SCARP RETREAT RATES IN SEMIARID ENVIRONMENTS FROM TALUS FLATIRONS (EBRO BASIN, NE SPAIN)

Triangular slope facets or talus flatirons are presented as a useful t ool for the calculation of scarp retreat rates in arid regions. in the central sector of the Ebro Basin, sequences of talus flatirons have b een developed around mesas capped by Miocene Limestone. These talus fl atirons are grouped in four stages of slope evolution (S-2, S-3, S-4, S-5). BY the extrapolation of the relict talus profiles with the adjus ted log-functions, the position of the scarp for each one of the stage s is located. The mean values obtained indicate a scarp retreat of 23 m between stages S-3 and S-2, 8 m between S-4 and S-3 and 26 m between S-5 and S-4. The ages obtained by radiocarbon dating are 2529 +/- 52 B.P. and 2930 +/- 60 B.P. for stage S-2, 27 864 +/- 444 B.P. for S-3 a nd 35 570 +/- 490 B.P. for S-4. Consequently, the mean scarp retreat r ates are 0.9 mm/yr for the S-2-S-3 interval and 1 mm/yr for S-3-S-4. T he scarp retreat rate is related to the lithological and structural fe atures of both the caprock and the underlying clay sediments and to th e palaeoclimatic evolution of the region that controls the intensity o f weathering and water erosion processes that affect the slope system. (C) 1998 Elsevier Science B.V. All rights reserved.