PLEISTOCENE BEDROCK INCISION FROM FLUVIAL RECORDS IN THE PYRENEES AND EBRO BASIN (SPAIN)

In the south central Pyrenees and adjacent Ebro basin, stepped terrace sequences provide stratigraphic markers for analysis of tectonic and climatic controls on long-term landscape evolution. Comparing a modern valley profile with paleo-longitudinal profiles, we measure the spatial and temporal variation in incision orthogonal to the orogen. Measurements of strath height, soil stratigraphy, and optically stimulated luminescence (OSL) dates allow correlation of Pleistocene strath terraces along 145 km of the Cinca River and 95 km of the Gallego River. Along the Cinca, fluvial sands of the Qt5, Qt6, Qt7, Qt8, and Qt9 terraces give mean OSL dates of 178 ± 21 ka, 97 ± 16 ka, 64 ± 4 ka, 41 ± 4 ka, and 11 ± 1 ka. Qt3 deposits show normal magnetic polarity, as do lower terraces. Qt2 sands, however, show reversed magnetic polarity, therefore pre-dating the Brunhes normal epoch and providing an upper age limit on the Qt3 of ~780 ka. For same-aged straths, bedrock incision rates are higher in the upper versus the lower reach of the Cinca. Highest bedrock incision rates occurred during formation of the Qt5 and Qt7 straths, coinciding with isotope stage 6 and 4 glacial events. Spatial variation in stream incision along the Cinca reflects the flexural response to erosional denudation, which decreases away from the axis of the Pyrenees. Temporal variations appear to reflect transient response to climatic changes, which influence discharge and therefore erosivity. Although changing Quaternary climates may have decreased profile steepness or increased profile concavity, accounting for some of the observed headwards divergence, grain size analyses do not support this. Along the Gallego River, fluvial sands give weighted mean OSL dates of 150 ± 9 ka, 67 ± 7 ka, and 44 ± 5 ka. Correlated with the Qt5, Qt7, and Qt8 terraces on the Cinca River, straths diverge upstream on the Gallego as well. Resistant lithologies at the mountain front form a prominent knickpoint that appears to exert a strong control on incision upstream. Along the lower reach, incision increases dramatically at 44 ka, associated with changes in channel form, and suggests a tectonic influence.