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Depositional influences on Re-Os systematics of Late Cretaceous–Eocene fluvio-deltaic coals and coaly mudstones, Taranaki Basin, New Zealand
International Journal of Coal Geology ( IF 5.6 ) Pub Date : 2021-02-01 , DOI: 10.1016/j.coal.2020.103670
Enock K. Rotich , Monica R. Handler , Richard Sykes , David Selby , Sebastian Naeher

The factors controlling Re-Os systematics and potential for geochronology in organic-rich sedimentary rocks deposited in fully terrestrial to paralic environments are not well understood. Here we present Re-Os, bulk pyrolysis and sulfur data for coals and coaly mudstones from the Late Cretaceous Rakopi and North Cape, Paleocene Farewell and Eocene Mangahewa formations, Taranaki Basin, New Zealand, to investigate a range of depositional controls on the behaviour of Re and Os in coaly rocks. These rocks were deposited in various fluvial, estuarine and coastal plain environments, and exhibit varying degrees of marine influence, as indicated by total sulfur content, presence of dinoflagellate cysts, and other parameters. The Taranaki coaly rocks have low Re (0.1–1.3 ppb) and Os (14.2–66.2 ppt) concentrations, even for strongly marine-influenced, high‑sulfur samples. These low concentrations are similar to those reported for entirely terrestrial coals, but are up to two orders of magnitude lower than in marine-influenced coals from the Carboniferous Matewan coal seam, USA. Unlike the Taranaki coaly rocks and other coals analysed for Re and Os, the Matewan coal seam is directly overlain by a fully marine shale. This suggests that such juxtaposition of depositional environments may be required for enhanced Re and Os enrichment in coals, for example, through drowning of the precursor peat mires by Re- and Os-rich seawater during the deposition of the overlying marine shale.

The initial 187Os/188Os (Osi) compositions of the Taranaki coaly rocks show significant variation. Samples from the Rakopi Formation exhibit radiogenic Osi values (0.8–1.2), which is expected for coals deposited in fully terrestrial settings and which source Os from weathering of surrounding upper continental crust. In contrast, samples from the progressively younger North Cape, Farewell and Mangahewa formations exhibit significantly less radiogenic Osi values (0.3–0.5). We attribute this to variable levels of marine influence from moderately radiogenic contemporaneous seawater and a change in sediment source composition following eruption and weathering of nearby subaerial volcanoes between 78 and 72 Ma, after deposition of the Rakopi Formation. The 187Re/188Os vs 187Os/188Os relationship for coaly rocks from the Farewell Formation exhibits significant scatter without any linear trend, precluding Re-Os geochronology. This scatter appears to have resulted from a combination of heterogeneous Osi (0.1 units) and limited variations in 187Re/188Os (145 units). Improved Re-Os isochroneity is noted in strongly marine-influenced coaly rocks from the Mangahewa Formation, with the 187Re/188Os vs 187Os/188Os relationship yielding an isochron age of 28 ± 16 Ma. Although this age is imprecise, it is within uncertainty of the estimated biostratigraphic age (37 ± 1 Ma) of the rocks.



中文翻译:

沉积对新西兰塔拉纳基盆地晚白垩世-始新世河流-三角洲煤和煤质泥岩的Re-Os系统的影响

人们还不太了解控制Re-Os系统学的因素以及在完全陆地到顺带环境中沉积的富含有机物的沉积岩中的年代学潜力。在这里,我们介绍了新西兰塔拉纳基盆地晚白垩世拉科比和北开普,古新世告别和始新世Mangahewa地层的煤和煤质泥岩的Re-Os,整体热解和硫数据,以研究一系列控制沉积行为的方法。煤岩中的coal和O。这些岩石沉积在各种河流,河口和沿海平原环境中,并表现出不同程度的海洋影响,如总硫含量,正鞭毛囊肿的存在以及其他参数所示。塔拉纳基(Taranaki)煤岩的Re(0.1–1.3 ppb)和Os(14.2–66.2 ppt)浓度较低,即使在受到海洋强烈影响的情况下,高硫样品。这些低浓度与报道的整个陆地煤相似,但比美国石炭纪马特万煤层的海相影响煤低两个数量级。与Taranaki煤岩和其他针对Re和Os进行分析的煤不同,Matewan煤层直接被完整的海相页岩覆盖。这表明沉积环境的这种并置可能是增强煤中Re和Os富集所必需的,例如,通过在上覆的海相页岩的沉积过程中,富含Re和Os的海水淹没前泥炭泥潭。美国。与Taranaki煤岩和其他针对Re和Os进行分析的煤不同,Matewan煤层直接被完整的海相页岩覆盖。这表明沉积环境的这种并置可能是增强煤中Re和Os富集所必需的,例如,通过在上覆的海相页岩的沉积过程中,富含Re和Os的海水淹没前泥炭泥潭。美国。与Taranaki煤岩和其他针对Re和Os进行分析的煤不同,Matewan煤层直接被完整的海相页岩覆盖。这表明沉积环境的这种并置可能是增强煤中Re和Os富集所必需的,例如,通过在上覆的海相页岩的沉积过程中,富含Re和Os的海水淹没前泥炭泥潭。

塔拉纳基煤质岩石的最初187 Os / 188 Os(Os i)组成表现出明显的变化。Rakopi组的样品显示出放射源的Os i值(0.8-1.2),这对于沉积在整个陆地环境中的煤来说是预期的,并且其Os来自周围上陆壳的风化。与此相反,从积极进取的年轻北角,告别Mangahewa地层样品表现显著少放射O的值(0.3–0.5)。我们将其归因于中等放射性的同期海水对海洋的影响程度的变化,以及在拉科比组沉积之后,附近的空中火山喷发和风化之后,沉积物来源成分的变化,在78至72 Ma之间。告别组煤质岩的187 Re / 188 Os与187 Os / 188 Os的关系表现出明显的散射,没有任何线性趋势,这排除了Re-Os的年代学。这种散布似乎是由于异质Os i(0.1个单位)和187 Re / 188的有限变化共同造成的Os(145个单位)。在Mangahewa组受到强烈海相影响的煤质岩石中,Re-Os的等时性有所改善,其187 Re / 188 Os与187 Os / 188 Os的关系产生等时年龄为28±16 Ma。尽管这个年龄是不精确的,但是在岩石的估计生物地层年龄(37±1 Ma)内是不确定的。

更新日期:2021-02-01
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