Abstract Glacial strata are excellent stratigraphic markers in Paleozoic basins worldwide. The eastern Parnaiba Basin in northern Brazil is a highly favorable setting for studying episodes related to the Ordovician-Silurian glaciation. Previous studies in this region led to the recognition of unequivocally glaciogenic sediments associated with pre- and postglacial deposits, favoring the understanding of lower Paleozoic events that occurred in West Gondwana. Outcrop-based facies and stratigraphic analyses of the ~170 m-thick siliciclastic succession of the Serra Grande Group indicate fluvial, glacial, and coastal depositional systems. Seventeen sedimentary facies are grouped into seven facies associations (FAs): FA1, interpreted as intermediary sheet braided plain deposits that consist of tabular cross-bedded coarse-grained sandstones and conglomerates; FA2-FA4, which correspond to subaqueous glacial deposits comprising massive to stratified diamictites, cross-laminated sandstones, diamictites and organic matter-rich black shales with dumpstone and dropstone structures; FA5-FA6, constituting organic matter-rich black shales, fine- to medium-grained sandstones with sigmoidal cross-bedding and occurrences of the ichnotaxon Arthrophycus interpreted as delta front deposits; and FA7, composed of medium- to coarse-grained pebbly sandstones interpreted as channelized braided plain deposits. This interpretation partially confirms the previous paleoenvironmental interpretation discounting the presence of subaerial alluvial fan and outwash plain deposits for this unit. The sequence stratigraphy of the Serra Grande Group is refined mainly using the coherent interpretation of the key surfaces and comparing the systems tracts with the global sea-level curve, providing a more robust third-order sequential evolutionary model that includes three depositional sequences. The Middle Ordovician sheet braided deposits are the lowstand sediments of Sequence 1 truncated by an extensive unconformity that possibly removed the transgressive and highstand strata. This unconformity generated by glacial dynamics represents 23 Myr during the Late Ordovician to early Silurian. The second sequence started with the advance of ice sheets in marginal areas of the basin, causing the deposition of ablation diamicton during the lowstand. The postglacial transgression is marked by ice-rafted debris from icebergs deposited onto organic matter-rich mud in the shoreface-offshore zone. The maximum flooding during the middle to late Silurian was succeeded by the highstand phase marked by the progradation of deltaic deposits. Biological activity is marked by the presence of polychaetes that reworked the marine seafloor. The Early Devonian featured extensive sea-level fall and renewed continental drainage with the formation of a braided plain marking a lowstand phase of Sequence 3. These new interpretations allow the global correlation of the Serra Grande Group and provide an improved understanding of the role of ice sheets and postglacial transgressions that affected West Gondwana during the Ordovician-Silurian periods.

中文翻译：

---

巴西帕纳伊巴盆地东部志留纪冰川作用：古环境、层序地层学以及对西冈瓦纳河演化和古地理的见解

摘要 冰川地层是世界范围内古生代盆地中极好的地层标志。巴西北部的东部 Parnaiba 盆地是研究与奥陶纪-志留纪冰川作用相关的事件的非常有利的环境。先前在该地区的研究导致人们明确认识到与冰前和冰后沉积物相关的冰川沉积物，有利于了解发生在西冈瓦纳的下古生代事件。基于露头的相和约 170 米厚的 Serra Grande 群硅质碎屑层序的地层分析表明存在河流、冰川和海岸沉积系统。十七个沉积相被分为七个相组合（FA）：FA1，解释为由板状交错层状粗粒砂岩和砾岩组成的中间片状编织平原沉积物；FA2-FA4，对应于水下冰川沉积物，包括块状至层状混叠岩、交错叠层砂岩、混叠岩和富含有机质的黑色页岩，具有倾卸岩和落石结构；FA5-FA6，构成富含有机质的黑色页岩、具有 S 形交错层理的细粒至中粒砂岩，以及被解释为三角洲前缘沉积物的 ichnotaxon Arthrophycus；和 FA7，由中至粗粒卵石砂岩组成，解释为沟状辫状平原沉积物。这种解释部分证实了先前的古环境解释，该解释不考虑该单元存在地下冲积扇和冲刷平原沉积物。Serra Grande Group 的层序地层主要通过对关键表面的连贯解释和系统域与全球海平面曲线的比较进行细化，提供了一个更强大的包含三个沉积层序的三阶序贯演化模型。中奥陶统片状辫状沉积物是层序 1 的低位沉积物，被广泛的不整合面截断，可能去除了海侵和高位地层。这种由冰川动力学产生的不整合面代表晚奥陶世至早志留世期间的 23 Myr。第二个序列开始于盆地边缘区域冰盖的推进，导致低位期间消融直径的沉积。冰后海侵的标志是来自冰山的冰筏碎片沉积在岸面-近海区富含有机物的泥浆上。以三角洲沉积物进积为标志的高水位阶段接替了中晚期志留纪的最大洪水。生物活动的标志是多毛类动物的存在，它们改造了海洋海底。早泥盆世以广泛的海平面下降和更新的大陆排水为特征，形成辫状平原，标志着序列 3 的低位阶段。这些新的解释使大山群的全球相关性成为可能，并提供了对冰作用的更好理解在奥陶纪-志留纪时期影响西冈瓦纳的冰层和冰后海侵。以三角洲沉积物进积为标志的高水位阶段接替了中晚期志留纪的最大洪水。生物活动的标志是多毛类动物的存在，它们改造了海洋海底。早泥盆世以广泛的海平面下降和更新的大陆排水为特征，形成辫状平原，标志着序列 3 的低位阶段。这些新的解释使大山群的全球相关性成为可能，并提供了对冰作用的更好理解在奥陶纪-志留纪时期影响西冈瓦纳的冰层和冰后海侵。以三角洲沉积物进积为标志的高水位阶段接替了中晚期志留纪的最大洪水。生物活动的标志是多毛类动物的存在，它们改造了海洋海底。早泥盆世以广泛的海平面下降和更新的大陆排水为特征，形成辫状平原，标志着序列 3 的低位阶段。这些新的解释使大山群的全球相关性成为可能，并提供了对冰作用的更好理解在奥陶纪-志留纪时期影响西冈瓦纳的冰层和冰后海侵。