Geomorphic and stratigraphic evidence of incision-induced halokinetic uplift and dissolution subsidence in transverse drainages crossing the evaporite-cored Barbastro-Balaguer Anticline (Ebro Basin, NE Spain)

The evaporite-cored Barbastro-Balaguer Anticline, located in the northeastern sector of the Ebro Tertiary Basin, NE Spain, is traversed perpendicularly by several drainages coming from the Pyrenees, including the Cinca, Noguera-Ribagorzana and Segre rivers. The terraces associated with these discordant fluvial systems, deposited upon evaporitic and detrital formations across the anticline, allow the analysis of geomorphic and stratigraphic anomalies related to the soluble and ductile nature of the halite-bearing rocks in the core of the anticline. Geomorphological mapping reveals that the gravels discontinuously capping the evaporitic core of the anticline, together with some alluvial mantles disconnected from the present-day river valleys, constitute the oldest sediments deposited under exorheic conditions in this sector of the Ebro Tertiary Basin. These alluvial deposits inset into the basin fill record an early post-capture palaeogeographic stage during which the drainage was dominated by unconfined distributary channels developed on extensive alluvial fans fed by perennial rivers coming from the Pyrenees. The terraces associated with the present-day fluvial valleys correspond to a later palaeogeographic stage characterized by a well-integrated drainage confined to fluvial valleys entrenched in the basin fill. These terraces show evidence of both dissolution-induced subsidence and halokinesis restricted to areas underlain by the evaporites of the anticline core. The deposits of the highest terrace levels of the Noguera-Ribagorzana and Segre rivers and its tributaries, Lo Reguer Creek and Farfanya River, are locally thickened filling basins generated by dissolution-induced synsedimentary subsidence up to several kilometers long and more than 100 m deep. Subsidence caused by the karstification of the evaporites reached higher magnitude and extent during the early stages of development of the new external drainage network, When the more soluble halite-bearing units were closer to the new incisional base level. Most likely, the development of a thick caprock, rich in less soluble sediments, has hampered dissolution and subsidence processes in subsequent stages. Incision-induced halokinetic deformation is locally recorded by differentially uplifted terraces dipping away from the valley. The increasing differential loading caused by the entrenchment of the fluvial systems has driven the lateral and upward flow of salt-rich evaporites from the valley margins towards the valley bottom. A minimum long-term uplift rate of 0.3 mm/yr has been calculated for a titled OSL-dated Upper Pleistocene terrace of the Cinca River valley. The capture of the Ebro Basin and its change to exorheic conditions have played an instrumental role in the development of the Quaternary gravitational deformations associated with the evaporitic bedrock. The new incisional drainage network propitiated the karstification and evacuation of large amounts of evaporites, and its entrenchment has created adequate differential loading conditions for the salt to flow towards the valley floors. (C) 2012 Elsevier B.V. All rights reserved.