Late Holocene evolution of playa lakes in the central Ebro depression based on geophysical surveys and morpho-stratigraphic analysis of lacustrine terraces

Abstract

The origin and morpho-stratigraphic evolution of the largest playa-lake system (La Playa-El Pueyo) in the Bujaraloz-Sástago endorheic area, located in the semiarid central sector of the Ebro Depression, are analysed. The enclosed depressions are developed on gypsiferous Tertiary bedrock and show a prevalent WNW-ESE orientation parallel to the direction of the prevalent strong local wind (Cierzo). Yardangs have been carved in bedrock and unconsolidated terrace deposits in the leeward sector of the largest lake basins. A sequence of three lacustrine terrace levels has been identified by detailed geomorphological mapping. The treads of the upper, middle and lower terrace levels are situated at +. 9. m, +. 6. m and +. 0.5. m above the playa-lake floors, respectively. Seismic refraction and electrical resistivity profiles acquired in La Playa reveal a thin basin fill (~. 2. m) with a planar base. These data allow ruling out the genetic hypothesis for the depressions involving the collapse of large bedrock cavities and support a mixed genesis of combined widespread dissolution and subsidence by groundwater discharge and eolian deflation during dry periods. The 5. m thick deposit of the middle terrace was investigated in hand-dug and backhoe trenches. Six AMS radiocarbon ages from this terrace indicate an aggradation phase between 3.9. ka and ca. 2. ka. These numerical ages yield a maximum average aggradation rate of 2.6. mm/yr and a minimum excavation rate by wind deflation of 3. mm/yr subsequent to the accumulation of the middle terrace. The latter figure compares well with those calculated in several arid regions of the world using yardangs carved in palaeolake deposits. The aggradation phase between 4 and 2. ka is coherent with other Iberian and Mediterranean records showing relatively more humid conditions after 4. ka, including the Iron Ages and the Iberian-Roman Period