Hydrogeological implication of the Pliocene-Pleistocenetorrential and debris flow succession around the LanzoUltramafic Massif (Western Alps)

A thick succession of deposits associated with torrential and debris flow processes preserved around the Lanzo Ultramafic Massif are described in this study. They represent the oldest Pliocene-Quaternary complex of continental sediments locally outcropping close to the inner margin of the Western Alps. These sediments, outcropping into the deepest fluvial incisions, show prevalent gravel facies with clast supported texture and trough cross bedding. The decimetric clasts are derived locally because of the reworking of peridotite substratum. The matrix consists of a mixture of sand, silt and clay. Both the sedimentological features and the distribution into the watercourse incisions indicate torrential and debris flow genesis. In the sectors not incised by watercourses the succession is prevalently constituted by angular or poorly rounded clasts, derived from the local supply, with a scarce matrix and a stratification parallel to the slope. These features are significant evidence that the gravitative facies prevail. Silty sediments are observed locally, either without clasts or containing small peridotite fragments, with a bedding parallel to slope. These sediments are connected to colluvial processes and supplied by the reworking of mature soils developed at the expense of peridotite bedrock. The definite stratigraphic position of the torrential and debris flow succession, located at the base of the different sedimentary Pliocene-Pleistocene units, and the strong weathering of sediments, indicated by their argillification and cementation by iron oxides, reflect an ancient age. This assessment is in agreement with the local interfingering with the Lower Complex of “villafranchian succession” (CARRARO Ed., 1996) that is related to the Middle Pliocene. The strong weathering of this succession produces the peculiar hydrogeological significance. The hydrogeological conceptual model of the area is described. In the different areas some springs linked to permeability contrast and piezometric surface emergence are signalled. These springs are prevalently used for drinking water, irrigation or domestic purpose. Some examples of springs around the Lanzo Ultramafic Massif are presented in a detailed geological map. The springs result from the difference in permeability between the ancient torrential and debris flow succession and the glacial, outwash and detrital cover. In these areas the ancient torrential and debris flow sediments have the hydrogeological role of a separation element between the shallow aquifer in the glacial, glaciofluvial and detrital deposits and the deep aquifer in the network of the crystalline rocks.