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The Tertiary Madrid Basin is located in the center of Spain and covers an area of more than 10,000 km2. The basin is filled by Tertiary sediments that are up to 3.5 km thick on its northwestern side. Neogene sediments account for the upper third of the Tertiary record and are divided into four main stratigraphic units. The lowermost unit, called the "Lower Unit" or "Saline Unit," is of Miocene age and consists mainly of evaporite sediments. It is divided into two subunits, the Lower Saline Subunit and the Upper Saline Subunit, based on sedimentary criteria.

In the inner part of the basin, the Lower Saline Subunit is crudely laminated with alternating laminae of anhydrite-magnesite, glauberite-anhydrite, and polyhalite-halite. In the outer parts of basin, this subunit is rich in early diagenetic glauberite with many textural varieties. In the Upper Saline Subunit, on the south side of the basin, massive thenardite beds with minor glauberite crystals overlie homogeneous reddish muds with halite and/or glauberite.

The Lower Saline Subunit is interpreted to represent deposition in an elongate, arid closed basin, oriented north to south. A perennial saline lake surrounded by a broad mudflats probably occupied the central part of the basin. In these saline mudflats abundant glauberite formed, both by replacement of other salts and displacively. The mudflat deposits merge laterally into a coalescent alluvial–fan system. The alluvial fans along the eastern margin of the basin may be associated with emplacement of nappes from the east. The erosion and weathering of Upper Cretaceous-Paleocene marine evaporites that crop out along the thrust zone appear to have been the main source of solutes for the lake brine. A paleotopographic change resulting from tectonic movements may explain the different character of the brines and stratigraphic record of the Upper Saline Subunit.

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