THE PLIOCENE AND EARLY PLEISTOCENE DRAINAGE NETWORK EVOLUTION IN THE MONFERRATO HILLS (PIEDMONT, NW ITALY)

Abstract

The drainage network in the Monferrato Hills is, in general, quite complex. However, in the western and eastern end of the Monferrato there are areas where the drainage is less complex. Morphological analysis suggests that the differences in drainage were mainly produced by captures that have affected the catchments of some streams. Geological and geomorphological studies have shown that during the early Pliocene, a portion of the Monferrato, along with part of the Torino Hill, was an island in which there were some catchment basins draining towards the southern and northern sea. The tectonic evolution caused the capture of rivers that have been dated from the Piacenzian onwards using stratigraphic and morphological data. The head of the Stura, Colobrio, Rotaldo and Grana valleys, that flowed southwards after the emersion of the island, were captured by headward erosion of streams draining towards the northern basin. The post-Zanclean uplift of the western Monferrato, some gentle post-Zanclean anticlines lying south of the hills, the subsiding basin lying north and the tectonic structures transversal to the Monferrato thrust front, played a dynamic role in the evolution of the drainage. On the contrary, the tectonic structures displacing the sediments that form the core of the Monferrato hills, active mainly before the Pliocene, influenced, mostly passively, the drainage network when the area emerged from the sea. In fact, the initial direction of the streams was conditioned by the different strike, dip and erodibility of the pre-Pliocene sediments.