Significance of different glacial facies in the Pleistocene geological evolution of the Ivrea Morainic Amphitheatre (Piedmont)

Abstract

: It is known that the Ivrea Morainic Amphitheatre consists of a group of morainic ridges: the corresponding glacial sediments form different allostratigraphic units referred on magnetostratigraphic and pedostratigraphic data to various moments of Pleistocene. The present article is about a detailed study of the 90 m thick glacial deposits forming the external morainic ridge and laying on fan-delta pliocenic sediments: inside one allostratigraphic unit (San Michele-Borgo Unit) four different facies have been recognised and relationships among them have been analysed. The morphological evidence of this complex of sediments is represented by a E-W 10 km long ridge, named Coste della Lace, 100 m higher than the present valley incisions: to the North it is bounded by the T Ingagna that divides it from the Donato relief corresponding to an ancient alluvial fan, and to the South by the T Viona that separates it from the more recent morainic ridge of Bornasco. Sedimentological features and facies distribution suggest that the complex of the described sediments forms a succession consisting of lodgement till (facies a) at the bottom, lodged at the base of the glacier, and of melt-out till (facies b) at the top, located essentially on the glacier. The presence of a thin but strongly weathered soil developed on facies b suggests that the relief was originally shaped on this facies and that it has been successively moulded on the underneath facies a so deeply as to reach the Pliocene sediments and the crystalline bedrock. Only locally does facies c outcrop; it is considered as ice-marginal deformation till, transported by flow and located in little lacustrine basins developed between the ice and the alluvial sediments of the Donato relief. In a very small area outcrops facies d interpreted as ice-marginal gravity flowtill deposited at the lateral margin of the ice and mainly formed by gravity processes and slope wash. On the basis of the former observations it is possible to reconstruct the depositional and erosional events that characterised the Plio-Quaternary geological evolution of the area.