Were Ediacaran siliciclastics of South Australia coastal or deep marine?
Were Ediacaran siliciclastics of South Australia coastal or deep marine?
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DOI：
10.1111/j.1365-3091.2011.01302.x
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发表时间：
2012-06
期刊：
影响因子：
3.5
通讯作者：
G. Retallack
中科院分区：
文献类型：
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作者：
G. Retallack
南澳晚新元古代罗恩斯利石英岩Ediacara段被许多学者认为是风成、河流、潮间和深海沉积。古土壤不会被认为是深海解释，但一些古土壤应该是明显的风成-河流-潮间带的解释，这是第一个研究检查埃迪卡拉成员在岩相学和地球化学尺度适合识别潜在的古土壤。识别新元古代岩石中的古土壤和洪泛平原相是一个挑战，因为这些岩石太古老，无法诊断非海相化石，如根迹。Ediacara段红色粉砂岩和白色砂岩的不同厚度与横向持久的上覆和下伏灰色页岩和石灰岩以及疑源类、叠层石和其他海洋化石不同。砂岩呈槽状交错层状，充填古河谷。红色粉砂岩具有黄土特征的分选不良、高棱角、粉砂大小的颗粒。对特定的桑迪和粉砂层进行取样，以进行详细的岩相学和地球化学研究，因为它们包括干燥裂缝、砂晶、冰裂缝、碳酸盐结核和软沉积物变形，如古土壤。这些岩层中的化学和粒度变化揭示了表层粘土的形成和土壤中长石的氧化。岩相学研究还揭示了这些古土壤的丝状结构相媲美的微生物绳的生物土壤结皮的surprise中断。这一系列古土壤特征可用于识别新元古代硅质岩系中的古土壤。
The Late Neoproterozoic Ediacara Member of the Rawnsley Quartzite in South Australia has been considered aeolian, fluvial, intertidal and deep marine by various authors. Palaeosols would not be expected for the deep marine interpretation, but some palaeosols should be evident for the aeolian–fluvial–intertidal interpretations, and this is the first study to examine the Ediacara Member at a petrographic and geochemical scale appropriate to recognize potential palaeosols. Recognition of palaeosols and floodplain facies in Neoproterozoic rocks is a challenge because such rocks are too ancient for diagnostic non‐marine fossils such as root traces. The varied thickness of Ediacara Member red siltstones and white sandstones is distinct from laterally persistent overlying and underlying grey shales and limestones with acritarchs, stromatolites and other marine fossils. The sandstones are trough cross‐bedded and fill palaeovalleys. The red siltstones have poorly sorted, highly angular, silt‐size grains characteristic of loess. Particular sandy and silty beds were sampled for detailed petrographic and geochemical studies, because they include desiccation cracks, sand crystals, ice cracks, carbonate nodules and soft‐sediment deformation like those of palaeosols. Chemical and grain‐size variations within these beds reveal surficial clay formation and oxidation from feldspar as in soils. Petrographic studies also revealed surficial disruption of these palaeosols by filamentous structures comparable with microbial ropes of biological soil crusts. This array of palaeosol features may be of use for recognizing palaeosols in other Neoproterozoic siliciclastic sequences.