Depositional Systems andCyclic Sequences of Pink Yipu Formation Longmenshan Area

Zheng Rongcai; Liu Wenjun; Li Xianghui; Weng Hongfen; Chen Yuanren

The study area lies in the Longmenshan area on the western margin of the Yangtz Plate, where the Ping Yipu Formation, Lower Devonian are well exposed. Four depositional systems and three third-or-der T-R cyclic sequences have been distinguished from the formation. In this region, the paleo-geographic pattern along the ancient continental margin of the Yangtz Plate was formed of the transi-tional faoies zone alternating with littoral fades zone. In the vertical sections, the third-order cyclic se-quences were made of different depositional systems sparately in the two facies zones, which the se-quences of foreshore (or tidal)estuary inner-outer shelf delta were developed in the transitional fa-cies zone, and the other sequences of foreshore nearshorer inner shelf(yes or no) foreshore-backshore were developed in the littoral facies zone. The sedimentary evolutional characteristics of thetwo facies zones were different and controlled by different geological conditions, including tectonic activ-ity intensity, rate of tectonic descent and deposition, paleogeographic seat and world sealev el chang e sdining Early Devonian. Because of the world sea-level changes was the main and primary factor to con-trol the evolution of the three cyclic sequences in the formation, the third-order T-R cyclic sequence in the two sedimentary facies zones were synchronous and may be correlated with other oreas, such as theSouth China and the world. According to the evidences above mention, two cyclic sequence models ofancient continental margin of the Yangtz Plate have been advenced in this paper, which were of the ac-tiv e o r relatively stable tectonic descent setting.

Chengdu Institute of Technology, Chengdu, 610059

Received Date: 1996-07-01

Publish Date: 1997-09-10

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Abstract: The study area lies in the Longmenshan area on the western margin of the Yangtz Plate, where the Ping Yipu Formation, Lower Devonian are well exposed. Four depositional systems and three third-or-der T-R cyclic sequences have been distinguished from the formation. In this region, the paleo-geographic pattern along the ancient continental margin of the Yangtz Plate was formed of the transi-tional faoies zone alternating with littoral fades zone. In the vertical sections, the third-order cyclic se-quences were made of different depositional systems sparately in the two facies zones, which the se-quences of foreshore (or tidal)estuary inner-outer shelf delta were developed in the transitional fa-cies zone, and the other sequences of foreshore nearshorer inner shelf(yes or no) foreshore-backshore were developed in the littoral facies zone. The sedimentary evolutional characteristics of thetwo facies zones were different and controlled by different geological conditions, including tectonic activ-ity intensity, rate of tectonic descent and deposition, paleogeographic seat and world sealev el chang e sdining Early Devonian. Because of the world sea-level changes was the main and primary factor to con-trol the evolution of the three cyclic sequences in the formation, the third-order T-R cyclic sequence in the two sedimentary facies zones were synchronous and may be correlated with other oreas, such as theSouth China and the world. According to the evidences above mention, two cyclic sequence models ofancient continental margin of the Yangtz Plate have been advenced in this paper, which were of the ac-tiv e o r relatively stable tectonic descent setting.

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Reference (8)

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Depositional Systems andCyclic Sequences of Pink Yipu Formation Longmenshan Area

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