Relazione tra architettura deposizionale e composizione nei depositi di piattaforma terrigena: gli accumuli a minerali pesanti (placers) dell’ Ordoviciano della Sardegna e della Bretagna

The aim of the present study is to investigate the stratigraphic significance of some heavy minerals concentrations. It is based on a sequence analysis and a petrophysic characterisation (natural radioactivity) of placers located in ordovician formations of SE Sardinia (S Italy) and W Armorican Massif (W France). In these formations, the heavy minerals are mainly represented by titaniferous minerals (rutile and anatase), zircon and monazite; the tourmaline is less common. The modal analyses show that their concentration can reach 40 %. Several gamma ray facies have been identified. A high radioactivity facies is represented by sandy beds enriched in heavy minerals; the high radioactivity (until 88000 cpm) is linked to abundance of U and Th in zircon and monazite. Two litho facies with medium radioactivity (4000 to 6000 cpm) have been recognized; their radioactivity is linked to the K in the silty clayey intercalations. The gamma ray facies have been located in the depositional sequences and a stratigraphic model is proposed. This model is based on the sediment volume partitioning and on the superposition of eustatic signals of different frequencies. On the shallow environments of shoreface and proximal inner shelf, the storm wave action favours the reworking of high-density minerals into storm beds. During major-order cycles sea-level rises, the decreasing of terrigenous inputs in the marine environments and the amalgamation of several very high frequency sequences lead to concentrations of the heavy minerals. On the contrary, during sea-level falls, the high-density minerals are diluted by abundant terrigenous flux.