DETRITAL ZIRCON AGES FROM THE MOLASSE BASIN OF UPPER AUSTRIA: INSIGHTS INTO DEEP-WATER SEDIMENTATION IN THE OLIGOCENE-MIOCENE PUCHKIRCHEN FORMATION

Age-dating of detrital zircons from subsurface strata of the Molasse basin in Upper Austria using the Sensitive High-Resolution Ion Microprobe with Reverse Geometry (SHRIMP-RG) has yielded insight into the Oligocene-Miocene paleogeographic setting, and Eastern Alpine sediment source areas. Various (seismically mapped) conduits fed sediment into the deep-water (> 1000 m water depth) basin, and sandstone was sampled from each; all are characterized by a distinctive detrital zircon age spectra character. Sediment that fed into the basin from the west (ultimate sediment source being the western part of the Eastern Alps) is distinguished by a high proportion (> 20%) of Caladonian-aged (500-400 Ma) zircons. Perhaps the greatest contributor of coarse-grained material to the Austrian Molasse basin was the Inntal fault drainage complex, which sourced a vast hinterland to the southwest of the study area that included the Ötztal nappe, and extended as far south as the Periadriatic Line. A well-distributed zircon age spectra characterizes sediment sourced from this conduit, with the most important ages eroding from Austroalpine basement terrains of Variscan (350-250 Ma), Caledonian, and Cadomian (650-550 Ma) affinities. Sand that entered the basin directly from the fold-thrust belt to the south is, within our data, devoid of late Variscan (300-250 Ma) zircons, but otherwise similar to sandstone that fed into the basin via the Inntal drainage. Early Oligocene zircons (30-40 Ma) are present in all sands derived from the Alps, as are Precambrian-aged grains (1000-650 Ma), which make up 27% of all the zircons dated in this study. Sandstone from a shallow marine unit that was deposited proximal to the northern basin margin consists of > 80% early Variscan (350-300 Ma) zircons, which originated from the Bohemian Massif. This feature is not considered to have been a major source of sediment to the deep-water Molasse basin in Upper Austria. Two samples were analyzed from turbiditic deposits of the basin axis that contain zircon-ages (1.85-1.66 Ga) not present in the other sands examined; it is possible that these beds originated from an unmapped conduit on the southern basin margin.