Abstract:
Geomorphological, sedimentological and geochronological analyses have been
undertaken to decipher the effects of Quaternary environmental change on the
development of the Rio Bergantes, Ebro basin, northeast Spain. Field mapping and
profiling utilising large-scale aerial photography and airborne LiDAR data has
identified a sequence of six river terraces. Age control, provided by luminescence
dating methods, has revealed two major phases of valley floor aggradation at ~ 183 -
130 ka (Terrace BT1, elevated ~ 25 m above the modern river channel) and ~ 111-
26.8 ka (BT2, ~ 18-20 m), punctuated by ~ 20 m of incision in the intervening period.
Following these two phases of large-scale aggradation, the Bergantes river system
adjusted to a more sediment limited regime and responded to environmental changes
through a series of smaller cut and fill cycles. This resulted in four lower elevation
terraces (BT3-BT6) inset within the older (BT1 and BT2) valley fills. These lower
terraces have been temporally constrained at ~ 24.5-14.2 ka (BT3); 10.6-9.8 ka (BT4);
8.3-6.2 ka (BT5) and ~ 3 ka (BT5). Field sedimentological and stratigraphic analysis
of the alluvial fills, combined with sediment storage and export calculations, indicates
the large influence of tributary and hillslope derived sediment supply during the
aggradation of BT1 and, to a lesser extent BT2. This contrasts with the predominantly
trunk river derived deposits which make up the fill of BT3-BT6, when the hillslopes
and tributaries in the study area appear to be relatively de-coupled with the trunk
river. A lack of lithological variation in the Bergantes catchment, combined with large
amounts of carbonate, has restricted the possibility of deciphering any spatial or
temporal changes in sediment source areas. However, a new and innovative
methodology has been designed and implemented which calculates automatic
sedimentary section-scale grain size measurements from Terrestrial Laser Scan (TLS)
data. This methodology can now be applied to other ancient fluvial sedimentary
successions.
Data presented in this thesis suggest that the Bergantes river system has responded to,
and has preserved, a record of environmental changes operating both in sync with,
and at higher frequency than Milankovitch-driven glacial-interglacial cycles. Major
valley floor aggradation phases (or those associated with relatively large amounts of
fluvial sediment e.g. BT1 and BT2) correlate with Pleistocene glacial conditions in
MIS 6 and MIS 5, when the climate on the Iberian Peninsula was cool and dry.
Intervening phases of major valley incision appear to coincide with warmer and
moister climates in Pleistocene interglacials. The younger terraces (BT3-BT6) are
characterised by much smaller amounts of fluvial sediment. Dating control suggests
that their development correlates with stadial (aggradation) and interstadial (incision)
events during the Late Pleistocene and Holocene. Many phases of aggradation appear
to coincide with the North Atlantic Heinrich Events, which are also recorded in
marine cores off the west coast of the Iberian Peninsula, and also to other securely
dated phases of aggradation elsewhere in the Mediterranean basin.
