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Carbonates in the Sete Lagoas Formation (São Francisco craton, Brazil) preserve a record of chemical, biological, and oceanographic changes that occurred during the Ediacaran Period. The base of this formation constitutes a post-glacial cap carbonate, which contains seafloor precipitates (carbonate and barite crystal fans) as well as various authigenic and diagenetic minerals (apatite, pyrite, and barite). Here, we present petrographic and geochemical data on this unit, and discuss the significance of its mineral association for marine environments following the Marinoan ('Snowball Earth') glaciation. For the first time, we report well-developed apatitic cements in a Neoproterozoic cap carbonate. Isopachous and intergranular void-filling cements encrust and surround seafloor-precipitated fan crystals that precipitated as aragonite. We propose a model for the origin of this mineral association, which relates phosphogenesis and aragonite fan formation to a single set of environmental conditions. According to this model, the boundary between oxic and anoxic conditions was located at or just below the sediment-water interface. Burial of iron (oxyhydr)oxides below this boundary liberated phosphate to pore water and provided fuel for iron reduction. Iron reduction released Fe 2+ , which inhibited nucleation of carbonate and allowed for aragonite growth on the seafloor. Concurrently, 'iron-pumping' shuttled phosphate from the water column to the sediment, and perhaps in conjunction with organic phosphorus remineralization via anaerobic microbial pathways, created conditions conducive to phosphate mineralization. This model corroborates the hypotheses that aragonite crystal fan formation requires the presence of an inhibitor to carbonate nucleation, in addition to high alkalinity, and that Fe 2+ serves as this inhibitor. Overall, our work documents a close association between aragonite crystal fan formation and phosphogenesis at the beginning of the Ediacaran, illuminates the paleoenvironments of cap carbonates with seafloor precipitates, and contributes to understanding of phosphogenesis following low latitude glaciations.

Carbonate rocks from the Macaé Group (Albian) represent an example of carbonate sedimentation related to the drift phase in Campos Basin. This study presents depositional features, integrating them with diagenetic and stratigraphic aspects of the Macaé Group carbonates including the upper part of the Quissamã Formation and the lower part of the Outeiro Formation. Macroscopic analyses in cores and microscopic ones in thin sections allowed the recognition of eleven sedimentary facies -nine of them corresponding to the Quissamã Formation and two of them representing the Outeiro Formation. These facies were grouped into five facies associations. Oolitic grainstones and oncolitic grainstones are interpreted to be deposited in shallow depth probably in shoals above the fair weather wave base. The interbanks between shoals were formed in less agitated waters and characterized by deposition of peloidal bioclastic packstones and wackestones representative of sedimentation in calm waters. Bioclastic packstones and oolitic packstones/wackestones represent allochthonous deposits related to the beginning of the regional drowning that occur in upper Quissamã Formation. Pithonellids wackestones and bioclastic wackestones with glauconite are related to deep water deposits, characteristics of the Outeiro Formation. Post-depositional features revealed the action of diagenetic processes as, micritization, cimentation, dissolution, compaction, dolomitization and recrystallization occurred during the eoand mesodiagenesis phases. Vertical facies analysis suggests shallowing upward cycles stacked in a sequence progressively deeper towards the top (from the Quissamã Formation to the Outeiro Formation). KEYWORDS:Carbonate Rocks; Quissamã Formation; Outeiro Formation; Facies; Diagenesis. RESUMO: As rochas carbonáticas do Grupo Macaé (Albiano) representam a sedimentação carbonática relacionada ao início da fase drifte na Bacia de Campos. Discutem-se aqui aspectos deposicionais, diagenéticos e estratigráficos das rochas carbonáticas do Grupo Macaé, englobando a porção superior da Formação Quissamã e a porção basal da Formação Outeiro. Análises macroscópi-cas em testemunhos e microscópicas utilizando-se lâminas delgadas permitiram o reconhecimento de onze fácies sedimentares -nove delas correspondentes à Formação Quissamã e duas à Formação Outeiro. Estas foram agrupadas em cinco associações de fácies, relacionadas às diferentes posições fisiográficas de uma plataforma carbonática. Os bancos carbonáticos estão relacionados à deposição de grainstones oolíticos e grainstones oncolíticos em profundidades rasas, acima do nível de base de ondas de tempo bom (NBTOB). Os baixios oncolíticos ou interbancos foram formados em águas menos agitadas

In the Ediacaran marine succession of the Cerro Negro Formation (Tandilia System, NE Argentina), abundant microbially induced sedimentary structures indicate general conditions of substrate biostabilization. Numerous discoidal structures in this succession were previously interpreted as moulds of soft-tissue holdfasts of sessile organisms, within the form genus Aspidella. In this study, we performed a detailed re-analysis of some of these features and discuss two alternative hypotheses to explain their genesis: (1) as the result of soft-sediment deformation and fluid injection structures; and (2) as structures of active animal–sediment interaction (i.e. trace fossils). We show that the dome-shaped discs are internally laminated, with a cylindrical to a funnel-shaped vertical tube at their central region. The presence of these downwards vertical extensions and other intricate internal arrangements cannot be explained under the taphonomic spectrum of discoidal fossils, but shows striking similarities to Intrites-like structures and other sand-volcano-like pseudofossils (e.g. Astropolithon). However, some structures are hard to distinguish from vertical dwelling burrows with funnel-shaped apertures and thick-lined walls, commonly produced by suspension- and detritus-feeding invertebrates (e.g. Skolithos isp., Monocraterion isp. and, less likely, Rosselia isp.). Since reliable age constraints are unavailable, and further investigation concerning other palaeobiological indicators is needed, the most parsimonious hypothesis is that of a structure derived from fluid-escape processes. Our study demonstrates the importance of detailed investigation on discoidal structures in either upper Ediacaran or lower Cambrian strata.