Chronostratigraphic Division of Yanchang Formation in Southern Ordos Basin and Response of Chang 73 to Episode Ⅰ of Indosinian Movement
Zhang Wen
Li Yuhong
Zhang Qiao
Chen Gaochao
Han Wei
Zhao Fenghua
Abstract：The main controversies of Triassic Yanchang Formation in Ordos basin are focused on the following two aspects:whether the age of Yanchang Formation covers the Middle Triassic;the confirmation of stratigraphic boundary between Middle and Upper Triassic,especially the attribution of the Chang 73.Therefore,the tuff samples of H2-2 and H2-1 were taken from the bottom and upper-middle Chang 73 in H2 Well respectively.The chronological (zircon LA-ICP-MS U-Pb) and petrographical features of H2-2 and the petrographical and geochemical features of H2-1 are researched.The results indicate that the zircons in H2-2 belong to magmatic zircons and their weighted average 206 pb/2a8 U ages are 239.8± 2.0 Ma,which is classified into Middle Triassic Ladinian age and corresponds to Episode Ⅰ of Indosinian Movement.The tuff of Chang 7a is derived from continental granite,in accordance with the tectonic setting of West Qinling region.Combined with previous results,it is thought that Yanchang Formation spans the Middle and Late Triassic epochs.Chang 73 belongs the 2nd member of Yanchang Formation and its top surface (i.e.top surface of Zhangjiatan shale) is the boundary between Middle and Upper Triassic.The magma activities in Episode Ⅰ of Indosinian Movement brought about the Chang 7 oil group,the tuff and other event sediments in Ordos basin.And the granitic magma activities in West Qinling region are coupled with the tuff of Chang 73 in distribution thickness,ages,magma features,and tectonic environment.It is likely that the tuff of Chang 7s derives from the magma activities in West Qinling region.
Keywords：Ordos basinYanchang FormationChang 73tuffchronostratigraphic divisionepisode Ⅰ of indosinian movementgeochemistry
Publication Date：2017-01-01
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：13( 1565-1577 )