The influence of tectono-volcanism on the depositional environment of the Early Cretaceous Jiufotang Formation in the western Liaoning—A case study of the Yanshan scientific drilling project borehole(YSDP-1)
The Jiufotang Formation in western Liaoning consists of organic-rich oil shales and hosts severa well-preserved Jehol fossils.The depositional environment was affected by tectono-volcanic events caused by destruction of the North China Craton.In this study,we analyzed the geochemical data of the sedimentary rocks of the Jiufotang Formation in the YSDP-1 borehole of the Yanshan Scientific Drilling Project.We used geochemical indicators to distinguish different lithologies,determine the provenance and tectonic backgrounds of the different lithologies,and clarify the effects of different volcanic materials on depositional environments.We found that the sedimentary rocks of the Jiufotang Formation in the core contain fine-grained sedimentary rocks deposited in the water body and volcanic clastic rocks and tuffs formed by the input of volcanic materials.The provenance results showed that both rock types had felsic igneous rock sources and a tectonic setting characterized by the superposition of an active continental margin and island-arc tectonic backgrounds.These results indicate that the two lithologies originated from volcanism-sedimentation in the same tectonic setting,which was mainly controlled by subduction of the Paleo-Pacific plate.These volcanic materials have a complex impact on depositional environments.The volcanic debris flows allow a large amount of dissolved oxygen to enter deep water,leading to the oxidation of the water body,and a number of terrestrial debris dilute the organic matter.Each input of volcanic debris flows represents the initiation of tectonic extension activities and the deepening of basins.The volcanic ash input leads to the eutrophication of the water body and the formation of a persistent anoxic condition in deep water,resulting in the formation of organic-rich oil shale,which in turn is favorable for the preservation of Jehol fossils.
tectono-volcanismJiufotang Formationprovenance tracedepositional environmentdestruction of the North China Craton
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