Dolomitización y calcitización en facies lacustres carbonatadas y yesíferas (Eoceno superior, Formación Deza, cuenca de Almazán, España) Dolomitization and calcitization in gypsiferous and carbonate lacustrine facies (upper Eocene, Deza Formation, Almazán basin, Spain)

The lacustrine Deza Formation shows a complex association of early diagenetic facies represented by dolomite, interstitial gypsum and calcite. Dolostone beds are predominant at the lowest part of the unit (first 40 m), in the Cihuela area where they are part of sequences of 1-3 m thick that begin with clayey to marly levels. Dolostones consist of crystal mosaics rangingfrom2to30µmacrossthatcontainnumerousdolomiteandcalcite pseudomorphs (30 µm to 1 cm long) after lenticular interstitial gypsum; limnicfossilsaredispersed.Texturalandcompositionalrelationshipsindicate two gypsum generations; the first one, not always present, consists of microlenticular gypsum (< 200 µm) that was replaced by dolomicrosparite, whereas the second one (macrocrystalline gypsum, 200 µm - 1 cm) was replaced by sparite occupying a continuous network surrounding darker dolomitic pseudobreccias. It is suggested that the first gypsum generation occurredwithintheoriginallacustrinecarbonatemudswhenthelakeswere shrunk; dolomitization probably occurred linked to this process. The subsequent widespread second generation of gypsum took place during a drier stage generating the characteristic pseudobrecciated texture. A later wetter stage is represented by the calcitization of this gypsum and dedolomitization.