Timing, drivers and impacts of the historic Masiere di Vedana rock avalanche (Belluno Dolomites, NE Italy)

The “Masiere di Vedana” rock avalanche, located in the Belluno Dolomites (NE Italy) at the foot of Mt. Peron, is reinterpreted as historic on the base of archeological information and cosmogenic 36Cl exposure dates. The deposit is 9 km2 wide, has a volume of ∼ 170 Mm3 corresponding to a pre-detachment rock mass of ∼ 130 Mm3, and has a maximum runout distance of 6 km and an H/L ratio of ∼ 0.2. Differential velocities of the rock avalanche moving radially over different topography and path material lead to the formation of specific landforms (tomas and compressional ridges). In the Mt. Peron crown the bedding is subvertical and includes carbonate lithologies from Lower Jurassic (Calcari Grigi Group) to Cretaceous (Maiolica) in age. The stratigraphic sequence is preserved in the deposit with the formations represented in the boulders becoming younger with distance from the source area. In the release area the bedding, the SSE-verging frontal thrust planes, the NW-verging backthrust planes, the NW–SE fracture planes, and the N– S Jurassic fault planes controlled the failure and enhanced the rock mass fragmentation. The present Mt. Peron crown still shows hundreds-of-metres-high rock prisms bounded by backwall trenches. Cosmogenic 36Cl exposure ages, mean 1.90± 0.45 ka, indicate failure occurred between 340 BCE and 560 CE. Although abundant Roman remains were found in sites surrounding the rock avalanche deposit, none were found within the deposit, and this is consistent with a late Roman or early Middle Ages failure. Seismic and climatic conditions as landslide predisposing factors are discussed. Over the last few hundred years, earthquakes up to Mw = 6.3, including that at 365 CE, have affected the Belluno area. Early in the first millennium, periods of climate worsening with increasing rainfall occurred in the NE Alps. The combination of climate and earthquakes induced progressive long-term damage to the rock until a critical threshold was reached and the Masiere di Vedana rock avalanche occurred.

Sampling was based on the size of the boulders and stable position, single stage of exposure, continuous exposition in the same position (not shifting), no coverage; minimal surface of weathering or erosion (karren encased for at maximum 0.5 cm).
The samples VB3a, VB3c, VB14 were taken on the Mt. Peron slope at different altitude, VB2 on the right bank of the Cordevole, in the Vedana area and VB12, VB13a and VB13b come from the southernmost part of the deposits, in the Roe Alte sector.
The main characteristics of the dated samples are here reported.
VB2. The sample comes from a grey metric boulder on the right bank of Cordevole river. The rock belongs to Calcari Grigi Group and had a network of black calcite veins. Thin section shows that the rock is a peloidal packstone, with micritic matrix. It is possible to recognize fragments of echinoderm, bivalves, spicules of sponges, algae, foraminifers and peloids.
VB3a. The sample comes from a metric boulder of Upper Rosso Ammonitico Fm. on the left side of Cordevole River, at the foothill of Mt. Peron, 300 m over the Peron village. The sample is a pinkish packstone with nodular structure and fragments of Saccocoma.
VB3c. The sample comes from a decametric boulder of Fonzaso Fm. on the left side of Cordevole River, at the foothill of Mt. Peron, 300 m over the Peron village. The boulder is next to the VB3a sample but is way bigger. On this section the sample appears to be a bioclastic peloidal packstone with fragment of echinoderm, spicules of sponges, bivalve and calcareous algae.

VB12.
Sample comes from a grey Vajont limestone metric boulder with evident crinoids and algae situated in the further part of Roe Alte deposits. Thin section shows a packstone with fragment of echinoderm, bryozoan and algae, subordinately bivalves, peloid, and foraminifers. Note that oolites still show a very well preserved radial structure.   Table 2, are expressed as percentage concentrations of element oxides for major and minor elements and as parts per million (ppm) for trace elements. In order to include the LOI value (expressed as %) into the sum of major element oxides, analyses were normalised to 100% minus LOI value.
Instrumental precision (defined by several measurements performed on the same sample) is within 0.6% relative for major and minor elements, and within 3% relative for trace elements. The XRF accuracy was checked by reference standards (Govindaraju, 1994) and was within 0.5 wt% for Si, lower than 3% for other major and minor elements, and lower than 5% for trace elements. The lowest detection limits of XRF were within 0.02 wt% for Al2O3, MgO and Na2O, within 0.4 wt% for SiO2, within 0.005 wt% for TiO2, Fe2O3, MnO, CaO, K2O and P2O5 and within a range between 3 and 10 ppm for trace elements.

S3. XRF
Trace metals and REE were determined by Inductively Coupled Plasma-Mass Spectrometry (Thermo Elemental, mod. X-Series II ). Optimisation of the instrumental parameters was performed to achieve best sensitivity, low levels of oxides (CeO + /Ce + <2%) and double charged ions (Ba ++ /Ba + <3%). Mass calibration of the quadrupole was also performed. Major instrumental parameters are as follows:  From the bottom up:

S4. ICP-MS
-Bedrock consisting of the Bolago Marl: thickness varies from a minimum of 3 m in the northern side to a maximum of 5 m in the southern margin, with an undulated upper limit that roughly corresponds to the strata surface.
-Glacial till, 0.5-to-2 m thick, composed of rounded to sub-rounded decimetric clast (b-axis spanning from 0.5 to 40 cm) of various lithologies (i.e. flysh, limestone/dolostone and volcanic/metamorphic). Clasts show evidence of incisions and striae and little surficial alteration. The deposit is characterized by the presence of a high amount of silty-clayey matrix, brown-to-grey in colour. No evidence of clasts organization. Upper limit is extremely undulated, erosive in origin.
-The uppermost unit is monogenic, being constituted by angular carbonate clast, varying in size from 1 cm to 1 m, with a lot of coarse sandy matrix. Smaller clasts are more abundant than larger ones, that are grouped in the uppermost part of the deposit. This unit is 2-to-20 m thick