四川盆地北缘下寒武统页岩生物标志化合物特征及其地质意义

Characteristics of biomarker compounds and their implications for Lower Cambrian black shale on the northern margin of Sichuan Basin

针对四川盆地绵阳—长宁拉张槽北段米仓山地区下寒武统筇竹寺组黑色页岩开展了系统的有机地球化学分析,以正构烷烃、类异戊二烯烃、萜类化合物、甾类化合物等生物标志化合物为重点,探究绵阳—长宁拉张槽的成因机理及沉积环境。扬子板块北缘下寒武统黑色页岩中的有机质主要来自低等浮游生物与细菌的共同贡献;盐度一定的情况下,分层的海水不仅为水体表层浮游生物的繁殖发育提供良好环境,还给厌氧的喜盐细菌的繁殖提供了有利条件;筇竹寺组黑色页岩中检测出的4-甲基甾烷为典型热液喷口微生物的分子化石,其产出为绵阳—长宁拉张槽的拉张成因提供了证据。据此构建了该区古海洋模型,甲烷氧化菌、硫酸盐还原菌等热液喷口微生物,不仅为有机质提供了生物来源,还促进了浅部水体藻类的繁殖,提高了古生产力。

The formation of the sag in the central Sichuan Basin may have influenced the development of major reservoir and source rock formations, such as the Lower Cambrian Qiongzhusi (Niutitang) Formation. This study analyzed the organic geochemistry, including n -alkanes and isoprenoids, steranes, terpanes (hopanes series), of organic matter in the Qiongzhusi Formation on the northern margin of the Yangtze Plate to investigate the depositional environment and formation mechanism of the intracratonic sag. The results indicate that the organic matter in the study area was sourced from eukaryotic phytoplankton and bacteria. The presence of gammacerane and low pristane/phytane ratios indicates that the top of the water column contained enough oxygen to provide favorable conditions for eukaryotic phytoplankton to thrive. The presence of 4-methyl steranes, a typical molecular indicator of hydrothermal vent microbes, in the Qiongzhusi Formation suggests anaerobic organisms thrived at the bottom of the water column as a result of extension during the formation of the intracratonic sag. A paleoceanographic model of the study area was constructed. Both methane oxidizing and sulfate reducing bacteria along with other hydrothermal vent microbes may not only serve as sources of organic matter, but also promote the propagation of algae in shallow water to improve organic productivity in this ancient environment.