The Maramures Basin, in the Carpathian mountain belt of northern Romania on the border with the Ukraine, belongs to the eastern part of the Pannonian Basin. In the study area, extensional tectonic movements during the Miocene were coeval with silicic and intermediate volcanism in the inner part of the Eastern Carpathians. Throughout this region, explosive events have resulted in the deposition of pyroclastic flows and ash-fall deposits interbedded with marine sediments.
Several tuff units of Badenian (15–13.6 Ma) age occurring throughout the area are extensively zeolitized. These rocks occur as massive homogeneous beds, white to pale greenish-blue, and are commonly extensively jointed. In the Bârsana-Calinesti area and along the Morii Valley, two conspicuous tuff units that can be traced over many km are separated by a calcareous sandstone bed. Most tuffs have a vitroclastic texture in which former glass shards are pseudomorphed by clay minerals and clinoptilolite. Opal-CT commonly occurs as clumps of radiating rods that produce a spherical morphology. Also, rare celadonite is found in the lower greenish tuffs. Pyrogenic crystal fragments are quartz, plagioclase and biotite. Folded muscovite plates and fragments of basement rocks are dominant among the lithic clasts. Above the Bârsana Formation, a second series of white zeolitized tuff, the Ocna Sugatag Formation, is represented by at least two different units overlying an evaporite salt deposit. A large outcrop of a massive white tuff at this locality contains abundant fine-grained clinoptilolite and cation-exchange capacity values of >160 meq/100 g. Clinoptilolite-Ca is also present in the Sighetu tuff unit in the northern part of the Maramures Basin where a distinctive horizon contains plant remains preserved in spherical concretions. Plant material and algal limestones in the same succession strongly suggest that the marine depositional environment was close inshore, and shallow- rather than deep-water conditions are inferred. A mineralogically similar, unaltered, volcanic tuff found in the Coaş area suggests that the precursor glass was rhyolitic (72–74% SiO2) with a high-K calc-alkaline affinity. We conclude that pervasive zeolitization is due to the interaction between seawater and vitroclasts at an early stage after deposition.