Geological interpretation and numerical modelling of salt movement in the Barbastro–Balaguer anticline, southern Pyrenees

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)

“…Halokinesis in the halite-bearing Barbastro Formation started 28-26 Ma ago (Gil and Jurado, 1998). Initially, rock salt flowed from north to south driven by the overload exerted by the southernmost Pyrenean thrust sheets on the northern sector of the Barbastro Formation.…”

“…This salt anticline has an asymmetric geometry, with steeply dipping beds in the southern forelimb (up to 85°between the Cinca and Noguera-Ribagorzana valleys) and a low dip in the northern backlimb. Some authors propose a complex thrust structure including: a blind thrust detached near the base of the Barbastro Formation, a backthrust and secondary south-verging thrusts (Senz and Zamorano;Sans et al, 1996;Gil and Jurado, 1998;Fig. 2).…”

“…The shortening achieved by these thrust faults has been calculated at 6 km and 2.6 km by Senz and Zamorano (1992) and by Sans et al (1996), respectively. Folding in the Barbastro-Balaguer Anticline began in the Late Oligocene (Senz and Zamorano, 1992;Sans et al, 1996;Gil and Jurado, 1998) coevally to the sedimentation of the Sariñena Formation and after the deposit of the Peraltilla Formation, which displays a constant thickness across the anticline that is indicative of homogeneous subsidence in the foreland and limited shortening in the South Pyrenean Front Range. Gil and Jurado (1998) consider that three mechanisms were involved in the development of the anticline.…”

“…According to some authors, the required confining pressure for rock salt to change from fragile to ductile behavior at 25°C varies between 6 MPa (Balk, 1949) and less than 20 MPa (González de Vallejo et al, 2002). Considering an average density for the evaporitic Barbastro Formation of 2350 kg/m 3 (Gil and Jurado, 1998) There are other factors that reduce the pressure at which rock salt changes from fragile to ductile behavior, such as: (a) the existence of small amounts of brine (Goudie, 1989;Jackson, 1995;Talbot and Pohjola, 2009) Since the denser evaporitic caprock overlaying the halite-rich sequence is thinner beneath the Cinca River valley than in the more elevated valley margins, due to river incision, the rock salt under the river valley is less loaded than the underground salt underneath the divides. The load gradient related to variations in the caprock thickness would be responsible for the ductile halite-rich rock to flow laterally and upwards from the valley margins towards the valley floor.…”

“…the halite-rich sequence is some 160 m thicker in the valley margins than in the valley axis. Considering an average density of 2350 kg/m 3 for the Barbastro Formation (Gil and Jurado, 1998), 160 m of overburden implies a load of 3.6 MPa. Thus, a differential load of 3.6 MPa acting on the halite-bearing Barbastro Formation along 4 km, represents a load gradient of around 1 MPa/km, which could have triggered the flowage of the ductile underground salt from the valley margins towards the valley bottom, with the consequent progressive uplift and back-tilting of the T4 and T6 Cinca River terraces (Figs.…”