La Unidad Salina (Mioceno) en el área de Madrid. Características petrológicas y mineralógicas

Resumo

The petrology and mineralogy of the Saline Unit (Miocene) in the northeastern side of the Madrid Basin have been determined from the study of evaporite deposits occuring in two sections located at the urban area of Madrid. The two drillings cut saline deposits throughout 107 m. and 104 m., respectively. They form the lowest deposits in both drillings. In the northernmost drilling (A), the Saline Unit is made up mainly by more or less gypsified anhydrites that occur alternating with magnesite marls and illitic shales. This set of sediments show some episodic terrigenous intercalations formed by smectitic shales and biotite sands. A few kilometers to the South (drilling B), the Saline Unit shows a lesser marginal character, also occurs interbeded with the anhydrite-magnesite deposits, glauberite and halite-polyhalite. Glauberite and halite-poly - halite occurs as primary lenticular forms, as well as neoformed on polyhalite. This latter mineral shows spherulitic textures and fascicular bundles. Sometimes, polyhalite crystals occur as polycristalline or penetrative forms on cube-shaped halite crystals. Finally, the halite crystals commonly exhibit morphologies that are attributed to subaequous bottom nucléation.