RIVER RESPONSE TO QUATERNARY SUBSIDENCE DUE TO EVAPORITE SOLUTION (GALLEGO RIVER, EBRO BASIN, SPAIN)

The stream terrace evolution of the Gallego river during the Quaternar y was controlled by both climatic change and subsidence. Quaternary te rrace deposits, overlying Tertiary clay and limestone, are between 2 a nd 5 m thick, whereas above evaporite formations the alluvial deposits may be as much as 110 m thick. Chronologically, the first period of a lluvial thickening involved the stream terraces T2 (+105 m above the p resent thalweg), T3 (+95 m) and T4 (+85 m), which have been dated by p aleomagnetic reversals as Matuyama (pre-780,000). The second subsidenc e period affected the deposits of the stream terrace T9 (+30), dated a s Brunhes (post-780,000). In both thickening periods, the subsidence w as due to solution of the underlying evaporite formations (halite and gypsum), presumably, during intervals of high water flow. In the propo sed model, the valley subsidence was balanced by stream aggradation ma intaining the river equilibrium profile. The subsidence recorded in th e alluvial deposits shows a complex spatial and temporal evolutionary pattern and total subsidence was assumed to be equal to the alluvial t hickening for each subsidence period, reaching up to 165 and 25 m, res pectively. (C) 1998 Elsevier Science B.V.