Caracterització geològica, geofísica i hidrogeològica de zones morfològicament deprimides associades a estructures tectòniques de la depressió de l’Alt Empordà, entre Figueres i el riu Fluvià

Abstract

This work focuses on a series of depressions located in inland areas of Alt Empordà (NE Catalonia), which in the past have been permanent ponds. The geological materials and structure of the subsoil and the existing hydrogeological relation with groundwater aquifers have been characterized. Geological, geophysical and hydrogeological methodologies have been applied in this study. First, a geological bibliographic and cartographic synthesis of the region has been done, followed by the consultation of geotechnical reports available in the area, which allowed to identify and characterize the materials present in the subsoil. A morphometric analysis of the depressions established a direct relationship between them and the tectonic structures of the area. Five geophysical electrical tomography profiles in the ponds of Closa Gran, Estanyet, Palol, Pradell and Púdol have led to characterize their internal geological structure. All of them show a similar pattern consisting of an endorheic sedimentary filling in the uppermost part of the depression. Below this filling, older sedimentary formations which also are present in the edges and the surrounding areas are fractured, deformed and vertically displaced towards depth. Three seismic refraction profiles in Closa Gran, Palol and Pradell, have served to determine the minimum thickness of the endorheic sediments in the central area. The geophysical results have allowed estimating the period of time when subsidence and sedimentation phenomena have been active, and the rate at which they took place.

The hydrogeological relations between ponds and aquifers have been surveyed by means of nine piezometric campaigns regularly spaced in time (June to December, 2014). A general flow of groundwater running from west to east has been identified. This action has been complemented by a hydrochemical study of groundwaters in the area, which has confirmed the presence of a deep, regional water flow. As a conclusion, a conceptual model of formation and evolution of the studied depressions has been proposed. It is based on the existence of a water flow through deep limestone and evaporite units. This flow would rise across the discontinuities that represent the normal faults in this area and reach the Neogenen permeable sediments, where it woud have spread laterally. Periodical oscillations of the hydraulic levels would have caused suffosion processes, that is, the removal of the fine fraction of the sediments, and gravitational instabilities. These phenomena woud have been manifested in the surface as subsidence dolines