Study Area - Response of Flood Events to Land Use and Climate Change - page 115

4.7 Geology and Geomorphology

The Barcelonnette Basin is geologically characterized by two Eocene sheet thrusts
(Autapie and Parpaillon), made of strong limestones or sandstones, and overlaying
an autochthonous bedrock of soft autochthonous Oxfordian-Callovian black
marls. The thickness of the black marls reaches 250-300 m. The black marls have
a thickness of 250-300 m and can be divided into three subsets: (a) The Lower
Callovian black marl with detrital plates (50-60 m thick); (b) The Middle and
Upper Oxfordian black marls (150-250 m thick); (c) with centimeter argillaceous-
limestone banks; Some rare outcrops of Argovian black marl (15-20 m thick) [ 10 ].
Due to this predisposing geological structure, among some affects of this
geological structure are: the hillslopes are affected by (a) severe gullying; (b)
shallow landslides; and (c) deep-seated large landslides (Fig. 4.18 ).
The geomorphological (Fig. 4.19 ) setting of the Barcelonnette basin is com-
plex, and has strong control of landslide distribution. The complexity of this
geomorphological setting has a strong control of landslide distribution [ 10 , 11 ].
Hilltops consist of very steep slopes (slope gradient ranging between 30 and 70)
and more gentle slopes (slope gradient ranging between 10 and 20). Alternating

Fig. 4.18 Geological and geomorphological cross-section of the Barcelonnette Basin [ 10 ]