Petroplinthite formation in a pedosedimentary sequence along a northern Mediterranean coast: from micromorphology to landscape evolution

The petroplinthic horizon is a layer of indurated material in which Fe is an important cement, and it pertains to the pedogenetic material called "laterite". The aim of this paper is to document the evolution of a coastal pedosedimentary sequence that developed in NW Italy during the Quaternary and discuss the genesis of its petroplinthic horizon within the context of environmental changes that should not have been favourable to lateritisation processes. The palaeosol profile was described, and the soil horizons were grouped into pedostratigraphic levels. The horizons were characterised using laboratory routine analysis, X-ray diffraction and thin section micromorphology. In addition, a scanning electron microscope examination of the back-scattered images and an elemental analysis were performed on the petroplinthic horizon only. The micromorphological evidence and mineralogical analyses suggest a polygenetic origin for the profile that reflects the influence of separate processes acting on distinct parent material under different environmental conditions. The petroplinthic horizon results from a pedogenetic process that occurs during seasonal fluctuations of the water table, whereas the plinthite has no pedogenetic link with the weathered bedrock. The plinthitisation/ferrugunisation derived from iron enrichment and accumulation from an external upslope source and/or by post-depositional precipitation of "secondary" iron phyllosilicates (e.g. hisingerite) is a result of the dissolution of pre-existing hematite in inherited detrital laterite fragments.