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On a Mechanism of Clay Smear Emplacement in Synsedimentary Normal Faults

LEHNER, F. K., Koninklijke/Shell Exploratie en Produktie Laboratorium, Rijswijk, The Netherlands, and W. F. PILAAR, Shell Internationale Petroleum Maatschappij, The Hague, The Netherlands

Faults that contain clay smears 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. A detailed examination of the exposures shows that the very soft and plastic shale beds are affected by synsedimentary normal faulting very soon after their deposition. A flexure first forms in the shale bed and as it enters the (possibly stationary) depth of incipient (growth) faulting, the flexure ruptures and is traversed by the fault that emerg s at the top of the shale slightly offset towards the downthrown block. This 'pull apart' mechanism creates the space that is continuously filled by soft clay material extruded from the hanging and foot wall sections of the intersected shale bed. As faulting progresses, massive clay smears can be generated in this manner and bridge substantial throw intervals, before becoming disconnected. 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 appears from this study that clay smears start to form at shallow depth, field observations reported elsewhe e suggest that they may grow and preserve their sealing properties during subsequent burial.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)