Abstract Tafoni and honeycombs remain some of the most enigmatic and puzzling geomorphological phenomena. Their globally widespread occurrence across a wide range of lithologies and environmental conditions suggests that their formation is determined by a factor that overarches variations in these conditions and weathering processes. Based on a study of tafoni and honeycombs in the Crimean Piedmont, this paper demonstrates that the primary factor in their formation is the pre-exposure alteration of rocks along fractures and karst conduits as a result of fluid–rock interactions. Such alteration is commonly induced by ascending flow and is related to hypogene karstification. The morphological expression of cavernous features through removal of the alterite can occur under subsurface conditions, but is more frequent on exposure to atmospheric weathering. The local or regional characteristics of a weathering system are irrelevant or of only secondary importance in determining the localization and morphology of cavernous features. New features do not form on rock surfaces where the alteration zone was totally denuded or never present. The proposed model resolves major issues inherent to previous interpretations of cavernous features. It has general applicability and important implications for geodynamic and palaeohydrogeological reconstructions. Typical tafoni and honeycombs may indicate past events of ascending flow and the potential presence of hypogene karst systems.

Abstract Oligocene and Lower Miocene deposits in the Paratethys are important source rocks, but reveal major stratigraphic and regional differences. As a consequence of the first Paratethys isolation, source rocks with very good oil potential accumulated during Early Oligocene time in the Central Paratethys. Coeval source rocks in the Eastern Paratethys are characterized by a lower source potential. With the exception of the Carpathian Basin and the eastern Kura Basin, the source potential of Upper Oligocene and Lower Miocene units is low. In general, this is also valid for rocks formed during the second (Kozakhurian) isolation of the Eastern Paratethys. However, upwelling along a shelf-break canyon caused deposition of prolific diatomaceous source rocks in the western Black Sea. Overall, Oligocene–Lower Miocene sediments in the Carpathian Basin (Menilite Formation) can generate up to 10 t HC m −2 . Its high petroleum potential is a consequence of the interplay of very high productivity of siliceous organisms and excellent preservation in a deep silled basin. In contrast, the petroleum potential of Oligocene–Lower Miocene (Maikopian) sediments in the Eastern Paratethys is surprisingly low (often <2 t HC m −2 ). It is, therefore, questionable whether these sediments are the only source rocks in the Eastern Paratethys.