The Quaternary succession of the Bulè Valley (Po Valley, Piedmont) as possible supply for prehistoric jade axes

This contribution presents a detailed geological mapping and preliminary data on the Quaternary succession of the Bulè and low Alpetto tributary valleys (Po Valley, Piedmont). The investigated area, in the altitude range between 3016 and 1487 meters, is located in the Monviso Massif Ophiolitic Complex and in the adjacent carbonatic units of the Dora-Maira Massif. The Quaternary sequence consists mainly of glacial and landslide deposits with local lacustrine, fluvial, debris and colluvial bodies. The detailed investigation of the petrographic composition of the glacial deposits allowed to recognize the occasional occurrence of jadeitite boulders, exploited during the Neolithic age for axe heads production. Four main jadeitite blocks have been found, with sizes up to 1 m3, some of them already reported by previous geological literature (Compagnoni et al., 2007) and other found during the present fieldwork (Avondetto, 2014). In detail, a Neolithic site of jade polishing has been discovered and referred to the middle of V millennium B.C. (Allisio, 2012) in the high Bulè Valley at 2880 meters. The jadeitite blocks have been mainly found in the glacial deposits. The petrographic characterization of the new jadeitite samples is in progress, and might potentially shed light on the still controversial genesis of these peculiar rocks. The distribution of moraines (between 2450 and 1250 meters) and glacial diffluence saddles have led to the reconstruction of the glacial expansion episodes, reported in four major glacial steps. An ancient step (1), probably related to the Last Glacial Maximum, implies a confluence between the Alpetto Glacier and a left lobe of the Bulè Glacier. The subsequent step (2), related to the start of the glacial withdrawal, defines three progressive phases of glacial diffluence of the Bulè Glacier left lobe in the Alpetto Valley. The further step (3) suggests a right lobe diffluence of the Alpetto Glacier in the Vallone Bulè. Finally, the last step (4) is due to the drastic retreat of glacier fronts towards the valleys head. At the end of the last glacial period, some rock falls and sliding gravitational phenomena occurred, partly covering the glacial record. Small deep-seated gravitational slope deformations also developed, resulting in a loose or disjointed substrate and some morphological features, as doubled ridges, minor scarps and trenches.