The Itaboraí Basin, located in Rio de Janeiro state, is the smallest segment of the Southeastern Brazil Continental Rift, and the first to open, during the Palaeocene. Numerous studies focused on its fossiliferous content, but few systematically approached its depositional and diagenetic processes and products. A detailed description of the basal sedimentary deposits that remained after nearly fifty years of exploitation for cement production allowed an interpretation of the initial tectono‐sedimentary conditions responsible for the formation of the Basin. The macroscopic and petrographic description of outcrops and well core samples of the calcretes and travertines that correspond to the initial basin infill was integrated with stable isotope geochemistry. The calcretes were separated into different facies (powdery, nodular, massive, platy, laminated and brecciated), and classified according to morphogenetic stages. Their main microscopic features include nodules, contraction cracks, vadose pisoliths, rhizocretions, infiltrated clays, corroded grains, expanded micas and clay aggregates with bimasepic–plasmic microfabric. The travertines exhibit diverse forms of abiotic calcite crystal aggregates (plumose, lamellar to prismatic‐radial, spherulitic) and a minor proportion of microbial microcrystalline precipitates (bacterial shrubs). Tectonically active periods, characterized by intense fault movements associated with humid climate, promoted hydrothermal activity, formation of travertines and modification of the alluvial deposits and calcrete facies by ascending fluids. During periods of tectonic stability under arid to semi‐arid climate, the exposed alluvial and hydrothermal deposits were influenced by pedogenetic processes. Sulphide‐rich carbonate hydrothermal breccias occur in the main fault zone at the southern border. The characterization of extremely active pedogenetic and hydrothermal processes during opening and early evolution of the Itaboraí Basin is essential to the understanding of the initial evolution of the Southeastern Brazil Continental Rift. Additionally, some isotopic and petrographic similarities of the Itaboraí carbonates with the vast South Atlantic Aptian Pre‐Salt lacustrine deposits may offer an interesting contribution to the understanding of that important petroleum province.

中文翻译：

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来自古新世Itaboraí盆地的钙质和钙华，作为巴西东南部大陆裂谷早期演化的证据

位于里约热内卢州的Itaboraí盆地是巴西东南部大陆裂谷的最小部分，也是古新世以来第一个开放的盆地。许多研究都集中在它的化石含量上，但是很少有系统地研究它的沉积，成岩过程和产物。对水泥生产进行近五十年开采后剩余的基础沉积物的详细描述，可以解释造成盆地形成的初始构造沉积条件。对应于最初盆地充填物的露头和钙华的露头和岩心样品的宏观和岩相描述与稳定同位素地球化学相结合。各个小岛被分为不同的相（粉状，结节状，块状，板状，层压和角砾化），并根据形态发生阶段进行分类。它们的主要微观特征包括结节，收缩裂纹，渗流石块，根状分泌，浸润的粘土，腐蚀的颗粒，膨胀的云母和具有双张型质微织物的粘土聚集体。石灰华表现出多种形式的非生物方解石晶体聚集体（羽状，层状至棱柱形，球状，球状）和少量的微生物微晶沉淀物（细菌灌木）。构造活动期，其特征是与潮湿气候有关的强烈断层运动，促进了热液活动，钙华的形成以及上升流体对冲积沉积和钙质相的改变。在干旱至半干旱气候的构造稳定时期，裸露的冲积物和热液沉积物受到成岩作用的影响。富含硫化物的碳酸盐热液角砾岩发生在南部边界的主要断层带。Itaboraí盆地开放和早期演化过程中极活跃的成岩作用和热液作用的特征对于理解巴西东南部大裂谷的初始演化至关重要。此外，Itaboraí碳酸盐岩与广阔的南大西洋Aptian Pre-Salt湖相沉积岩的同位素和岩石学相似性可能为对该重要石油省的理解提供有趣的贡献。Itaboraí盆地开放和早期演化过程中极活跃的成岩作用和热液作用的特征对于理解巴西东南部大裂谷的初始演化至关重要。此外，Itaboraí碳酸盐岩与广阔的南大西洋Aptian Pre-Salt湖相沉积岩的同位素和岩石学相似性可能为对该重要石油省的理解提供有趣的贡献。Itaboraí盆地开放和早期演化过程中极活跃的成岩作用和热液作用的特征对于理解巴西东南部大裂谷的初始演化至关重要。此外，Itaboraí碳酸盐岩与广阔的南大西洋Aptian Pre-Salt湖相沉积岩的同位素和岩石学相似性可能为对该重要石油省的理解提供有趣的贡献。