Origin of paleovalleys on the Rio Grande do Sul Shield (Brazil): Implications for the extent of late Paleozoic glaciation in west-central Gondwana
Origin of paleovalleys on the Rio Grande do Sul Shield (Brazil): Implications for the extent of late Paleozoic glaciation in west-central Gondwana
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DOI：
10.1016/j.palaeo.2018.04.013
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发表时间：
2019-10
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通讯作者：
Nicholas D. Fedorchuk;J. Isbell;N. Griffis;I. Montañez;F. Vesely;R. Iannuzzi;R. Mundil;Q. Yin;K. Pauls;Eduardo L.M. Rosa
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作者：
Nicholas D. Fedorchuk;J. Isbell;N. Griffis;I. Montañez;F. Vesely;R. Iannuzzi;R. Mundil;Q. Yin;K. Pauls;Eduardo L.M. Rosa
冈瓦纳中西部晚古生代冰中心的位置、寿命和地理范围仍不清楚。巴西最南端的里约热内卢Grande do Sul地盾上的古山谷以前被解释为由源自非洲的出口冰川雕刻而成的峡湾，这些冰川流入帕拉南<e:1>盆地（巴西）。在这项研究中，研究了沉积学、地层学和两个这样的古山谷（马里亚纳皮门特尔古山谷和莱<s:1>奥古山谷）的沉积物来源，以验证这样一个假设，即在石炭纪和二叠纪期间，今天纳米比亚上空的一个冰中心在巴西最南端干涸。与先前的发现相反，古山谷内的相组合与峡湾环境不一致，没有观察到明确的冰川作用证据。该相经历了从非冰川湖/河口环境到河流主导环境，再到受限制的海洋/河口环境的过渡。碎屑锆石结果显示，来自古山谷充填物的单一新元古代年龄（约800-550 Ma）与下伏火成岩和变质基底（Dom Feliciano带）的年龄相匹配，与含有丰富的更古老（中元古代、古元古代和太古代）锆石的非洲来源不一致。石炭-二叠纪期间新元古代基底构造的活化控制了古峡谷的形成及其充填物的沉积。这些古山谷中缺乏冰川作用的证据，这突出表明需要对假定的晚古生代冰川沉积物进行详细研究。这些结果支持了一种假设，即在里约热内卢大南盾（帕拉南<e:1>盆地的南缘）上建立良好的冰川沉积物可能是一个向北横跨乌拉圭的独立叶的产物，而不是一个从非洲向西流出的单一的巨大冰盖。
The location, longevity, and geographic extent of late Paleozoic ice centers in west-central Gondwana remain ambiguous. Paleovalleys on the Rio Grande do Sul Shield of southernmost Brazil have previously been interpreted as fjords carved by outlet glaciers that originated in Africa and emptied into the Paraná Basin (Brazil). In this study, the sedimentology, stratigraphy, and provenance of sediments infilling two such paleovalleys (the Mariana Pimentel and Leão paleovalleys) were examined in order to test the hypothesis that an ice center over present day Namibia drained across southernmost Brazil during the Carboniferous and Permian. Contrary to previous findings, the facies assemblage from within the paleovalleys is inconsistent with a fjord setting and no clear evidence for glaciation was observed. The facies show a transition from a non-glacial lacustrine/estuarine environment, to a fluvial-dominated setting, and finally to a restricted marine/estuarine environment. Detrital zircon results present a single population of Neoproterozoic ages (c. 800–550 Ma) from the paleovalley fill that matches the ages of underlying igneous and metamorphic basement (Dom Feliciano Belt) and is incongruent with African sources that contain abundant older (Mesoproterozoic, Paleoproterozoic, and Archean) zircons. Furthermore, results suggest that the formation of the paleovalleys and the deposition of their fill were controlled by the reactivation of Neoproterozoic basement structures during the Carboniferous and Permian. The lack of evidence for glaciation in these paleovalleys highlights the need for detailed studies of supposed late Paleozoic glacial deposits. These results are supportive of the hypothesis that well-established glacial sediments on the Rio Grande do Sul Shield (southern margin of the Paraná Basin) may be the product of a separate lobe extending north across Uruguay, rather than a single, massive ice sheet draining west from Africa.