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José Bandeira Júnior, Universidade Federal do Pará (Brazil)
Afonso Nogueira, Universidade Federal do Pará (Brazil)
Alcides da N. Sial, Universidade Federal de Pernambuco (Brazil)
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Dolomite is the main cement in Neoproterozoic sandstones from the Raizama Formation, basal unit of Alto Paraguai Group, exposed in the southwestern Amazon craton. In the last decades, these rocks have been interpreted erroneously as depositional carbonate of the Araras Group that are overlain by Alto Paraguai Group. Outcrop-based facies analysis in combination with petrography and carbon and oxygen isotopes was carried out in the sandstones of Raizama Formation exposed in the Império quarry, district of Gloria, state of Mato Grosso, Brazil. The storm-influenced littoral deposits of Raizama Formation are overlain by offshore mudstone of Sepotuba Formation, and consist of fine to coarse-grained quartzarenites composed of predominantly monocrystalline quartz grains with syntaxial overgrowths. The dolomite cement forms an idiotopic texture with occurrence of saddle dolomite. This cement at the base of studied succession is less frequent (10%), while upsection it increases up to 90%, near to the contact with the Sepotuba Formation. δ13C values of dolomite vary from of -1,70 to -3,56‰, and the most depleted values are smaller than -3 ‰, performing two-thirds of the analyzed samples. δ18O values of dolomite range from -3,78 to -5,17‰, with predominant values below of -5 ‰. The relation δ13C/δ18O does not show covariance and does not change stratigraphically. The origin of the dolomitic cementation could be related to hydrothermal or diagenetic fluids associated to a later remobilization of the carbonates from the top of the Araras Group. This explained why the δ13C values of the dolomitic cement are more depleted than those found in the carbonates from the upper part of the Araras Group, with values around -2 to 0 ‰.The most negative values of carbon indicate that the dissolution of original dolomitic material was produced from a mixture of lighter isotopic water. Faults, fractures and megacavities, filled by dolomite and calcite cements probably were fluid conduits during later remobilization and burial diagenesis, indicated too by the occurrence of saddle dolomite. The dolomite cement is restricted to the top of the Formation Raizama, limited by not dolomitized mudstone of the Formation Sepotuba. These mudstone exercised a function of a stratigraphic trap that favored the imprisonment and restricted percolation of the carbonate enriched fluid. The dolomite cement and absence of depositional marine carbonate in the top of the Raizama Formation exclude definitively the comparison of this unit with the Araras Group.
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