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Faulting of an Alternating Sandstones and Shales Turbidite Sequence: Fault Mechanism and Architecture and Their Roles in Fault Seal

Abstract

Characterization of the faulting mechanism and fault architecture of a 5 km-long, N40°E-striking fault zone in a thick turbidite sequence of sandstones and shales exposed along the flank of an anticline in the Simi Hills, southern California, provides new insights into the interplay between the fault and the groundwater. Numerous monitoring boreholes across and along the fault within the sequence consistently dipping 20° to 30° to NW define the 3D distribution of the groundwater table in the area. Except for a small lateral step along the fault, hydraulic head measurements consistently show differences of 10s of meters from one side to another side of the fault. The fault is made up of at least three segments named here as northern, central and southern segments. Key information about the faulting mechanism has been collected at two sites. The first one is an outcrop of the central segment and the second one is a 194 m-long borehole, which intersects the northern segment at near its bottom. At the first site the fault zone juxtaposes sandstones against shales and consists of a 13 meter-wide fault rock including a highly deformed sliver of sandstone. The central core is 8 m-wide and contains mostly shale characterized by diffuse deformation with a complex texture. At the southeastern edge of the fault exposure, a shale unit parallel the fault zone and dipping 50° NW towards the fault zone provides the key evidence that the shale unit was incorporated into the fault zone in a manner consistent with shale smearing mechanism. At the second site, based on an optical televiewer image supplemented by rock cores, a juxtaposition plane (dipping 75° SE) between a highly fractured sandstone and a intensely deformed shale fault rock has been interpreted as the southeastern boundary of the fault zone. The shale fault rock is noticeably folded and brecciated with locally complex cataclastic texture. The observations and interpretations presented above suggest that the drop of hydraulic head detected across the fault segments is primarily due to the low-permeability shaly fault rock incorporated into the fault zone by the shale smearing process. Interestingly, at around the lateral step between the northern and the central fault segments, where the fault offset is expected to diminish (no hard link, no significant shaly fault rock and possible sandstone-to-sandstone juxtaposition), the groundwater levels measured on either sides of the fault zone are more-or-less equal.