Relating lithofacies, petrofacies, diagenesis, and mechanical behaviour of sandstones of the Ranzano Formation (Piedmont Tertiary Basin, Northwest Italy). Among sedimentary rocks, turbiditic sandstones are very common within orogenic belts, especially those with low relief composed mostly of sedimentary tectonic units; in this respect the Northern Apennines belt is a typical example. The mechanical behaviour of these rocks is extremely variable, depending on the degree and character of the diagenesis, which in turn depends on interactions between different factors such as the tectonic setting, the degree and rate of burial, the textural characters and composition of sediments. The relations between depositional facies, detrital composition, diagenesis, and mechanical behaviour are considered for the case of turbidite sandstones of the Ranzano Formation (Eocene-Oligocene), which crops out in the eastern Piedmont Tertiary Basin (BTP). Understanding these relationships is also important because of the extensive use of these rocks for buildings forming the cultural heritage of Northern Apennine and western Po Plain villages and towns. Samples collected from three different members (Pizzo d'Oca, Val Pessola and S. Sebastiano Curone Members) of the Ranzano Formation, having strongly contrasting petrofacies and extremely variable depositional facies grading from conglomerates to mudstones, were analysed in order to check their relations with diagenetic characters in a quite small area, having an homogeneous diagenetic regime. The data show that the relative amount of interstitial components (cements and pores), does not seem to be correlated with the framework composition (petrofacies), and also show no correlation between the occurrence of authigenic versus detrital carbonates. In contrast, there is a clear correlation of the amount of pores and cements with depositional facies. In turn, the incidence of the authigenic minerals and, in particular, their abundance relative to the volume of residual intergranular pores, seems to be fundamental in determining the resistance of the rocks to compression. As a result, depositional facies, rather than framework composition, seem to determine the diagenetic evolution of turbiditic arenites. It suggests that thin arenitic intervals interbedded with marls in thin-bedded arenaceous-pelitic facies, worked as conduits for carbonate-rich fluids expelled from the adjacent marly sediments during compaction, allowing calcitic cements to precipitate. Conversely, in thick massive sandstone beds amalgamated in tens of meter thick bodies, fluid circulation had been less intensive, with consequently reduced precipitation of authigenic carbonates.

You do not currently have access to this article.