GNGTS 2013 - Atti del 32° Convegno Nazionale

(San Pietro di Ragogna conglomerate) (Zanferrari et al. , 2008; Poli et al. , 2009). The recent tectonic activity of the Arba-Ragogna thrust system is also testified by drainage anomalies and gentle scarps connecting uplifted paleolandscapes of Quaternary age (Galadini et al. , 2005; Monegato et al. , 2010). A vertical slip-rate of about 0.19 mm/y has been calculated during the last 21 kys (Poli et al. , 2009). The sedimentary units and terraced staircase at the outlet of the Meduna valley. De- tailed stratigraphical and morphotectonic studies of the terraced surfaces at the outlet of the Meduna valley (south of Ponte Racli locality) allow to detect 9 depositional units (Q1 - Q9 in Fig. 2) linked to alluvial (Q1, Q2, Q3, Q5, Q6, Q7, Q8, Q9) or glacial (Q4) paleo-enviroinments. Starting from the available chronological data for the Meduna alluvial fan (Avigliano et al. , 2002; Zanferrari et al. , 2008), the geometric relationships between the observed units and the terraced surfaces pinpointed to a succession of depositional events from the Early Pleistocene to the Holocene (Fig. 2). Q4, Q5, Q7, Q8 e Q9 units are terraced but the thick- ness of the deposits is not steady, for which a distinction between “strath terrace” and “fill terrace” ( sensu Bull, 1991) can be adopted. A “strath terrace” is characterized by thin deposits (<3m) above an ero- sion surface on the bedrock; where- as the “fill terrace” is characterized by thicker preserved deposits. Ac- cording Wegmann and Pazzaglia (2009) this subdivision is a key pa- rameter for discussing the genesis of the terrace staircase in tectonically active areas. Deformational events. The re- construction of the deformative events is based both on morpholog- ical (piracy and valley deepening) and tectonic evidence. Starting from the older we identified four deform- ative events (Fig. 2). 1. Early Pleistocene (Gelasian). This event can be morphologically recognized by the main changes in the valley drainage that took place in the Meduna catchment: from the path across the present Forchia di Fig. 2 – Chrono-stratigraphical sketch of sedimentary units (Q1-Q9) and tectonic events (1-4) recognized at the outlet of Me- duna Valley. Unit Q4 is not dated and can be ascribed to only one of the glacial maxi- ma linked with arrows. 195 GNGTS 2013 S essione 1.2