The Cihuela Carbonate unit is located in the eastern part of the Tertiary continental Almazan Basin, which is connected to the Duero Basin. This unit is 200 hundred metres thick and it consists of five increasing-upward carbonate sequences. Each megasequence is made up of a lower part of mudstones or marls, an upper carbonate part (calcitic and dolomitic facies) and transitional facies between the two. These sequences were formed in mud flats and in the associated ephemeral carbonate lakes with characteristic formation of interstitial gypsum during the frequent periods of exposure.
The clay mineral species identified by X-ray diffraction are: illite, two types of smectite (Di and Trioctahedral), palygorskite, seplolite and minor kaolinlte, chlorite and mixed-layer chlorite-smectites. Illlte and dioctahedral smectite rich levels represent the detrltal material deposits of the distal alluvial fan (mud flat) without later transformations under saline influence. The Mg-smectite and palygorskite are ofdiagenetic origin, possibly formed by transformation of the detrltal phyllosillcates. Mg-smectites, which occurs exclusively In the mudstone layers interbedded with the dolomites In the first megasequence, were formed In the saline lakes. Palygorskite is recorded in the rest of the megasequences and it was formed in the mud flat, in conditions of lower salinity