Stratigraphy of the Araí Group: record of paleoproterozoic rifting in Central Brazil

The Araí Group is characterized by a succession of low-grade metassedimentary and metavolcanic rocks that overlaps the Aurumina Suit and the Ticunzal Formation. The related basin fill-succession of the Araí Group was probably deposited in an intracontinental rift, which evolution initiated before 1,77 My in the Paleoproterozoic. Originally this unit was subdivided in the Arraias Formation (continental sequence) and Traíras Formation (marine sequence). However, the analysis, compilation and integration of the geologic data, mainly related to the sedimentary geology and the tectono-stratigraphy, had shown the necessity to formalize four unit subdivisions, including the Água Morna, Arraias, Caldas and Traíras formations. The Água Morna Formation represents a braided river fluvial deposicional system, evolved in an intracontinental SAG basin type, during a pre-rift phase, and representing the Água Morna Tectonosequence submitted to thermal subsidence processes. The Arraias Formation represents a thick package of continental sediments, subdivided in four members: Cubículo (aluvial fans), Prata (braided river), Mutum (aeolian), Ventura (lacustrine) and Buracão (volcanic and pyroclastic), representing the Arraias Tectonosequence, which constitute the infilling of the main rift phase, under mechanic subsidence. The Caldas Formation represents the transitional depositional sequence of the Araí Group, subdivided in two informal members related to beach system: Lower Member (backshore and foreshore) and Upper Member (shoreface). The Traíras Formation represents the marine sequence, subdivided in the Boqueirão Member (mixed platform dominated by tides), Rio Preto Member (siliciclastic platform dominated by tides) and Rosário Member (mixed platform dominated by tides), integrating the Traíras Tectonosequence and developed during flexural subsidence (post rift phase). Here the rewied stratigraphy has shown well functional for mapping and regional correlations in different areas of occurrence of the Araí Group in Central Brazil.

阿拉伊群（Araí Group）以一套低变质沉积岩与变质火山岩系的连续序列为特征，其超覆于奥鲁米纳岩套（Aurumina Suite）与蒂昆萨尔组（Ticunzal Formation）之上。阿拉伊群相关的盆地充填序列可能沉积于陆内裂谷环境，该裂谷的演化始于古元古代的17.7亿年前（原文标注为1.77 My，结合古元古代地质背景推测应为1.77 Ga，即17.7亿年）。最初该地层单元被划分为阿拉亚斯组（Arraias Formation，陆相序列）与特拉伊拉斯组（Traíras Formation，海相序列）。然而，通过对主要涉及沉积地质学与构造地层学的地质数据进行分析、整合与汇编，研究表明有必要将其正式划分为四个地层单元，即阿瓜莫尔纳组、阿拉亚斯组、卡尔达斯组与特拉伊拉斯组。阿瓜莫尔纳组代表一套辫状河河流沉积体系，形成于陆内SAG盆地的裂前阶段，对应经历热沉降作用的阿瓜莫尔纳构造层序（Água Morna Tectonosequence）。阿拉亚斯组代表一套厚层陆相沉积组合，被划分为五个成员：库比库洛成员（冲积扇相）、普拉塔成员（辫状河相）、穆图姆成员（风成相）、文图拉成员（湖相）与布拉卡昂成员（火山及火山碎屑相），对应阿拉亚斯构造层序，该层序构成了机械沉降作用下主裂谷阶段的充填物。卡尔达斯组代表阿拉伊群的过渡沉积序列，被划分为两个非正式成员，均与海滩体系相关：下成员（后滨与前滨相）与上成员（临滨带相）。特拉伊拉斯组代表海相序列，被划分为博凯朗成员（潮汐主控混合台地）、里约普雷托成员（潮汐主控硅质碎屑台地）与罗萨里奥成员（潮汐主控混合台地），归入特拉伊拉斯构造层序，形成于挠曲沉降的裂后阶段。经修订的地层学方案在巴西中部阿拉伊群出露的不同区域的地质填图与区域对比中展现出良好的适用性。