The Lavini di Marco rock avalanche deposit (Italy): 36 Cl exposure dating of a landslide of historical age

The Lavini di Marco rock avalanche deposit (“Marocca di Marco”) is located along the left side of the middle Adige valley, south of the town of Rovereto (NE Italy). This deposit is estimated to amount to ~2x108 m3 of volume and is distributed on an area of ~6.8 km2, comprised between 1260 m (crown) and 170 m of altitude (toe) on the NW face of the Mt. Zugna Torta. The Lavini di Marco deposit is composed of at least two different rock avalanche bodies, named the Lavini di Marco (the principal) and the Costa Stenda bodies separated by the Costa Stenda ridge. These deposits are composed of boulders of different size, up to 100-200 m3, the bigger ones generally located in the lower portion of the deposit. Samples for 36Cl exposure dating were collected at different sites: i) big boulders (>50 m3) at the base of the deposit; ii) big boulders within the deposit; iii) sliding plane bedrock; iv) walls at the upper landslide scarp. Sampling was performed paying attention to consider rock weathering (i.e., karst erosion) and vegetable cover. Exposure ages range from 800 ± 210 yrs to 5490 ± 330 yrs BP. Ages of 1620 ± 120 yr and 1360 ± 100 yrs BP were obtained on bedrock surfaces along the sliding plane; ages of 3090± 290 yrs and 5490 ± 330 yrs BP were instead obtained on boulders at the base of the Marco deposits. An age of 21310 ± 680 yrs BP, more than 15000 years older than any other boulder ages, was achieved from the top of the biggest boulder on the Marco rockslide deposit suggesting that it belonged to the pre-slide bedrock. Based on the ages of the Marco deposit boulders, a mean of 5220 ± 245 yrs BP (samples LdM2, LdM11) can be calculated for the main Lavini di Marco landslide. However, the youngest ages of 1260 ± 143 yrs BP (samples LdM 5, LdM 6, LdM 8) suggest a re-activation during Middle Age. Several landslides occurred during the transition to warmer and wetter conditions during early Holocene. The youngest Lavini di Marco rockslide activation dates a catastrophic flood event of Adige River in Verona city, as reported in the Fulda Annales, back to 883 AD, possibly due to the removing of a portion of the Lavini di Marco avalanche deposits.