River response to Quaternary subsidence due to evaporite solution (Gállego River, Ebro Basin, Spain)

Abstract

The stream terrace evolution of the Gállego river during the Quaternary was controlled by both climatic change and subsidence. Quaternary terrace deposits, overlying Tertiary clay and limestone, are between 2 and 5 m thick, whereas above evaporite formations the alluvial deposits may be as much as 110 m thick. Chronologically, the first period of alluvial thickening involved the stream terraces T2 (+ 105 m above the present thalweg), T3 (+ 95 m) and T4 (+ 85 m), which have been dated by paleomagnetic reversals as Matuyama (pre-780,000). The second subsidence period affected the deposits of the stream terrace T9 (+30), dated as Brunhes (post-780,000). In both thickening periods, the subsidence was due to solution of the underlying evaporite formations (halite and gypsum), presumably, during intervals of high water flow. In the proposed model, the valley subsidence was balanced by stream aggradation maintaining the river equilibrium profile. The subsidence recorded in the alluvial deposits shows a complex spatial and temporal evolutionary pattern and total subsidence was assumed to be equal to the alluvial thickening for each subsidence period, reaching up to 165 and 25 m, respectively.