Scarp retreat rates in semiarid environments from talus flatirons (Ebro Basin, NE Spain)

Triangular slope facets or talus flatirons are presented as a useful tool for the calculation of scarp retreat rates in arid regions. In the central sector of the Ebro Basin, sequences of talus flatirons have been developed around mesas capped by Miocene limestone. These talus flatirons are grouped in four stages of slope evolution (S2, S3, S4, S5). By the extrapolation of the relict talus profiles with the adjusted log-functions, the position of the scarp for each one of the stages is located. The mean values obtained indicate a scarp retreat of 23 m between stages S3 and S2, 8 m between S4 and S3 and 26 m between S5 and S4. The ages obtained by radiocarbon dating are 2529±52 B.P. and 2930±60 B.P. for stage S2, 27 864±444 B.P. for S3 and 35 570±490 B.P. for S4. Consequently, the mean scarp retreat rates are 0.9 mm/yr for the S2–S3 interval and 1 mm/yr for S3–S4. The scarp retreat rate is related to the lithological and structural features of both the caprock and the underlying clay sediments and to the palaeoclimatic evolution of the region that controls the intensity of weathering and water erosion processes that affect the slope system.