On a Mechanism of Clay Smear Emplacement in Synsedimentary Normal Faults

Abstract

Faults that contain clay smears that may act as effective barriers against fluid flow. It is therefore of interest to understand the mechanism by which clay smears are emplaced in fault zones. Such a mechanism was inferred by the authors from observations made in the openpit browncoal mines at Frechen near Cologne, Germany, where synsedimentary (growth) faults were exposed by openpit mining in a paralic sequence of gravel, slightly consolidated sands, shales, and lignite of the Late Tertiary Rhine delta. The fault zones at Frechen contain persistent clay intercalations, a few millimeters to as much as 1 m in thickness, derived from extremely plastic shale beds in the upthrown and downthrown blocks, and smeared-out-usually without substantial admixing of sandy material from the fault walls - over distances as great as 70 m in dip direction. In the Frechen mine, these 'clay smears' are found to seal faults against transverse groundwater flow. The effectiveness of this important fault sealing mechanism, which is likely to operate only in soft sediments and at sufficiently slow fault slip velocities, can be quantified in an approximative way in terms of a dimensionless ratio of the clay extrusion rate over fault slip velocity. While it appearsmore » from this study that clay smears start to form at shallow depth, field observations reported elsewhere suggest that they may grow and preserve their sealing properties during subsequent burial.« less