Publication: Datos geofísicos y evolución sedimentaria de la Depresión de la
Janda (Cádiz)
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Publication Date
2001
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Sociedad Geológica de España
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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.