Stratigraphy, petrography and palaeogeographic significance of the Early Oligocene “menilite facies” of the Tarcău Nappe (Eastern Carpathians, Romania)

The analyzed “menilite facies” (Early Oligocene Lower Menilites of the Tarcaˇu Nappe, Romanian Carpathians, formed by “bedded cherts” and black shale-like deposits), document the upward evolution of a mainly turbiditic sequence. The stratigraphical relationships with the underlying turbidites are marked by a sudden and sharp transition to a predominantly pelitic menilite-bearing succession, probably as a consequence of a drastic decrease in the terrigenous supply. This type of sedimentation ceased at the Rupelian-Chattian boundary, when new turbiditic flows occurred. The multi-source provenance characterizing the basal turbidites (quartzarenite and litharenite sandstones which were probably derived from external cratonic areas and from inner crystalline belts respectively), is here interpreted as closely linked to tectonically induced palaeogeographic modifications. This hypothesis is in agreement with literature data, which relate these “menilite facies” to an Eocene-Oligocene widespread anoxic event that occurred in the western and central Paratethys, linked to drastic palaeogeographical modifications and to a global climatic deterioration. These palaeogeographical modifications may have corresponded to the Paleogene microplate reorganization and progressive exhumation of the Alpine-Dinaric-Balkan fold-thrust belt, which was responsible for the increasing isolation of the Paratethian basin from the World Ocean. Early Oligocene cooling events, consequent stratification of different salinity water layers and/or upwelling currents, could have produced anoxic conditions at the bottom of the flysch basin and the deposition of the “menilite facies”.