Intramontane basin opening and erosive unloading: A 10Be-supported model for the Usia drainage basin, southern Pyrenees.
Final Report Abstract
Intramontane basins function as sediment traps and provide important insight into erosion and sediment dynamics in mountain ranges. Aiming at establishing the drivers and phasing of basin erosion in the southern central Pyrenees, adjacent 10–100 km2 catchments were investigated as to their morphogenetic evolution. The Valle de la Fueva is a highly erodible drainage basin which comprises a number of catchments in the Ainsa depression – i.e. an Eocene piggy-back-type basin situated in the south-Pyrenean thrust belt. Field observations revealed distinctive morphogenetic units (or complexes) of high-lying talus slopes (i), extensive pediments and glacis (ii), and deeply incised ravines with "cut-in-fill"-type terraces (iii). The basin opening and long-term drainage connectivity of the La Fueva watersheds is not well constrained, but outlet cross-sections, feature geometries, and associated 10Be exposure ages argue for a relatively recent drainage separation at or before Marine Isotope Stage 4. Three main morphogenetic complexes reflect successive basin development stages from extensive surface denudation to intense fluvial dissection, and late-stage terrace formation. 10Be exposure ages for talus slopes and glacis cluster between 18 and 45 ka (MIS 3–MIS 2) and testify to a lasting cold-climate genesis of both the present talus surfaces (i) and extensive denudation plains (ii). The lower complex (iii) which comprises deeply cut ravines and lower aggradation surfaces developed during late MIS 2 climatic warming and denotes an Early Holocene termination of intense catchment erosion (ca 8–12 10Be-ka), followed by relative quiescence and minor fluvial incision and terrace formation during Middle-Late Holocene. The preservation of pronounced cold and warm climate morphogenetic features speaks for a transient disequilibrium of the present La Fueva basin landscape. Non-uniform stream gradients and distribution of knickpoints indicate that incision magnitudes are mainly a function of catchment size and directly linked to local precipitation and runoff volumes. Our findings suggest that non-glaciated mountain catchments seem to have high preservation potential for (at least) the latest landscape rejuvenation cycle. They could therefore play a key role in assessing the direct impact of major climate transitions on surface processes and landscape transformation. Improvements in quantifying (paleo-) surface process rates are essential because common glaciofluvial chronologies and long-term basin scale erosion rates tend to be affected by systematic time-averaging bias.
Publications
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(2018). Direct response of small non-glaciated headwater catchments to Late Quaternary climate change: The Valle de la Fueva, southern Pyrenees. Geomorphology, 318, 187-202
Stange, K.M., Midtkandal, I., Nystuen, J. P., Kuss, H.-J., and Spiegel, C.
