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Importance of Slickenlines

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Slickenlines are scratches on fault surfaces resulting from shear motion (Figures 1, 2). Remember that shear fractures are smaller versions of faults that you’ll find in core - so slickenlines record the shear motion of fractures. You won’t find slickenlines on joint surfaces as these are opening mode fractures. Sometimes it is possible to tell the direction of shear by running your fingers over the slickenlined surface - this would be the smooth direction (top right to bottom left for missing block in Figure 1). Slickenfibres on the fault plane are mineral fibres that grow during fault movement.

Its very likely you’ll come across rubble zones when looking at rock core from oil and gas fields. Its well worth checking the pieces of rubble (as well as the intact core of course !) for evidence of slickenlines. The reason for this is that you’ll know you’re looking at natural (not induced) fractures. Figure 3 shows bagged rubble from North Sea appraisal well that was drilled through a seismically resolvable fault in sandstones. The piece of sandstone that the pencil is pointing at has a particularly well developed polished surface and slickenlines - indicating natural shear fractures and consistent of course with the seismic evidence in this case.

Figure 1. Slickenlines in playa lake siltstones, North Sea. Direction of travel of missing block is (relatively) from top right to bottom left. The white patches are calcite slickenfibres.

Figure 1. Slickenlines in playa lake siltstones, North Sea. Direction of travel of missing block is (relatively) from top right to bottom left. The white patches are calcite slickenfibres.

Figure 2. Slickenlines in Cretaceous shale core, West of Shetland, UK.

Figure 2. Slickenlines in Cretaceous shale core, West of Shetland, UK.

Figure 3. Slickenlines and polished surface in Jurassic faulted sandstones from the North Sea.

Figure 3. Slickenlines and polished surface in Jurassic faulted sandstones from the North Sea.

Slickenlines can be studied directly on fault planes in rock outcrops - easier to take measurements than on core as the core has to be oriented. The normal fault in Figure 4 has had some oblique movement - this is illustrated by the plunge of the slickenlines on the fault plane (slightly oblique to the pencil scale). In other words, there has been some movement along strike as well as in the direction of dip.

Figure 4. Slickenlines on a fault surface in outcrop - running slightly oblique to the pencil scale. Hopeman Sandstone, Inner Moray Firth, Scotland, UK.

Figure 4. Slickenlines on a fault surface in outcrop - running slightly oblique to the pencil scale. Hopeman Sandstone, Inner Moray Firth, Scotland, UK.