Controls of sea-level fluctuations on the formation of Ordovician siliceous nodules in terrigenous offshore environments

Abstract Siliceous nodules of the Ordovician Period, of the Armorican Massif and of southwest Sardinia are made of a mixture of terrigenous origin material (quartz and phyllosilicates) and of biogenic origin components (microcrystalline quartz, calcium phosphate and calcite). They are associated with fine-grained sequences, which were deposited between the distal part of the inner shelf and the outer shelf. Within these sequences, two facies can be distinguished, clayey–silty facies without nodules related to episodes of high sedimentation rate and nodule-bearing clayey facies linked to episodes of reduced sedimentation. The genesis of the siliceous nodules can be integrated into a model taking account of the fluctuations in terrigenous flux due to eustatic variations: during periods of rising sea level, the terrigenous flux decreases and the sedimentation is dominated by bioclastic elements (calcareous, siliceous and phosphatic). Stability of the sediment–water interface facilitates biogenic silica dissolution. During early diagenesis, silica precipitation leads to the formation of proto-nodules. The distribution of the two facies can be explained by the superposition of two eustatic signals, one of very high frequency (VHF) and the other of high frequency (HF): the silty–clayey facies could be accumulated during the period of falling sea level within the HF cycle and the nodule-bearing clayey facies could correspond to amalgamation of several VHF cycles related to periods of low sedimentation rate in the HF cycle.