An early würmian age for the inneralpine halldorf site, salzach valley, Austria

The Salzach Valley is one of the major valleys in the Eastern Alps which was occupied by a large ice stream during glacial maxima. In contrast to the Inn and Enns valleys, dated Pleistocene sediments predating the last glacial maximum are rare in the interior of this valley. The only known site is a former gravel pit near Halldorf, close to the conspicuous turn of the valley, where reworked lignite fragments were previously dated to 32 to 55 ka BP. In this study we re-examined these and additional lignite fragments in order to clarify the chronostratigraphic position of this site. Observations made at the time when the quarry was in operation showed that the lignite fragments were well rounded and compressed, and occurred in poorly sorted and poorly bedded deltaic foresets which lacked evidence of over-consolidation. The lignite also contains wood which is also compressed. Radiocarbon analyses performed on twelve individual wood samples yielded 14 infinite C dates (with one exception) indicating that they were most likely older than the Middle Würmian. Pollen showed high arboreal pollen percentages including Picea (dominant), Alnus, Pinus, as well as low percentages of Quercus, Fagus, Abies and Corylus in some samples. Osmunda was also found in some samples. Wood anatomical studies performed on fourteen samples revealed a dominance of Pinus, which, however, likely reflects the poorer preservation potential of soft wood genera such as Picea. The pollen data confirm the radiocarbon dates and document the former presence of a forest vegetation, consistent with Early Würmian records from the northern alpine rim including Mondsee. Although the pollen data do not permit to unequivocally assign these lignite samples to a known stratigraphic interval, they favour a First Early Würmian Interstadial age, whereby different samples record different parts of this long period during which the former peat bog formed. Strong compaction of the peat and wood probably reflects ice loading during the last glacial maximum, while subsequent erosion, transportation and re-deposition by meltwater streams occurred during the deglaciation phase.