Sediments of small lakes along creeks in Northern Apennines (NW-Italy) as evidence of ancient slope instability

Abstract

In the last decades the erosion of some creeks in the Ligurian Apen- nines has revealed sediments of stagnant waters rich in plant remains. These small “fossil lakes” represent the effects of phases of slope instabil- ity that caused barrages from landslides at different times, since at least 10,000 BP until the 19th century. This paper presents a review of studies in the central Ligurian mountains concerning the formation of temporary ponds along streams, which is documented for different sites in this area for prehistoric times, the Middle Ages and later. It also aims to deepen – by means of biostratigraphyc analysis – the knowledge of a site along the creek Rio Dell’Orso, near the village of Senarega (710 m, Scrivia Valley, NW-Italy). There, erosion has exposed sediments of a disappeared lake, containing logs, branches and pollen. A small basin has formed, after a landslide that had dammed the stream in prehistoric times, for an esti- mated period of about three centuries. Radiocarbon dating allowed to attribute the deposits to a period around 4000 BP. More than 40 branches and trunks were identified and pollen analysis was carried out. Among the macroremains, silver fir (Abies) clearly prevails in number and size; also Fagus and Fraxinus were identified. Even within the pollen assemblages, unfortunately rather badly preserved, the silver fir is dominant in all layers. These data are consistent with what is known about the Holocene history of the Ligurian Apennines: also the upper Scrivia Val- ley was long characterized by forests dominated by silver fir with beech, which were still widespread throughout the region until the Roman pe- riod and the Middle Ages.