First discovery and characterizations of late Cretaceous seep carbonates from Xigaze in Tibet, China
First discovery and characterizations of late Cretaceous seep carbonates from Xigaze in Tibet, China
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DOI：
10.1007/s11434-012-5434-2
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发表时间：
2012-09
影响因子：
--
通讯作者：
Hongpeng Tong;Duofu Chen
中科院分区：
文献类型：
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作者：
Hongpeng Tong;Duofu Chen
油气渗漏广泛存在于大陆边缘，由于其与极端环境中的天然气水合物、强温室气体甲烷和生物资源的密切关系而日益受到关注。根据渗漏碳酸盐岩或与渗漏有关的化石化合成组合，世界范围内泥盆纪至第四纪地层中已识别出古油气渗漏。然而，在中国大陆的古地层中却很少发现与渗漏有关的矿床。本文报道了在西藏日喀则地区首次发现的古渗漏存款，特别是晚白垩世渗漏碳酸盐岩。日喀则弧前盆地上白垩统浊积岩地层中包裹着以结核状产出的冷泉碳酸盐岩。这些碳酸盐岩由自生碳酸盐（平均56.2%）、碎屑石英和长石（平均27.3%）和粘土矿物（蒙脱石、伊利石和蒙皂石，平均16.5%）组成。在现代渗漏碳酸盐岩中常见的有凝块泥晶岩、泥质岩和框状黄铁矿。碳酸盐岩的δ 13 C值为负值，介于−27.7‰至−4.0‰（V-PDB）之间，表明热成因甲烷是主要碳源。通过消除La影响修正的Ce/Ce* 值显示没有真实的Ce异常，表明碳酸盐主要在弱还原环境中沉淀。这些特征为日喀则弧前盆地晚白垩世海底发育油气渗漏提供了明确的证据。
Hydrocarbon seeps, widely occurring in continental margins, have become increasingly focused owing to their close relationships with gas hydrates, strong greenhouse gas methane, and biological resources in extreme environments. Ancient hydrocarbon seeps have already been recognized from Devonian to Quaternary strata worldwide based on seep carbonates or seep-related fossil chemosynthetic assemblages. However, seep-related deposits are rarely found from ancient strata in the mainland China. Here, we report the first discovery of an ancient seep deposit, specifically late Cretaceous seep carbonates from Xigaze in Tibet, China. Xigaze seep carbonates, occurring as nodules, are enclosed in upper Cretaceous turbidite strata in Xigaze forearc basin. These carbonates are composed of authigenic carbonate (56.2% on average), clastic quartz and feldspar (27.3% on average), and clay minerals (chlorite, illite and smectite, 16.5% on average). Clotted micrites, peloids and framboid pyrites are frequently observed, all of which are common in modern seep carbonates. The carbonates have negativeδ13C values varying from −27.7‰ to −4.0‰(V-PDB), suggesting that thermogenic methane is the primary carbon source. Ce/Ce* values revised by eliminating La effects show no real Ce anomaly, indicating the carbonates were primarily precipitated in a weak reducing environment. Overall, these features provide unequivocal evidences that the seafloor of Xigaze forearc basin developed hydrocarbon seeps in late Cretaceous.