Dolomita bacteriana en facies carbonato-yesíferas de lagos efímeros (Formación Deza, Eoceno superior, Cuenca de Almazán)

Abstract

The lower sequence of the Deza Formation represents an ephemeral carbonate lacustrine environment revealed by the presence of limnic fossil and evidence of dry periods marked by the presence of numerous carbonate pseudomorphs, after lenticular interstitial gypsum crystals. In this work, the results of a petrological study of some dolomite facies in the lower sequence are shown to highlight the characteristics of dolomite crystals and to demonstrate their origin by biomineralization. The dolostones have massive or grumelar texture, with microfossils (gastropods and charophytes less than 2%) and carbonate pseudomorphs (calcite or dolomite) after interstitial lenticular gypsum crystals (20-60%); they have always palyigorskite as cement and may have undergone dedolomitization and silicification. Under SEM dolomite crystals are cylindrical, with sizes up to 40 microns in high. Due to their concentric inner vacuolar structure, high fluorescence, and the occasional occurrence of "dumbbell" forms, the dolomite is interpreted as formed as a result of sulfate reducing bacteria and EPS activity. This dolomite occurred in a micrite/biomicrite mud with interstitial gypsum crystals formed during drought periods. The biodolomite was formed at the expenses of the original calcareous matrix by replacement of gypsum crystals.