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an arid environment has more resistance to erosion, such as gypsum. The landslide features
vary in style from deep-seated rotational slips to block topples and may be up to 1 million cubic
metres in volume (Griffiths et al. 2002).
The spatial and temporal distribution of the above features in the landscape is governed
by their geographical location to the upper Pleistocene river capture site. The beheaded river
(Rambla de los Feos in Case Fig. 5.1a) experienced a reduction in erosion rates. In contrast the
captured drainage system (Upper Aguas; Ramblas Góchar, Cinta Blanca and Chopos in Case
Fig. 5.1a & b), and the area below the capture site (Lower Aguas), incurred dramatic increases
in net erosion. At the capture site base level was lowered by 90 m, starting the propagation of
a rejuvenating wave of incision up the catchments and accelerating sediment production and
delivery to the fluvial system (Mather et al. 2002; Stokes et al. 2002). The river capture event
was thought to have occurred at around 100 ka. More recent U-series dating, however, sug-
gests that it may be much younger, before 77.7
4.4 ka (Candy et al. 2005). Over this time
frame the impact of the river capture has reached 20 km upstream, at a decaying rate. Near the
capture site a tenfold increase in incision was experienced. This changed the sediment delivery
processes. Initially the generation of steep, rapidly unloaded slopes generated mass movement
processes such as landslide failures (Mather et al. 2003). In weaker lithologies the dominant
sediment delivery process later became dominated by more progressive slope erosion by surface
runoff and subsurface piping processes (pseudokarst). Most of this accelerated erosion is still
restricted to the main valley side-slopes, and has not yet reached the main drainage divides so
that overall
surface
lowering is much less than that recorded by the localized valley incision
(Case Fig. 5.1b).
The change in processes driven by the tectonically induced river capture has affected the
Pleistocene to Holocene sedimentation within the study area in a number of ways. In the pre-
capture state the then linked Upper Aguas and Feos fluvial terraces were dominated by large
material (pebbles, cobbles and boulders) sourced from the Sorbas Basin and routed to the sedi-
mentary basins to the south. Post-capture, however, the beheaded section of the river system
(the Feos) underwent a reduction in sediment and water discharge and became dominated by
localized and smaller scale slope erosion. This is reflected in a change in sedimentology of the
fluvial terrace deposits in terms of sediment calibre, provenance and sedimentary style. Post-
capture fluvial terrace deposits are dominated by more localized, typically smaller (granule and
pebble) material than the pre-capture terraces and no longer receive sediment sourced from
within the Sorbas Basin. In addition the pre-capture river terrace deposits are frequently
buried by fine-grained colluvium in the more open parts of the valley and coarser, alluvial-fan
material in the more valley constrained, basement sections of the river. In contrast the captured
Upper Aguas and Lower Aguas have seen enhanced sediment production and delivery to
the fluvial system, together with the re-routing of the high water and sediment discharges
sourced from the Sorbas Basin to the sedimentary basins to the east. Adjacent to the capture site
the sediment delivery from the valley sides has been dominated by a combination of landsliding
and gullying.
±
Relevant reading
Candy, I., Black, S. & Sellwood, B.W. (2005) U-series isochron dating of immature and mature calcretes as a
basis for constructing Quaternary landform chronologies for the Sorbas Basin, southeast Spain.
Quaternary
Research
64
, 100 -11.
