Combining outcrop gamma-ray logging and sequence analysis in the study of Ordovician placer deposits:Sardinia (S Italy) and Armorican Massif (NW France)

Marco Pistis (1) , Marie-Pierre Dabard (2) , Alfredo Loi (1)

1 Dipartimento di Scienze Chimiche e Geologiche
2 GR - Géosciences Rennes

This work is aimed at acquiring knowledge and understanding of some placer deposits of the Ordovician of SE Sardinia (S Italy) and W Armorican Massif (NW France). These shallow water heavy mineral concentrations were deposited in a siliciclastic storm-dominated shelf environment where the stratigraphic evolution shows an apparently random distribution of such deposits in identical facies. Heavy minerals are mainly represented by titaniferous minerals (rutile and anatase), zircon and monazite; the tourmaline is less common. The study is based on high resolution sequence stratigraphy analysis in outcrops and backstripping procedure, supported by a petrophysical characterization and petrographic study. Gamma-ray logging has been performed on the basis of the presence of zircon and monazite minerals which are natural radioactive. Several gamma-ray facies have been identified. A high radioactivity facies is represented by sandy beds enriched in heavy minerals, in which modal analyses show that their concentration can reach 50%. The high radioactive signal is linked to the abundance of Uranium and Thorium in zircon and monazite. Two facies with medium radioactivity have been recognized; their radioactivity is linked to the Potassium in the silty-clayey intercalations. Stratigraphic key surfaces previously identified by the sequence analysis, are well highlighted also by the gamma-ray signal. Total Counts and K mark fourth-order sequences and stratigraphic key surfaces of thirdorder sequences (Maximum Regressive Surface and Maximum Flooding Surface), whereas the U and Th signal point out placer beds. The comparison between gamma-ray facies located within the depositional sequences and the results of the backstripping analysis point out that heavy mineral concentrations occur during major-order base-level rises in high-energy depositional environments (shoreface and upper offshore). In the shallow environments of shoreface and proximal inner shelf, the storm wave action favours high-density minerals deposition, but this process seems to be controlled by allocyclic factors such as base-level variations. We proposed a model based on the sediment volume partitioning and on the superposition of cycles of distinct frequencies. During major-order cycles sea-level rises, the decreasing of terrigenous inputs in the marine environments and the amalgamation of very high frequency sequences lead concentrations of the heavy minerals. On the contrary, during sea-level falls, the high-density minerals are diluted by abundant terrigenous flux. Acknowledgements: This work was financed by the Sardinia Regional Government (LR7-2007, N.7: “Promozione della ricerca scientifica e dell’innovazione tecnologica in Sardegna”).

Domaines: Sciences de la Terre

Marco Pistis, Marie-Pierre Dabard, Alfredo Loi. Combining outcrop gamma-ray logging and sequence analysis in the study of Ordovician placer deposits:Sardinia (S Italy) and Armorican Massif (NW France). 19th International Sedimentological Congress from 18 to 22 august 2014 in Geneva, Switzerland Sedimentology at the crossroads of new frontiers, Université de Genève; International Association of Sedimentologists, Aug 2014, Genève, Switzerland. 1 p. ⟨insu-01104236⟩