Tracing the drainage change in the Po basin from provenance of Quaternary sediments (Collina di Torino, Italy)

Abstract

Quantitative provenance analysis of the Quaternary sediments from the Collina di Torino and the Po alluvial plain was assessed by using statistical techniques. This method, based on high-resolution heavy mineral analysis on modern and ancient sands processed by compositional biplot and linear mixing model, allowed reconstruction of the drainage change of the western Po River and its tributaries in the last 1 Ma. Mineralogical and statistical analyses discriminate three groups in the Quaternary samples. Sediments from the Po alluvial plain are rich in garnet, epidote and minor amphiboles. Detritus from the western slope of the Collina di Torino is characterized by epidote-rich assemblages, actinolite, blue-green hornblende and chloritoid. Heavy minerals from sediments of the northwestern slope include epidote-group minerals, glaucophane and hypersthene. A linear mixing model was used to compare detrital modes from modern and Quaternary detritus, and to identify the best modern analogue for any sample of ancient sedimentary deposit. Statistical analysis indicates that the first mineralogical group compares best with detritus of the modern Po River. Detritus from the second and third group compare best with sediments of the modern Dora Riparia River and Stura di Lanzo River, respectively. These results outline how the distribution of the Middle-Late Pleistocene fluvial deposits of the Collina di Torino is linked with the ancient Alpine watercourses, before the shifting of the Po River on the northern edge of the hill. During the Middle Pleistocene, rivers draining the Piedmont Zone of the Western Alps (PaleoDora Riparia and PaleoStura di Lanzo rivers) ran across the hill, while the PaleoPo River, which drained the Dora Maira Massif (Internal Massif), flowed to the south of the Collina di Torino. At the end of the Pleistocene, the PaleoPo River was diverted to the north to its present-day position in the alluvial plain. This case history shows how quantitative provenance analysis based on orogenic sediments is the key to reconstruct Quaternary drainage evolution of the foreland basin.