Pyrite-bearing non-marine ostracods from the Lower Cretaceous Maracangalha Formation, Recôncavo Basin, NE Brazil: Depositional controls and paleoecological implications

Abstract

Ostracods are aquatic microcrustaceans which use calcium dissolved in water to synthesize their carapaces, along with a low magnesium content. Depending on the conditions of the depositional environment and diagenetic processes, ostracods can be preserved in the fossil record, with or without alteration of the original chemical composition. In this study, samples from shales, siltstones and silty sandstones of the Maracangalha Formation (Recôncavo Basin) along the Gameleira and Manguinhos (Ilha de Itaparica) and Praia da Falha (Ilha dos Frades) outcrops, Baía de Todos os Santos, were analyzed. Results show that the pattern of ostracod carapaces preservation was controlled by distinct sedimentological processes. In the Manguinhos and Praia da Falha outcrops, ostracod specimens show total or partial replacement of the carapace by pyrite, with the more expressive pyritization observed in shale from Manguinhos. In the Gameleira outcrop, on the other hand, no pyritized carapaces were observed. Results of the Energy Dispersive Spectrometry (EDS) from Manguinhos and Praia da Falha outcrops indicate the presence of Fe and S pointing to pyrite mineralization. The Scanning Electron Microscopy (SEM) images revealed the euhedral and framboidal habit of pyrites present in the carapaces. The low recovery of specimens from Gameleira and Praia da Falha, as well as the abundance of specimens in Manguinhos, seem to have depositional controls. At Gameleira and at Praia da Falha, facies associated with low-to-high density turbidity currents may have hampered preservation, thus low recovery of ostracods. Despite the occurrence of fragmented carapaces, the anoxic conditions of the Manguinhos shale samples allowed a better preservation of specimens. The Manguinhos facies are interpreted as relatively deep lacustrine deposits. The presence of framboidal pyrite suggests early conditions of diagenesis at low burial depths. Pyrite mineralization may have occurred through a hydrogen sulfide environment, rich in organic matter, with dissolution of the carbonate carapace through the acidic pH of the medium and relative rapid post-mortem burial. The sedimentary facies described, along with the taphonomic results of ostracod preservation, corroborate a lacustrine environment with occasional sediment gravity flows to the Maracangalha Formation.