Sedimentary processes related to the groundwater flows from the Mesozoic Carbonate Aquifer of the Iberian Chain in the Tertiary Ebro Basin, northeast Spain

Abstract

In this work, the importance of groundwater in the formation and evolution of evaporitic lacustrine facies in the Iberian Chain and Ebro Basin contact (Spain) is studied. There are outcrops of geological materials from Palaeozoic to Quaternary times. These materials have been classified into eight hydrostratigraphic units. The Jurassic `carniolas' and limestones in the Iberian Chain (HU 4 and 5) are the materials which have the best hydraulic properties for underground water catchment and for forming the regional aquifer. The Triassic gypsum and marl of HU 3 form the impermeable substratum of the overlying Jurassic and Cretaceous carbonated aquifers, and they are therefore the impermeable base of the aquifers being studied. The equipotential line map of the regional aquifer shows the pattern of the groundwater flowing to specific points which mark the springs producing the underground drainage of the Iberian Chain. The volume of water throughout the aquifer is currently evaluated at approximately 250 hm3/year, which discharge through springs with high flowrates, or through diffuse discharges in the riverbeds and in wetlands (saline lakes). The water discharged in these springs has a high mineralisation, with a dry residue of over 1000 mg/l. Calcium sulphate compositions dominate, originating in the presence of soluble anhydrous materials within the Lias and Keuper formations. The current sedimentation in relation to the groundwater flows from the Iberian Chain can only be found in the areas of diffuse discharge where the evaporitic sedimentation can be observed because of the frequently endorheic character. On the right bank of the Ebro River more than 60 depressions are known, where very mineralised lakes form. These are locally referred to as `saladas'. During the Miocene, the hydrogeological functioning would be similar to the present one, leaving the groundwater and the dissolved salts in a non-marine basin. They would therefore accumulate in large areas of diffuse discharge, creating lakes where the evaporites would precipitate. The deep regional groundwater flows would ascend to the surface from the overthrust lamina fronts, going through thick Tertiary alluvial deposits in the Ebro Basin, and therefore the discharges were mainly diffuse.