Stratigraphic evolution of the Upper Carboniferous-Lower Triassic Sequence, Parnaíba Basin, Brazil

ABSTRACT: The Upper Carboniferous-Lower Triassic Sequence represents the third major sedimentary cycle of the Parnaíba Basin in northeastern Brazil and corresponds to the Balsas Group. The sequence is composed by a thick pack of siliciclastic rocks whose sediments were deposited under complex conditions, varying from clastic/evaporitic shallow marine to lacustrine and desert environment. Aiming to clarify the sedimentary sequence evolution, this work conducted a stratigraphic analysis of the Upper Carboniferous-Lower Triassic sediments. Modern and classic concepts of sequence stratigraphy were applied on a database of wells and seismic sessions. We identified three main depositional sequences. Sequence 1 corresponds to rocks initially deposited by a fluvial system which evolved to shallow marine conditions related to a transgressive stage, later evolving to a deltaic system. Sequence 2 corresponds to sediments deposited under lacustrine/arid conditions, during an important regressive stage, that evolved to a dominantly desert environment recorded in Sequence 3. Seismic stratigraphic analyses allow to infer key stratigraphic surfaces, and three main sequences. Seismic facies related with such sequences are dominantly parallel and sub-parallel, with good lateral continuity, suggesting that the sedimentary rates were homogenous along the interval analyzed in this sector of Parnaíba Basin.摘要：上石炭统-下三叠统地层序列为巴西东北部帕纳伊巴盆地（Parnaíba Basin）的第三套主要沉积旋回，对应巴尔萨斯群（Balsas Group）。该序列由一套厚层硅质碎屑岩系组成，其沉积物沉积于复杂多变的沉积环境中，涵盖碎屑/蒸发岩质浅海、湖相至沙漠等多种类型。为阐明该沉积序列的演化机制，本研究针对上石炭统-下三叠统沉积物开展了地层学分析。研究将层序地层学的经典与现代理论应用于测井与地震数据集，最终识别出三套核心沉积层序。层序1对应初始由河流体系沉积的岩石，该体系逐渐演化为与海侵期相关的浅海环境，后续进一步演化形成三角洲体系。层序2对应重要海退期内形成的湖相/干旱环境沉积物，其后续演化形成的以沙漠为主的沉积环境特征则被记录于层序3中。地震地层学分析可用于推断关键地层界面与三套主要层序。与上述层序相关的地震相以平行、亚平行结构为主，且具备良好的横向连续性，这表明帕纳伊巴盆地该研究区块内的沉积速率在本次分析的层段中保持均一。