The Steptoean Positive Carbon Isotope Excursion (SPICE), inorganic aragonite precipitation and seawater chemistry : Insights from the Middle - Late Cambrian Port au Port Group, Newfoundland

ACKNOWLEDGEMENTS The authors would like to thank the following bodies for funding this research: Shell Brazil through the ‘BG05: UoA-UFRGS_SWB Sedimentary Systems’ project at UFRGS, the Aberdeen Formation Evaluation Society and the University of Aberdeen. LAMIR, BGS and EPMA staff are also thanked for their assistance with stable isotope and electron microprobe analysis. Dr Ilse Kamerling and Dr. D. Kemp are thanked for their assistance with pXRF analysis, Marianna Skupinska with collection of point count data and Prof. D. McIlroy is thanked for his assistance during fieldwork. Reviews by Dr C. Pederson, Prof. P. Swart and two anonymous reviewers helped improve the manuscript significantly.Peer reviewedPublisher PD

Carbon isotopes, stratigraphy, and environmental change: the Middle–Upper Cambrian Positive Excursion (SPICE) in Port au Port Group, western Newfoundland, Canada

In many basins, Upper Cambrian carbonate successions display intervals with a positive carbon isotope excursion (CIE) of up to +5‰. In North America, this marks the boundary between the Sauk II–III super-sequences. A Steptoean positive carbon isotope excursion (SPICE) locality previously identified in the Port au Port peninsula, western Newfoundland, has been revisited and an additional potential SPICE locality found. In both locations, a CIE is found to be associated with a prominent bioherm and sandstone layer within a sequence of carbonate rocks. At March Point columnar stromatolites occur, whereas at Felix Cove thrombolites can be seen. In the latter, the sandstone immediately overlies the thrombolites coincident with the CIE, whereas at March Point a dolomitized grainstone occurs above the stromatolites. The sandstone at this locality post-dates the CIE. Although lower than the SPICE in some localities, a positive CIE is present in both sections: March Point (+1.1‰) and Felix Cove (+1.8‰). Additionally, δ13Corg rises from −30.0‰ to −22.0‰ at March Point and from −27‰ to −24.0‰ at Felix Cove and, in accordance with previously published work, we suggest that this could be the SPICE. Comparison of the stratigraphy and petrography between the two localities suggest that both depositional and diagenetic factors could have influenced the nature of the interpreted SPICE in Newfoundland. It is also possible that the local carbon isotopic signature may have been influenced by a semi-restricted depositional and early diagenetic environment related to the paleogeographic configuration rather than the global marine excursion

Quimioestratigrafia 87SR/86SR das rochas carbonáticas do grupo Bambuí.

O Grupo Bambuí cobre extensas áreas do Cráton São Francisco e da borda oriental da Faixa Brasília, e parte da Bacia do São Francisco, onde exsudações de gás são conhecidas e podem ter como rochas geradoras os folhelhos da Formação Sete Lagoas, na base do grupo. As idades do Grupo Bambuí, entretanto, permanecem controversas na literatura devido às dificuldades de datação diretas na sequência. Através de atividades como a localização de lentes de rochas carbonáticas presentes nas formações Sete Lagoas e Lagoa do Jacaré, no estado de Minas Gerais, e a amostragem destas rochas para análises, este trabalho propõe a determinação sistemática da composição isotópica de Sr (87Sr/86Sr) ao longo da maior parte da sequência Bambuí e sua comparação com a curva de variação isotópica mundial, com significado geocronológico. os resultados mostraram razões de estrôncio que variam de 0,70714 a 0,70804 para a Formação Sete Lagoas, e de 0,70746 a 0,7082 para a Formação Lagoa do Jacaré. Sua assinatura isotópica com razões de 87Sr/86Sr ascendentes da base para o topo do grupo concorda com o comportamento esperado para o período Ediacarano, sugerindo que a deposição tenha ocorrido entre 630 e 575 Ma. A comparação com a quimioestratigrafia do Grupo Una, correlato do Grupo Bambuí na porção norte-central do Cráton São Francisco, evidencia a diferente gênese entre estas bacias correlatas, com valores descendentes da base para o topo. Estas diferenças estariam relacionadas com os tipos de bacia em que cada grupo foi formado, aporte sedimentar e áreas fonte desses sedimentos.The Bambuí Group covers extensive areas of São Francisco Craton and the eastern edge of the Brasília Belt. It is part of the São Francisco Basin, where natural gas exsudation are known and can have as rocks source the shales of the Sete Lagoas Formation, at the base of the group. The ages of the Bambuí Group, however, remains controversial in the literature due to the difficulties of direct dating the sedimentary rocks in the sequence. Through activities such as the location of lens of carbonate rocks present in Sete Lagoas and Lagoa do Jacaré formations, in the Minas Gerais state, and sampling of these rocks for analysis, this paper proposes a systematic determination of the isotopic composition of Sr (87Sr/86Sr ) across the longest part of the Bambuí sequence and its comparison with the global isotopic variation curve, with a geochronological significance. The results showed strontium ratios ranging from 0.70714 to 0.70804 for the Sete Lagoas Formation, and from 0.70746 to 0.7082 for the Lagoa do Jacaré Formation. Its isotopic signature with 87Sr/86Sr ratios rising from bottom to top of the group agrees with the expected behavior for the Ediacaran period, suggesting that Bambuí deposition occurred between 630 and 575 Ma. The comparison with the chemical stratigraphy of Una Group, which is a correlate to Bambui Group situated at northcentral São Francisco Craton, reveals the different genesis between these basins correlated, with values descending from base to top. These differences are related to the types of basin were each group was formed, as the souce areas and the sediment input

Quimioestratigrafia 87SR/86SR das rochas carbonáticas do grupo Bambuí.

O Grupo Bambuí cobre extensas áreas do Cráton São Francisco e da borda oriental da Faixa Brasília, e parte da Bacia do São Francisco, onde exsudações de gás são conhecidas e podem ter como rochas geradoras os folhelhos da Formação Sete Lagoas, na base do grupo. As idades do Grupo Bambuí, entretanto, permanecem controversas na literatura devido às dificuldades de datação diretas na sequência. Através de atividades como a localização de lentes de rochas carbonáticas presentes nas formações Sete Lagoas e Lagoa do Jacaré, no estado de Minas Gerais, e a amostragem destas rochas para análises, este trabalho propõe a determinação sistemática da composição isotópica de Sr (87Sr/86Sr) ao longo da maior parte da sequência Bambuí e sua comparação com a curva de variação isotópica mundial, com significado geocronológico. os resultados mostraram razões de estrôncio que variam de 0,70714 a 0,70804 para a Formação Sete Lagoas, e de 0,70746 a 0,7082 para a Formação Lagoa do Jacaré. Sua assinatura isotópica com razões de 87Sr/86Sr ascendentes da base para o topo do grupo concorda com o comportamento esperado para o período Ediacarano, sugerindo que a deposição tenha ocorrido entre 630 e 575 Ma. A comparação com a quimioestratigrafia do Grupo Una, correlato do Grupo Bambuí na porção norte-central do Cráton São Francisco, evidencia a diferente gênese entre estas bacias correlatas, com valores descendentes da base para o topo. Estas diferenças estariam relacionadas com os tipos de bacia em que cada grupo foi formado, aporte sedimentar e áreas fonte desses sedimentos.The Bambuí Group covers extensive areas of São Francisco Craton and the eastern edge of the Brasília Belt. It is part of the São Francisco Basin, where natural gas exsudation are known and can have as rocks source the shales of the Sete Lagoas Formation, at the base of the group. The ages of the Bambuí Group, however, remains controversial in the literature due to the difficulties of direct dating the sedimentary rocks in the sequence. Through activities such as the location of lens of carbonate rocks present in Sete Lagoas and Lagoa do Jacaré formations, in the Minas Gerais state, and sampling of these rocks for analysis, this paper proposes a systematic determination of the isotopic composition of Sr (87Sr/86Sr ) across the longest part of the Bambuí sequence and its comparison with the global isotopic variation curve, with a geochronological significance. The results showed strontium ratios ranging from 0.70714 to 0.70804 for the Sete Lagoas Formation, and from 0.70746 to 0.7082 for the Lagoa do Jacaré Formation. Its isotopic signature with 87Sr/86Sr ratios rising from bottom to top of the group agrees with the expected behavior for the Ediacaran period, suggesting that Bambuí deposition occurred between 630 and 575 Ma. The comparison with the chemical stratigraphy of Una Group, which is a correlate to Bambui Group situated at northcentral São Francisco Craton, reveals the different genesis between these basins correlated, with values descending from base to top. These differences are related to the types of basin were each group was formed, as the souce areas and the sediment input

Interactions between Clays and Carbonates in the Aptian Pre-Salt Reservoirs of Santos Basin, Eastern Brazilian Margin

The giant Pre-salt reservoirs represent most of the oil production in Brazil. The main Aptian sag reservoirs were deposited in a unique and highly complex hyper-alkaline lacustrine setting. These deposits are essentially constituted by fascicular and spherulitic calcite precipitated in a magnesian clay matrix (stevensite, kerolite, and saponite/hectorite). Although vital for understanding the origin and main reservoir quality control, the genesis and interactions of clays and carbonates are still poorly constrained. The detailed petrographic description was focused on 812 thin sections from five wells drilled in the Santos Basin Aptian Barra Velha Formation, combined with cathodoluminescence, UV epifluorescence, and X-ray diffraction analyses. The main syngenetic processes were the deposition of finely laminated peloidal and ooidal Mg-clays, the formation of fascicular calcite crusts on the sediment–water interface, and the redeposition of these materials as intraclasts. Abundant clay peloids engulfed in syngenetic shrubs indicate that calcite and clay precipitation was concomitant, though with highly variable rates. Eodiagenetic phases include matrix-replacive and -displacive spherulites and fascicular shrubs; matrix-replacive blocky calcite and dolomite; lamellar carbonates filling matrix shrinkage pores; and microcrystalline calcite, dolomite, and silica replacing the Mg-clay matrix. The preferential dolomitization and calcitization of peloidal layers were most likely due to their higher permeability and larger specific surface. Matrix-replacive saddle dolomite, macrocrystalline calcite, and dawsonite are interpreted as mesodiagenetic or hydrothermal phases after significant matrix dissolution. Unraveling the processes of the formation and alteration of the carbonates and clays and their interactions in the Pre-salt deposits is essential for constraining the depositional and diagenetic conditions in their unique environments and their diagenetic overprinting and for decreasing the exploration risks and increasing the production of those extraordinary reservoirs