Anatomy of a normal fault with shale smear: Implications for fault seal

Abstract

We describe the geometry and structural attributes of an exceptionally well-exposed normal fault with shale smear in southern Israel. We discuss the mechanism by which the shale was emplaced into the fault zone and compare and contrast it with other shale emplacement mechanisms. The shaly unit, the Ora formation, is about 110 m thick in normal stratigraphic position and is composed of a lower shale member of approximately 60 m and an upper shale member of approximately 30 m, separated by a middle carbonate-bearing unit approximately 20 m thick. One or both of these shale units occur along the entire 2 km length of the fault, although with a drastically reduced thickness. The upper shale member vanishes along a large part of the fault, but the lower shale appears to survive in the fault rock with less than 0.5 m thickness. Both shale units have been stretched for more than 250 m (the fault throw) between nearly planar discontinuities defined by the footwall or hanging-wall cutoff planes (duplex). Thus, the fault geometry, position, and distribution of the remaining shale rocks reveal a smearing process by which the shale units reduce their thickness or vanish by thinning perpendicular to the fault and stretching parallel to the fault. The continuity of the lower shale unit as a fault rock appears to be barely maintained for a throw/thickness ratio of approximately 4 but not for the upper shale unit, which has a throw/thickness ratio of approximately 8. The brittle carbonate-bearing rocks within the shaly rocks are fractured and faulted and show boudinage at various scales, which result in significant variation in the lithological and mechanical character of the fault zone along the throw interval. The faults and joints, however, do not appear to degrade the integrity of the smeared shale as a lateral barrier along the fault zone.