The appearance of an oxygen-depleted condition on the Capitanian disphotic slope/basin in South China: Middle–Upper Permian stratigraphy at Chaotian in northern Sichuan
The appearance of an oxygen-depleted condition on the Capitanian disphotic slope/basin in South China: Middle–Upper Permian stratigraphy at Chaotian in northern Sichuan
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DOI：
10.1016/j.gloplacha.2012.01.002
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发表时间：
2013-06
影响因子：
3.9
通讯作者：
M. Saitoh;Y. Isozaki;Jian-xin Yao;Zhansheng Ji;Y. Ueno;N. Yoshida
中科院分区：
文献类型：
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作者：
M. Saitoh;Y. Isozaki;Jian-xin Yao;Zhansheng Ji;Y. Ueno;N. Yoshida
近年来，在世界各地的浅海剖面中，人们对与二叠纪末瓜达卢普世生物灭绝有关的全球环境变化进行了研究，但对沉积在较深的半透明带中的层序，以往的地层学研究还没有涉及。为了探讨这一时期弱光带的环境变化，对这一时期的岩石地层学和沉积环境的长期变化进行了分析。中国南方北方四川朝天区瓜达卢普统-乐平统（中-上二叠统）碳酸盐岩，厚150 m。上瓜达卢普统茅口组和下乐平统伍家坪组主要由透光陆棚相的生物碎屑灰岩组成，并含有丰富的藻类、珊瑚和纺锤虫等浅海生物化石。茅口组（Maokou Formation）11 m厚），因为它由薄层黑色泥岩/燧石组成，呈双辉斜坡/盆地相，含有丰富的放射虫和菊石。岩性和生物相的地层变化表明，在Capitanian两步海侵，随后在Guadalupian-Lopingian边界（G-LB）附近的大海退。此外，地层的生物扰动，总有机碳（TOC）的含量，并出现framboidal黄铁矿的变化表明，在朝天沉积环境的氧化还原急剧变化，根据新澄清的快速海平面波动。在与Capitanian海平面上升，沉积环境从一个氧化大陆架转移到一个缺氧的斜坡/盆地，并再次返回到一个氧化大陆架在接下来的大海退。四川北方卡庇塔尼亚分光斜坡/盆地的缺氧条件的出现尤其重要，因为它明显发生在瓜达卢佩末期灭绝事件之前。值得注意的是，贫氧海水首次出现在大陆边缘，大大早于著名的P-TB浅海缺氧现象。
The global environmental changes related to the end-Guadalupian (Permian) extinction have been recently studied in various shallow-marine sections in the world; however, no previous stratigraphic research focused on the sequence deposited in a relatively deep disphotic zone. In order to investigate the environmental changes in the disphotic zone during that interval, lithostratigraphy and secular changes in the sedimentary environment were analyzed for the ca. 150m thick Guadalupian–Lopingian (Middle–Upper Permian) carbonates at Chaotian in northern Sichuan, South China. The upper Guadalupian Maokou Formation and the Lower Lopingian Wujiaping Formation are mostly composed of bioclastic limestone of a euphotic shelf facies and contain abundant shallow marine fossils, such as algae, corals, and fusulines. The topmost Maokou Formation (ca. 11m thick) is unique in this section because it is composed of thinly bedded black mudstone/chert of a disphotic slope/basin facies with abundant radiolarians and ammonoids. The stratigraphic changes in litho- and bio-facies suggest a two-stepped transgression in the Capitanian followed by a great regression around the Guadalupian–Lopingian boundary (G–LB). Moreover, the stratigraphic changes in bioturbation, total organic carbon (TOC) content, and occurrence of framboidal pyrite suggest that the redox of the sedimentary environment at Chaotian changed drastically in accordance with newly clarified rapid sea-level fluctuations. In association with the Capitanian sea-level rise, the sedimentary environment shifted from an oxic shelf to an oxygen-depleted slope/basin, and again returned to an oxic shelf during the following great regression. The appearance of the oxygen-depleted condition on the Capitanian disphotic slope/basin in northern Sichuan is particularly important because it occurred clearly before the end-Guadalupian extinction event. It is also noteworthy that the oxygen-depleted seawater appeared for the first time on a continental margin significantly before the well-known P–TB shallow-marine anoxia.