Development of deep-seated gravitational slope deformation on a shale dip-slope: Observations from high-quality drill cores

•We observed the internal structures of a gravitationally deformed shale dip-slope.•Fracture opening, jigsaw-like structures, and less planar features are characteristic.•Gravitational shear zones develop to form a through-going master sliding zone.•Incipient landslide shear zones are formed by shearing along bedding planes.•Shear zone development is related to river incision.

We analyzed high-quality drill cores with 100% recovery drilled into a gravitationally deformed dip-slope of a shale-dominated sequence of sediments in Japan. The slope had undulating surfaces but no well-defined landslide scarps, suggesting that parts of the slope had undergone gravitational deformation but had not completely separated from the surrounding bedrock. The gravitational deformation of shale and sandstone is characterized by disintegration and brecciation of sediments and the formation of pulverized zones with poorly developed planar structures, jigsaw-like structures, and fracture openings. Shear zones are distributed intermittently within the slope but have not merged discernably into a through-going master sliding surface. Incipient landslide shear zones form openings within coherent rock by shearing along bedding, with these shear zones potentially developing downward in a stepwise manner, which may be related to stress redistribution induced by river incision.