Age and Paleoenvironmental Significance of the Frazer Beach Member—A New Lithostratigraphic Unit Overlying the End-Permian Extinction Horizon in the Sydney Basin, Australia
Age and Paleoenvironmental Significance of the Frazer Beach Member—A New Lithostratigraphic Unit Overlying the End-Permian Extinction Horizon in the Sydney Basin, Australia
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DOI：
10.3389/feart.2020.600976
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发表时间：
2021-01
期刊：
影响因子：
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通讯作者：
S. McLoughlin;R. Nicoll;J. Crowley;V. Vajda;C. Mays;C. Fielding;T. Frank;A. Wheeler;M. Bocking
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文献类型：
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作者：
S. McLoughlin;R. Nicoll;J. Crowley;V. Vajda;C. Mays;C. Fielding;T. Frank;A. Wheeler;M. Bocking
新近确定的月亮岛海滩地层的弗雷泽海滩成员广泛存在于悉尼盆地的露头和探井中。这个薄层单位是在舌叶蕨属植物群灭绝(定义了陆地二叠纪末期物种灭绝事件)后立即沉积的。该单元位于大多数地方的二叠系最上层煤层上。一种独特的颗粒-微角砾岩层在该段的底部局部出现。该单元还包括深灰色到黑色的粉砂岩、页岩、泥岩，局部还包括细粒砂岩和凝灰岩的薄透镜体。该段代表纽卡斯尔煤系的最顶端单元，在等级上被Doorlong页岩覆盖，或与粗粒砂岩冲刷(不整合)接触，与煤岩悬崖砂岩、Munmorah砾岩和横向相当的单元的砾岩相接触。该成员的特征是以高比例的降解木片、木炭、无定形有机物和真菌孢子为代表的孢粉学“死亡区”。在一些地区，单元上部的低多样性孢粉套代表了丰富的淡水藻类遗迹和陆生维管植物恢复植物群的初始阶段。根据单元内薄火山灰层的高精度化学磨损同位素稀释热电离质谱(CA-IDTIMS)测年，这些组合可作为Playfordiaspora crenulata Palynozone的参考，被解释为二叠纪最晚的时代。从弗雷泽海滩上段及其后续地层中发现的植物大型化石以鳞翅目(Peltaspermaceae)和狼尾目(Voltziales)为主，并辅之以侧柏类石松、蝶类植物和蕨类植物。稀疏的脊椎动物和无脊椎动物遗迹化石也出现在弗雷泽海滩成员或紧接在本单元上方的床上。弗雷泽海滩段部分与薄层富含有机质的泥岩相关，通常被称为“标志性泥岩”，覆盖了进一步向北的鲍文、加利利和库珀盆地的二叠系层序。这个厚度一般为5米的泥岩单元的广泛地理分布突出了冈瓦那东部在二叠纪末生物危机之后立即在冲积平原环境中发育了广泛的、短暂的浅湖系统和贫穷的生物群。
The newly defined Frazer Beach Member of the Moon Island Beach Formation is identified widely across the Sydney Basin in both outcrop and exploration wells. This thin unit was deposited immediately after extinction of the Glossopteris flora (defining the terrestrial end-Permian extinction event). The unit rests conformably on the uppermost Permian coal seam in most places. A distinctive granule-microbreccia bed is locally represented at the base of the member. The unit otherwise consists of dark gray to black siltstone, shale, mudstone and, locally, thin lenses of fine-grained sandstone and tuff. The member represents the topmost unit of the Newcastle Coal Measures and is overlain gradationally by the Dooralong Shale or with a scoured (disconformable) contact by coarse-grained sandstones to conglomerates of the Coal Cliff Sandstone, Munmorah Conglomerate and laterally equivalent units. The member is characterized by a palynological “dead zone” represented by a high proportion of degraded wood fragments, charcoal, amorphous organic matter and fungal spores. Abundant freshwater algal remains and the initial stages of a terrestrial vascular plant recovery flora are represented by low-diversity spore-pollen suites in the upper part of the unit in some areas. These assemblages are referable to the Playfordiaspora crenulata Palynozone interpreted as latest Permian in age on the basis of high precision Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-IDTIMS) dating of thin volcanic ash beds within and stratigraphically bracketing the unit. Plant macrofossils recovered from the upper Frazer Beach Member and immediately succeeding strata are dominated by Lepidopteris (Peltaspermaceae) and Voltziopsis (Voltziales) with subsidiary pleuromeian lycopsids, sphenophytes, and ferns. Sparse vertebrate and invertebrate ichnofossils are also represented in the Frazer Beach Member or in beds immediately overlying this unit. The Frazer Beach Member is correlative, in part, with a thin interval of organic-rich mudrocks, commonly known as the “marker mudstone” capping the Permian succession further to the north in the Bowen, Galilee and Cooper basins. The broad geographic distribution of this generally <5-m-thick mudrock unit highlights the development in eastern Gondwana of extensive, short-lived, shallow lacustrine systems with impoverished biotas in alluvial plain settings in the immediate aftermath of the end-Permian biotic crisis.