Dolomías primarias de ambiente lacustre salino: Mioceno de la Cuenca de Madrid

Abstract

The study focuses on dolomite carbonates that occur in a Miocene evaporitic succession, the Christmas tree gypsum unit. This unit outcrops in the eastern part of the Madrid Basin and it was deposited in a mudflatsaline lake complex during the Lower Miocene. Three types of dolostone facies have been considered: a) dolostone levels between mudstone or gypsum facies, b) patches and discontinuous levels of dolomite replacing gypsum crystals, and c) intra-and-inter-crystalline inclusions of dolomite in gypsum layers. Dolomites are mainly dolomicrites showing a low ordering degree and some Ca excess. Their isotopic composition [d l80, d13C) is characterized by light values. These dolostones are interpreted to be primary in origin and they are likely due to microbial activity as supported by facies relations, together with isotopic, textural and mineralógica! evidences.