Impact of halite dissolution subsidence on Quaternary fluvial terrace development: Case study of the Huerva River, Ebro Basin, NE Spain

This paper analyses the control of evaporite dissolution subsidence on the evolution of the lower 30 km-long reach of the Huerva River up to its confluence with the Ebro River. In the study area the Huerva River flows across different interfingered lithofacies of the subhorizontally lying Ebro Basin fill: (1) shales and sandstones upstream of Cadrete; (2) evaporites dominated by gypsum between Cadrete and Cuarte; (3) gypsum with glauberite and halite units in the subsurface downstream of Cuarte. The halite shows a marked downvalley increase in thickness. Twelve terrace levels and seven pediment levels correlative to some of the terraces have been mapped in the studied area. Upstream of Cadrete, the terrace deposits, with a relatively constant thickness less than 4 m and overlying unsoluble bedrock, remain undeformed. Between Cadrete and Cuarte the deposits of some terraces show local thickening (< 20 m). Downstream of Cuarte, a markedly asymmetric valley carved in gypsiferous bedrock and flanked by a prominent gypsum escarpment changes into a narrower and symmetric valley excavated in the > 60 m-thick terrace alluvium that fills a 5 km-long trough generated by synsedimentary subsidence phenomena caused primarily by the interstratal dissolution of halite. The older terraces (T1 to T4) show slight thickening (> 18 m) and locally truncate paleocollapse structures ascribed to the interstratal karstification of halite beds. The intermediate terraces (T5 to T7) correspond to degradation surfaces that grade downstream of Cuarte into aggradation flights underlain by the abruptly thickened deposits that fill this 5 km-long dissolution trough. Subsidence migrated episodically downstream during the generation of these terraces and the subsidence/aggradation rate was probably large enough to induce a base-level drop and knickpoint migration upstream generating strath terraces with convergent longitudinal profiles. The sedimentological changes that show the thickened terrace deposits in the subsidence area (high proportion of overbank fines (> 60%) including palustrine facies, gravel channels with lower width/depth ratio, multiple fining-upward cycles, decrease in the calibre of the channel gravels) indicate a change in the channel pattern from braided to meandering due to the lowering of the valley slope in the reach affected by subsidence. Some of the anomalous features of the lower terraces (T8 to T12) attributable to dissolution subsidence include: (1) steepening of the T8 terrace profile; (2) change of the T10 terrace from degradational to aggradational downstream Cuarte; and (3) thickening recorded beneath the T11 terrace surface.