PALEOGEOGRAPHIC AND PALEOTECTONIC IMPLICATIONS OF CLAY MINERAL DISTRIBUTION IN LATE JURASSIC EARLY CRETACEOUS SEDIMENTS OF THE PINDOS-OLONOS AND BEOTIAN BASINS, GREECE

A new method is proposed for solving the controversial problem of the location of oceanic areas from which the Pindos and Othrys ophiolites originated. Assuming that clay fractions, quickly disseminated over la rge areas, may be reliable markers of erosional processes, we studied these fractions in sediments from two ''external'' basins (Pindos-Olon os and Beotian) located between the main carbonate platforms of the Gr eek Hellenids. Chronologically, two different clay associations can be distinguished in the sedimentary sequences of the basins. The lower o ne is dominated by illite and subregular illite-smectite mixed-layers and poor in Mg, Ni and Cr; this association is consistent with common continental sources. The upper one, appearing in late Tithonian synchr oneously in both basins, consists of trioctahedral smectite (saponite type) enriched in Fe, Mg, Ni and Cr; they are considered to be derived from weathered ophiolites. Thus, the beginning of the ophiolite detri tal sedimentation, accepted as being middle to late Bathonian in the b asinal sequences of the Maliac or Othrys zone from Hydra or lower-midd le Oxfordian in all the internal units in the Argolis Peninsula, is mu ch younger (late Tithonian) in the external basins. Such a diachronism is not consistent with a model postulating initial ophiolite displace ments within the Pindos-Olonos basin by late Jurassic time. It suggest s that a platform was acting as a barrier between the site of the ophi olite first intra-oceanic deformation and the ''external'' basins. Thi s barrier cannot be the Parnassuss platform that kept subsiding in ner itic environments until late Cretaceous inside the ''external'' basins ; it is likely to be the Pelagonian platform, positively known to have been submerged during the Tithonian. Consequently, we conclude that t here was no genetic relationships between the ''external'' basins (Pin dos-Olonos basin in a broad sense) and the ''Pindos ophiolites'' of wh ich the zone of origin is to be located on the opposite eastern ''inte rnal'' side of the Pelagonian platform.