Late Quaternary large-scale rotational slides induced by river incision: The Arroyo de Gor area (Guadix basin, SE Spain)

Abstract

The Arroyo de Gor is a highly incised stream (up to 200 m) that has eroded into the Pliocene–Pleistocene infill of the Guadix basin in the Betic cordillera (SE Spain). This paper concentrates on a particular reach of this stream, namely the Cenascuras–Gorafe reach, where abundant inactive rotational slides can be recognized on both hillslopes of the canyon-shaped valley. The rotational slides have surface areas of 5–15 ha and volumes of mobilized material of 5–15 Hm 3. These rotational slides can be recognized by their main scarps, which are still well featured at both margins of the valley, as well as by the counter hillslope rotation of the strata in their bodies. The surfaces of rupture are concave, their width being greater than their length, i.e. the landslides are elongated in a direction perpendicular to the landsliding displacement and parallel to the stream. At depth, the surfaces of rupture developed on fine-grained sediments (mostly clays) underlying the conglomerates and sands affected by the rotational slides. The clays at the surface of rupture are smectite-rich and show several features produced by the landsliding, such as striations and sigmoidal-shaped structures. The Cenascuras–Gorafe canyon has been developed by a combination of entrenchment, fracturing and landsliding. Initial deep entrenchment of the stream is attributed to a base-level lowering related to the capture of the former endorheic basin by the Gualdalquivir river in the late Pleistocene. This river incision created a canyon with subvertical unstable walls, which due to gravitational instability would give way to vertical open tension cracks some distance from the canyon edge. The tension cracks would end at depths equal to the base of the canyon, which also coincides with the contact between the conglomerates and the underlying clays. The rotational slides are thought to have occurred during heavy rains by a combination of piping, which would have lengthened the tension cracks, and infiltration, which would reduce the shear strength along the gently dipping contact between conglomerates and the underlying clays. Thus, the blocks bounded by the tension cracks and the subhorizontal lithological contact would slide under their own weight on the subhorizontal surfaces of reduced shear strength. Several age constraints lead us to tentatively assign the landslides to the Isotope stage 5 (around 32 ka), an interglacial period agreed to be wetter and slightly warmer than the present-day semi-arid climate.