Il deposito pleistocenico di S. Michele (Ploaghe, Sardegna settentrionale) e i suoi rapporti con l’evoluzione morfologica del Logudoro

Pleistocenic deposit of S. Michele (Ploaghe, Sardinia) and its correlations with the morphological evolution of Logudoru

Abstract

Underlying a basalt flow dated 140,000 years, a sandy-clay deposit has been discovered intercalated with three buried soils. Chemico-physical analyses and micromorphological studies indicate that the clayey soil at the bottom formed in an area prone to frequent and long flooding, whole the two overlying sandy soils developed on materials already pedogenesed coming from eroded soils in neighbouring areas. The climate was characterized by a prolonged wet season followed by a dry season. The geomorphological study of the area showed that the site is located along a small incision which some 500,000 years ago was obstructed by a basalt flow where the deposit occurs. Reconstruction of the evolutionary phases that led to the formation of these soils, after deposition of the oldest flow, indicates that in the Middle Pleistocene climatic conditions were characterized by long wet periods alternated with short dry ones. By contrast, from 140,000 years onwards the climate became increasingly drier and gradually produced forms of erosion with surfaces (terraces, pediments) and deep incisions also associated with the general folding of the area.