Depositional environment of Middle Triassic organic-rich shales in the Ordos Basin, Northwest China,Geological Journal

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Depositional environment of Middle Triassic organic-rich shales in the Ordos Basin, Northwest China
Geological Journal ( IF 1.4 ) Pub Date : 2021-07-21 , DOI: 10.1002/gj.4215
Xiangdong Zhao _{1,

2} , Wei Wang ₁ , Guwei Xie _{3,

4} , Songqi Pan ₅ , Edmund A. Jarzembowski _{1,

6} , Daran Zheng _{1,

7}

Affiliation

The Middle Triassic Tongchuan Formation in the Ordos Basin of northwestern China is a typical lacustrine deposit. A major component of this formation is thick layers of organic-rich shales that are probably a potential hydrocarbon source and preserve the earliest known Mesozoic-type lacustrine ecosystem. The exact depositional environment of the shales in the Tongchuan Formation, however, remains unknown. To address this question, we carried out high-resolution carbon (δ¹³C_org) and sulphur (δ³⁴S_py) isotope analysis as well as undertook total organic carbon (TOC)/pyrite contents and pyrite morphology investigation, and framboidal pyrite size measurements in shales of the Bawangzhuang section of the southern Ordos Basin. Remarkably high TOC (23 ± 9%) and pyrite (7 ± 3%) contents were obtained from the shales, which indicate a large amount of organic carbon and pyrite burial during shale deposition. Framboids are the dominant pyrite morphology in the pyrite crystals and show large and variable mean diameters (7.0 ± 1.7 μm to 14.3 ± 6.8 μm) across the section, indicating oxic–dysoxic bottom water during shale deposition. δ¹³C_org and δ³⁴S_py revealed narrow and less variable values, ranging from −31.8‰ to −28.1‰, and −4.1‰ to 4.9‰, respectively. The δ¹³C_org values suggest balanced and consistent carbon cycles. Integrated with pyrite content and morphological patterns, consistent δ³⁴S_py values probably demonstrate a relatively open environment for the formation of sedimentary pyrite, and thus a shallow chemocline that was quite close to the water-to-sediment interface during shale deposition. Overall, the organic-rich shales of the Tongchuan Formation were probably deposited under oxic–dysoxic bottom-water conditions. Shallow chemocline depth combined with moderately high sedimentation rate and high primary productivity may have played crucial roles in the deposition and formation of the organic-rich shales in the Tongchuan Formation. The shallow chemocline also facilitates the fossil preservation in a lacustrine environment.

中文翻译：

鄂尔多斯盆地中三叠统富有机质页岩沉积环境

鄂尔多斯盆地中三叠统铜川组是典型的湖相沉积。该地层的一个主要组成部分是厚层富含有机物的页岩，它们可能是潜在的碳氢化合物来源，并保存着已知最早的中生代型湖泊生态系统。然而，铜川组页岩的确切沉积环境仍然未知。为了解决这个问题，我们进行了高分辨率碳 (δ ¹³ C _org ) 和硫 (δ ³⁴ S _py) 同位素分析，并在鄂尔多斯盆地南部霸王庄段页岩中进行了总有机碳 (TOC)/黄铁矿含量和黄铁矿形态调查，以及叶状黄铁矿尺寸测量。从页岩中获得了显着高的 TOC (23 ± 9%) 和黄铁矿 (7 ± 3%) 含量，这表明页岩沉积过程中埋藏了大量的有机碳和黄铁矿。Framboids 是黄铁矿晶体中的主要黄铁矿形态，并且在整个截面上显示出大且可变的平均直径（7.0 ± 1.7 μm 至 14.3 ± 6.8 μm），表明页岩沉积过程中底部水为含氧-缺氧。δ ¹³ C _org和 δ ³⁴ S _py揭示了狭窄且变化较小的值，范围分别为 -31.8‰ 至 -28.1‰ 和 -4.1‰ 至 4.9‰。δ ¹³ C _org值表明碳循环平衡且一致。结合黄铁矿含量和形态模式，δ ³⁴ S _py一致这些值可能表明沉积黄铁矿形成的环境相对开放，因此在页岩沉积过程中形成了非常靠近水-沉积物界面的浅层趋化层。总体而言，铜川组富有机质页岩可能沉积在含氧-缺氧底水条件下。较浅的趋化层深度结合较高的沉积速率和较高的初级生产力可能在铜川组富有机质页岩的沉积和形成中起关键作用。浅层趋化层也有利于湖泊环境中的化石保存。

更新日期：2021-09-02

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