USE OF TIDAL-CIRCULATION MODELING IN PALEOGEOGRAPHIC STUDIES - AN EXAMPLE FROM THE TERTIARY OF THE ALPINE PERIMETER

Tidal conditions in ancient shallow-marine basins, as revealed by pres erved sedimentary deposits, reflect the strong influence of the paleob athymetry. Use of a tidal-circulation model can test existing paleobat hymetric reconstructions. Meso- and macrotidal conditions are believed to have existed throughout most of the Miocene Molasse (France and Sw itzerland) seaway. Tidal simulations using external tidal ranges of 2 and 4 m entering from the south (Mediterranean) fail to generate mesot idal conditions in the majority of the seaway, because of a narrow con striction in the present Alpes du Dauphine and Drome areas. Meso-tidal conditions are simulated in the Rhone-Alp part of the seaway only aft er the postulated paleobathymetry is widened in the Alpes du Dauphine area; this change allows tidal penetration from the Mediterranean. Des pite this widening, the new simulations fail to generate mesotidal con ditions in the Swiss part of the seaway. Mesotidal conditions are simu lated in Switzerland with the addition of a second tidal input from ea stern Switzerland, favoring an interpretation that the Swiss part of t he seaway was connected with either the eastern Mediterranean or the I ndo-Pacific Ocean. The revised paleobathymetry brackets uplift events that affected the basin margin.