干旱区生态环境知识资源中心

Across the broad spatial and temporal expanse of the Alpine foreland basin, reliable correlation and synthesis of diverse stratigraphic, structural and paleontological data has been thwarted by the absence of a well defined chronologic framework. The Lower Freshwater Molasse of the Fornant and Findreuse sections in Haute-Savoie with a combined stratigraphic thickness of ca. 400 m yielded 31 vertebrate bearing horizons, 18 of which contained rich and diverse faunas representing the most complete micromammal faunal record of the Western Molasse Basin. Magnetostratigraphic dating of these sections provides a greatly improved temporal calibration of the depositional and faunal record between 26 and 21 Ma in this part of the Alpine foreland. Time-transgressive, lacustrine deposition began at approximately 26 Ma. During the subsequent 0.5 Ma of rapid subsidence, locally sourced, braided rivers flowed eastward toward the proximal foreland where lacustrine conditions prevailed. A similar facies geometry prevailed between 23.8 and 24.2 Ma, and both may be a response to Alpine thrust loading. In between those episodes, fluviatile conditions prevailed with meandering rivers transporting alpine detritus northwards into the basin. Lacustrine and ensuing playa conditions during late Chattian and early Aquitanian times may also correspond with a climatic change from moist to more arid and cooler conditions inferred from oxygen isotopes and plant macrofossils associated with the Oligo-Miocene boundary. The new chronologies allow a provisional calibration of the boundary ages of 6 assemblage zones: some of which can be specified to < 0.1 Ma and most of which require some modification of previously used assemblage-zone boundary ages. Assemblage zones are shown to vary in duration by at least 3-fold and may have durations of < 200 300 Kyr.