The Sarmatian (middle Miocene) “petrified forest” of Gramada (NW Bulgaria): role of the calcareous crusts

The Sarmatian (upper Serravallian, middle Miocene) layers cropping out at Gramada (North-West Bulgaria) contain calcareous cylinders of metric dimensions characterized by a central and cavity also cylindrical. Their detailed study shows both vertical and concentric structuring features. The various component elements of the cylinders result from sequences of micritic carbonate crusts with several components, mostly of algal origin (red algae) and to a lesser extent of microbial origin. Besides few ostracods, the lack of associated fauna is a peculiar feature that raises the issue of the depositional environment, in the context of the evolution of the Sarmatian sedimentation at the scale of the Paratethys area. The encrustation processes around possible vegetal supports (“petrified forest”) are also discussed.