Anatomy of ancient landslides along the front of the Daunia Sub-Apennine, Italy

In the Daunia Mountains (NW corner of the Puglia Region, in Southern Italy), tens of very large, previously unknown, very old and apparently relict landslides were recently recognized in the framework of a geomorphological landslide inventory covering an area of about 2000 km² . Mt. Cornacchia (1151 m asl) is the highest mountain of the Daunia Range, and its eastern slope preserves evidences of three impressive very old and relict mass movements which re-shaped the original thrust-related range front, and feeded a wide depositional area.

Recognition of very old and relict landslides is based on the identification of key but subtle anomalies in the geology and in the drainage. The main and oldest landslide is nearly 3,7 km long and 2,7 km wide, and can be considered as the first generation failure.  The flattened surface morphology and the extremely jagged perimeter of the deposit, suspended from the current drainage, suggest a long exposure to the erosion, about 350.000 years, according to tentative geomorphologic correlations with adjacent fluvial terraces. The other two landslides are respectively 3,5 and 3 km long and 0,7 and 0,3 km wide. They cover the central portion of the relict landslide, and are characterised by a E-W bulge about 1 km wide.  Despite similar geological and geomorphological characteristics, they appear much less dismantled then the first generation landslide and therefore they are not classified as relict, although they are likely pre-Holocene in age.

The overall thickness of colluvium is on average 25 m, but it overcomes 55 m. Larger thicknesses are found in the stratigraphic logs crossing all the three overlapping landslide bodies, usually separated by two levels of palaeosoils. Future dating of these palaeosoils will make it possible to clarify the timing of these main landslide events.

These ancient landslides were significant morphological modifiers of the environment. Holocene drainage follows the original landslide morphology, whilst the occurrence of more recent multigeneration mass movements is controlled by discontinuous multilayered aquifers and planes of weakness developed within the original landslide bodies. In addition, the ancient landslides dismantled much of the original thrust-related mountain front, facilitating a future retreat of the upper slope of Mt. Cornacchia.

Results show that remobilization of ancient landslide material did not occur with the magnitudes and kinematics of the initial events, suggesting that the relict and very old landslides occurred in different environmental condition then the present day. However, the long-term maintenance of the meta-stable conditions in the area is somehow connected with the geological and geomorphological features of these ancient landslides, recognized for the first time in our recent inventory.  We stress that this lack of knowledge has compromised, up to now, a correct management of landslide hazard in the study area, and we expect that the same may occur where ancient landslides are not recognized in the landscape and/or not considered in the chain of hillslope processes.