Corrêa-Martins Francisco J
Programa de Pós-Graduação em Geologia, Universidade Federal do Rio de Janeiro, Avenida Athos da Silveira Ramos, 274, Ilha do Fundão, 21910-200 Rio de Janeiro, RJ, Brazil.
Departamento de Geociências, Instituto de Agronomia, Universidade Federal Rural do Rio de Janeiro, BR-465, Km 7, 23897-000 Seropédica, RJ, Brazil.
An Acad Bras Cienc. 2019;91Suppl. 2(Suppl. 2):e20180730. doi: 10.1590/0001-3765201920180730. Epub 2019 May 6.
This study defines the neostratotype of the Lower-Middle Albian Itapecuru Formation in the Parnaíba Basin, Northeast Brazil. In this sedimentary succession along the right bank of the Itapecuru River near the Itapecuru-Mirim City, Maranhão State, three lithofacies associations are recognized. The first is silty claystone with some very fine sand, micaceous, reddish color, thin laminated, showing a tabular aspect, interpreted as floodplain fines in a distal position. The second is clayey siltstone, reddish color, with ripple cross-lamination, very thin cross-laminated wacke lenses, grading to thin parallel lamination, in apparently tabular layers, interpreted as crevasse splay and distributive channels deposits in successive avulsion events. The third is fine quartz wacke, reddish to grayish color, showing grouped trough cross bedding, interpreted as channel filling deposits. These facies associations correspond to part of a fluvial depositional system, with predominantly fine-grained rocks, and small lakes in the floodplain, in an oxidizing environment and marked by seasonality. These field data, reinforced by grain size and petrographic analyses showed that, contrary to previous descriptions, the Itapecuru Formation consists mainly of mudrocks. This research also shows that the medium to coarse sandstones and conglomerates, previously included in the Itapecuru Formation, belong to the underlying Grajaú Formation.
本研究确定了巴西东北部帕纳伊巴盆地阿尔比阶下-中统伊塔佩库鲁组的新层型。在马拉尼昂州伊塔佩库鲁-米林市附近伊塔佩库鲁河右岸的这一沉积序列中，识别出了三个岩相组合。第一个是粉质粘土岩，含一些极细砂，具云母，呈红色，薄层状，呈板状，被解释为远端位置的泛滥平原细粒沉积物。第二个是粘土质粉砂岩，呈红色，具波状交错层理、极薄的交错层理杂砂岩透镜体，渐变至薄的平行层理，呈明显的板状层，被解释为连续决口事件中的决口扇和分流河道沉积物。第三个是细粒石英杂砂岩，呈红色至灰色，具成群的槽状交错层理，被解释为河道充填沉积物。这些岩相组合对应于一个河流沉积体系的一部分，以细粒岩石为主，泛滥平原有小湖泊，处于氧化环境且具有季节性特征。这些野外数据，经粒度和岩石学分析强化后表明，与先前描述相反，伊塔佩库鲁组主要由泥岩组成。这项研究还表明，先前包含在伊塔佩库鲁组中的中粗砂岩和砾岩属于下伏的格拉若组。
