Dolomitización y calcitización en facies lacustres carbonatadas y yesíferas (Eoceno superior, Formación Deza, cuenca de Almazán, España)

Abstract

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 ranging from 2 to 30 μm across that contain numerous dolomite and calcite pseudomorphs (30 μm to 1 cm long) after lenticular interstitial gypsum; limnic fossils are dispersed. Textural and compositional relationships indicate 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 occurred within the original lacustrine carbonate muds when the lakes were 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.