A 33 metre thick pyroclastic-rich zone of the Mezardere formation of Oligocene age is exposed in the Keşan region of Thrace, Turkey. In this zone, vitreous tuffs of dacitic composition have altered primarily to zeolites, including mordenite, heulandite-clinoptilolite and analcime. Silicification and alteration to clay minerals are common. Zeolite minerals have developed from volcanic glass, whereas some mordenites have formed from dissolution of heulandite-group zeolites. Although authigenic mineral paragenesis does not vary laterally, there is a marked vertical variation, particularly in zeolites. Mordenite (+heulandite-clinoptilolite) and analcime do not coexist and have formed in different stratigraphic levels. This suggests that their chemical environment is controlled by different hydrologic systems. Whole rock composition shows the relationship between chemistry and secondary mineralogy. For example, whole rock trace element geochemistry indicates the natural selectivities of zeolites. There is also stratigraphic control on the chemistry and texture of mordenites. Specifically, (Na+K)/(Ca+Mg) ratios of mordenites decrease from the lower to the upper levels. Mordenites of the lower level show a fibrous habit while the upper level mordenites are needle-like in shape. The average Si/Al ratio in mordenites is 3.90, in heulandite-group minerals 3.95, and in analcimes 2.34.