequence beneath superficial deposits is extremely unusual although not unique. A model is proposed where this structure and internal deformation is explained by processes of pingo formation and decay in a part of the Lea Valley where the bedrock aquifer is confined by an aquitard as little as 3 m thick. Alternatively, it is possible that the structure formed during release of artesian groundwater pressure following fluvial scour. In either case, it is very likely that the structure lies above a fault zone including fractured, possibly karstic chalk, with high groundwater conductivity. The structure is most likely to have formed during Late Devensian times, during the deposition of the Shepperton Gravel, but it might be older, possibly pre-dating part or all of the Devensian Kempton Park Gravel.

[发布机构] NERC - Centre for Ecology and Hydrology

[学科分类] 地表过程