The emplacement of clay smears in synsedimentary normal faults: inferences from field observations near Frechen, Germany

Abstract

Background and methods : This paper reports on an outcrop study of clay smears in synsedimentary normal faults that were exposed in the open-cast lignite mines at Frechen near Cologne, Germany. The observations made are interpreted in terms of a mechanism of clay smear emplacement. Results and conclusions : The fault zones at Frechen contain clay fillings of up to 1 m in thickness, derived from extremely plastic shale source beds and smeared out over distances as much as 70 m in dip direction. The generation of substantial smears requires slow fault displacement rates and sufficient shale ductility. When a thick shale source bed is traversed by a normal fault, it is first flexed and eventually disrupted by a pull-apart mechanism that creates room for the emplacement of thick clay smears. Simple theoretical considerations suggest that the source bed thickness to some power n + 1 ≥ 2 may be a key parameter in the ranking of seal quality. The length of continuous smears increases with source bed thickness, but will ultimately be controlled by the smearing process. The latter remains to be investigated.