NEW DATA ON THE ORIGIN OF THE BEDDING OF THE GREZE - PALEOCLIMATIC IMPLICATIONS

New data on the origin of the bedding of the "greze": Paleoclimatic implications. Following a detailed study of a "greze litee" which presents the granulometric characteristics defined by Guillien, we have reviewed the various assumptions concerning the processes of deposition of these materials. The field data and the results of the sedimentological and the micromorphological analyses make it possible to better determine the main processes involved in the deposition of the "grezes". The dominant role assigned to slopewash by Guillien is not supported by sedimentary structures such as longitudinal graded beddings and oblique or cross beddings. The congelifluxion-slopewash hypothesis of Journaux does not explain vertical graded beddings and the washing alone cannot lead to the formation of a well defined table at the top of the diamicton or "lit gras". However, Journaux and Francou consider that solifluction may explain the "lit gras". The Andean model by Francou which favours frost-creep seems to be the best explanation for the bedding of the "grezes charentaises". Vertical graded beddings, contacts between layers, sedimentary features observed in the Andean environment have been recognized in the "grezes litees charentaises". The mecanism of solifluction sheet (nappe-coulee) seems to be the essential process of the stratogenesis. Thereby the climatic context of development of the "grezes" is characterized by a great frequency of short and superficial freezing and thawing cycles and the contribution of short-lived and renewed snow cover or patches providing the water required for active frost shattering and frost creep. The development of the "grezes litees" probably occurred during the dominant cold and humid periods of the last glacial cycle. The cold and dry terminal period is responsible for the reworking of the top of the "greze" (bendings and cryoturbations).