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Anatomy of a Normal Fault with Shale Smear: Implications for Fault Seal

¹ Shale Smear Project, Department of Geological and Environmental Sciences, Stanford University, California; aydin{at}pangea.stanford.edu
² Department of Geology and Mineralogy, Ben Gurion University, Beer Sheva, Israel

Atilla Aydin received his B.S. degree in geological engineering from Istanbul Technical University (Turkey) and his M.S. degree and Ph.D. in geology from Stanford University. After three years of teaching at Istanbul Technical University and ten years of teaching at Purdue University, he moved to Stanford University in 1991 as a research professor of structural geology and geomechanics. He is codirector of the Rock Fracture Project and director of the Shale Smear Project at Stanford. His research interest includes how rocks break and fluids flow through fractures and faults with a primary application to hydrocarbon entrapment, migration, and recovery.Yehuda Eyal is currently a professor of structural geology in the Department of Geological and Environmental Sciences at the Ben-Gurion University of the Negev, Israel. He received his B.S. degree (1965), M.S. degree (1967), and Ph.D. (1976) from the Hebrew University of Jerusalem, Israel. His current research interests are focused on development of the Syrian Arc and the relationships between the Syrian Arc and the Dead Sea stress fields in the Middle East; fractures and other mesostructures as strain and stress indicators; and kinematic analysis of shear zones, such as the eastern Sinai and central Sinai-Negev shear zones.

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.

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