Pisolithic ferricretes around the Cretaceous-Palaeocene boundary in southern Sardinia (Italy) as palaeoenvironmental proxies

Abstract

Some of the weathering deposits that formed during the emersive phase, which in Sardinia lasted from Late Cretaceous to Palaeocene–Early Eocene, consist today of ferruginous palaeosols (ferricretes), sometimes overlain by palustrine limestones. In this article, the two outcrops at Nuxis and Guardia Pisano, containing a ferricrete composed of subspherical goethitic-hematitic concretions, are studied in detail. The main components are Fe, Al, Si, along with Ba, which is very abundant in the Nuxis outcrop. With the exception of the barite, the pisolithic ferricretes of Sardinia can be compared, for their chemical and mineralogical composition, with the pisoplinthites that form in modern tropical soils. They are also comparable with the pisolithic ferricretes cropping out in southern France, that formed around the K/T boundary. The pisolithic ferricretes of southern France and Sardinia can be interpreted as the weathering product of identical warm and humid climatic conditions and, reasonably, of the same climatic event. Therefore, they provide further evidence that Sardinia and southern France formed part of the same palaeogeographic province. A tropical environment is, on the other hand, consistent with the barite leaching from the Palaeozoic substratum and with its reprecipitation into the iron pisoliths. Micro- and nanostructures, observed using SEM analysis within and between the concentric layers of the pisoliths, are consistent with biological processes involving bacteria during their deposition.