Hydrogeology of non-salt Gachsaran Formation in Iran: an example from the Zagros Range-Tang sorkh Valley

The study area of Tang sorkh Aquifer is located on the northern flank of the Derak anticline, a part of the Zagros Mountain Range, in the central-south of Iran. The study area is limited to undifferentiated non-salt Gachsaran Formation (NSGF), mainly composed of alternative layers of marl and marlstone, and gypsum. This paper is focused on the hydrogeological and hydrochemistry characteristics of the NSGF. A detailed geological cross section, perpendicular to the strike of the GF was prepared by detailed field surveying. The thickness of GF is 1,268 m, with 82 alternative units of marl, marlstone, gypsum, sandstone and limestone. The sub-aquifers are limited to four limestone, five gypsum, and one gypsum-halite units. The waters of these sub-aquifers emerge as springs or seepage into the Tang sorkh River (TR). The average discharge of springs emerging from limestone and gypsum units ranges between 0.3–8.4 and 0.5–0.75 l s−1, respectively. The source for sub-aquifers is mainly recharge from precipitation or river water. Marly layers disconnect the hydraulic connection between the sub-aquifers, directing the general flow parallel to the strikes. The lack of sinkholes and caves in the gypsum units is due to the small thickness of sub-aquifers, and consequently, low amounts of recharge water. The springs are classified into four groups based on their lithology, electrical conductivity, type of water, and trend of ion-concentration. The type of water in most of the springs is calcium-sulfate. The electrical conductivity is the main parameter to differentiate the hydrochemistry of the groups. The hydrochemistry of springs depends on the recharge from the TR, and the lithology of sub-aquifers and their adjacent units.