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Abstract

La Janda lake is located into a tectonic graben filled by Pleistocene and Holocene f1uvio-marine sediments. Geophysical survey consisting on Electric-Iogs and seismic refraction profiles aimed to determining the thickness of Quaternary sediments infilling the graben. Nevertheless, the results are significantly distorted by a saline aquifer that occupies most of the sedimentary filling. In any case it is possible to identify an assymetric subsident area reaching up to 300 m depth, characterised by very low apparent resistivities (1.5-2.4 W/m). This thick geoelectrical unit can be preliminary subdivided into 3 different subunits here called A, B 1, B2, characterised by resistivity differences. The shallow 4-6 m thick Unit A consists of a thin lacustrine and alluvial e/ay and silts of Holocene age easily recognized in seismic refraction profiles and drill cores. Unit B can be separated in two subunits; Both are saturated in brackish or saline waters; B1 is a 20-40 m thick unit that thins northward and correspond to the Plio-Pleistocene, B2 is a slightly more resistive unit that extends from this depth to 352 m and corresponds to deeply weathered mio-pliocene sandstones. The upper part of Sub-unit B1 correspond to estuarine sands recorded in a previous core which deposition finishes at ca. 3810 cal BP. A sharp normal fault limits the southern part of La Janda assymetric graben.