The Cretaceous Sedimentary Environments of Changtu Sag and Its Implications for the Rapid Uplifting of Yan-liao Orogen

XU Zheng yu; LIN Ge; LIU Chi yang; MA Xiao chang; HOU Lu

Changtu sag is Iocated in the northeast of Yan-Liao orogen as an important part of SongIiao Basin. It is fiIIed with voIcanic and sedimentary rocks, which can be grouped into three superseguences：SS!（corresponding to the rifting stage）, SS "（fauIt-rifting stage）and SS#（depressionaI stage）. During the earIy rifting, the sag was fiIIed with a huge of overfIowing and expIoding voIcanics. In the earIier fauIt-rifting stage, there deveIoped the Iowstand systems tract in Iower Jiufotang Formation that totaIIy refIects sedimentary characteristics of a thinner mudstone fIow and the expansion systems tract at the top of Jiufotang Formation that corresponded to a deep-water environment, incIuding fan-deIta and haIf-deep-Iake and deep- Iake. During the Iater fauIt-rifting stage, the depth of water became shaIIower with the predominant sedimentary facies of aIIuviaI apron, fan-deIta, swamp and offshore and shaIIow Iake and so on. The Jiufotang Formation was mainIy composed of muIti-compositionaI congIomerates and sandstones, of which the detritaI composition was dominated by voIcanic cIastics, suggesting their derivation from the earIier erupted voIcanic seguences which was rapidIy unroofed during the upIifting of the orogen. The Shahai Formation was mainIy composed of congIomerates, sandstones, interbeddings between sandstones and mudstone. DetritaI anaIysis shows that at the upper Shahai Fm., there occurred some proportion of metamorphic and granitic components, indicating the increase in the amount of upIifting and unroofing of the entire orogen whiIst the Iowering of upIifting rate. As the sedimentation continued, the centre of sediment and depression moved northwards, refIecting that Yan-Iiao orogen was continuousIy upIifted and separated during Iate Cretaceous. The emergence of the three Iowstand system tracts and three rifting activities demanded that the orogen had undergone at Ieast three rapid upIifting episodes during Cretaceous.

1. Changsha Institute of Geotectonics, Chinese Academy of Sciences, Changsha 410013;

2. Department of Geology, Northwest Universty, Xi'an 710069;

3. Jianghan Institute of Petrolum Administration Bureau, Qianjiang Hubei 433124

Received Date: 2001-03-05

Rev Recd Date: 2001-05-09

Publish Date: 2001-12-10

Key words:

Abstract: Changtu sag is Iocated in the northeast of Yan-Liao orogen as an important part of SongIiao Basin. It is fiIIed with voIcanic and sedimentary rocks, which can be grouped into three superseguences：SS!（corresponding to the rifting stage）, SS "（fauIt-rifting stage）and SS#（depressionaI stage）. During the earIy rifting, the sag was fiIIed with a huge of overfIowing and expIoding voIcanics. In the earIier fauIt-rifting stage, there deveIoped the Iowstand systems tract in Iower Jiufotang Formation that totaIIy refIects sedimentary characteristics of a thinner mudstone fIow and the expansion systems tract at the top of Jiufotang Formation that corresponded to a deep-water environment, incIuding fan-deIta and haIf-deep-Iake and deep- Iake. During the Iater fauIt-rifting stage, the depth of water became shaIIower with the predominant sedimentary facies of aIIuviaI apron, fan-deIta, swamp and offshore and shaIIow Iake and so on. The Jiufotang Formation was mainIy composed of muIti-compositionaI congIomerates and sandstones, of which the detritaI composition was dominated by voIcanic cIastics, suggesting their derivation from the earIier erupted voIcanic seguences which was rapidIy unroofed during the upIifting of the orogen. The Shahai Formation was mainIy composed of congIomerates, sandstones, interbeddings between sandstones and mudstone. DetritaI anaIysis shows that at the upper Shahai Fm., there occurred some proportion of metamorphic and granitic components, indicating the increase in the amount of upIifting and unroofing of the entire orogen whiIst the Iowering of upIifting rate. As the sedimentation continued, the centre of sediment and depression moved northwards, refIecting that Yan-Iiao orogen was continuousIy upIifted and separated during Iate Cretaceous. The emergence of the three Iowstand system tracts and three rifting activities demanded that the orogen had undergone at Ieast three rapid upIifting episodes during Cretaceous.

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The Cretaceous Sedimentary Environments of Changtu Sag and Its Implications for the Rapid Uplifting of Yan-liao Orogen

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