## 2 Description of the Study Region  
### Geological Setting  
Two geological units can be differentiated in the study area: the Paleozoic basement and the post-Hercynian sedimentary cover. The first unit is dominated by intensely folded phyllites intruded by a granitic batholith (Figure 1: 1,2). Subhorizontal to gently dipping, the post-Hercynian sedimentary cover unconformably overlies the basement. It is composed of Triassic, Jurassic, and Neogene formations (Figure 1). Sparse Quaternary alluvial sediments are also present.  
The Triassic strata belongs to the Chiclana de Segura Formation [11; 12], which extensively outcrops at the north of the Guadalimar River (Figure 1: 3). This formation varies in thickness (50-400 m) and is essentially horizontal or dips gently southward. In these rocks, their reddish hues and the presence of shales and sandstones are characteristic (Figure 2). The lower part of the Triassic series is mostly composed by sandstones levels. Towards the upper-middle part, the number and thick of clayey facies increases, and gypsum and salt levels intercalations appear (Figure 2).  
Figure 2: (**A**) Geological mapping of the studied sector. The positions of the electrical resistivity tomography (ERT) line of the seismic reflection profile S84-68 and the time-domain electromagnetic (TDEM) are marked. For this profile, 10 measuring stations (S1 to S10) were set up. Legend: (a) Triassic clays and sandstones (Chiclana de Segura Formation), (b) Jurassic dolomites, (c) Miocene (Tortonian)marl and marly limestones, (d) Miocene (Messinian) sandstones and marls, (e) boreholes, and (f) normal faults. (**B**) Synthetic stratigraphic column. Legend as (1) phyllites, (2) rudites, (3) sandstones, (4) clays, (5) gypsum, (6) dolomites, (7) gravel-rich sediments, (8) marls, and (9) marls and marly limestones.  
In the Guadalimar River valley, the Jurassic dolomites directly outcrop over the Triassic materials in apparent stratigraphic continuity (Figure 1: 4). They appear subhorizontal or with small dips, generally towards the south-southeast. These are strongly brecciated and dolomitized carbonates, thus causing an important secondary porosity that must affect the aquifer storability. The thickness, highly variable in the 0-70-m range, generally decreases towards the west. Although there are no biost stratigraphic criteria, this lithological unit has been attributed to the Lower Jurassic following facies and stratigraphic position criteria [12; 13].  
Jurassic strata dip slightly towards the south-southeast and they folded gently on NE-SW axes [5]. On the other hand, two NE-SW and NNW-SSE fault directions in both the basement and the dolomites. The orientations of theses faults overall influenced large-scale regional patterns of early Tortonian sedimentation [14; 15; 16]. Jurassic strata pinch out towards the west-northwest. Several pumping boreholes near Rus-Canena document this trend (Figure 1). West of Ibros, Jurassic strata are absent and the Miocene strata rest directly over the Triassic ones. The disappearance of Jurassic dolomites also marks the western limit of the aquifer [8].  
Three Miocene units are differentiated (Figure 1: 6): at the base, the Early Tortonian, which crops out locally, filling a small trough and pinching out laterally (Figure 2). These are gravel-rich sediments with dolomite fragments and siliceous sands, with little or no cement, alternating with marls [14]. Late Tortonian strata either directly overlie the Early Tortonian ones or they overlie the Jurassic dolomites. They average 200 m in thickness, attaining a maximum thickness of 500 m. These strata consist chiefly of marls and marly limestone, but there are also minor calcarenite intercalations [13; 17]. Messinian strata with more abundant calcarenites overlie the Late Tortonian strata [13; 17] (Figure 2).