Palaeo and present-day fluid flow through Eocene clay layers in Flanders. Fracturing and intra-formational faulting of the Ieper clay. Evidence for a major Palaeo-flow event

High-resolution reflection seismic investigations have revealed large-scale intra-formational fault patterns which involve faults with a throw of several metres, sometimes up to 10 m, which however are confined to the clay layer and fade away towards its base and top boundary. Fault patterns have been observed on quarry profiles in South Flanders, and are apparently similar to those observed on seismograms. The fault surfaces are generally characterized by a well-developed fault mirror with striae and a black fault gauge. A striking feature has been the discovery of micro-fossils within such gauge, originating from clay horizons about 60 m lower. These observations have led to a model, involving a build-up of under-compaction in the early burial history of the clays, and its subsequent relaxation through hydraulic fracturing, at fairly shallow depths. This field evidence of an early major hydraulic fracturing and dewatering event, which left its scars under the shape of a general network of faults and intense micro-fracturing, deserves attention in any evaluation of the seal capacity of the Ieper Clay under renewed hydraulic and/or thermal stresses. (R.P.)