Depositional Systems andCyclic Sequences of Pink Yipu Formation Longmenshan Area
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摘要: 本文首次从龙门山区下泥盆统平驿铺组中，划分出河口湾、三角洲、滨岸和陆棚四个沉积体系和三个Ⅲ级Ｔ－Ｒ旋回层序。平面上，四个沉积体系组成了扬子板块西侧的古大陆边缘由过渡相区和滨岸相区相间分布的古地理格局；垂向上，两相区中的三个Ⅲ级Ｔ—Ｒ旋回层序也由不同的沉积体系组成，旋回层序的演化虽然受构造差异沉降影响，但仍以Ⅲ级海平面升降变化为主要控制因素，并具同步演化规律，可分别代表古大陆边缘活动型和相对稳定型的两种Ⅲ级Ｔ—Ｒ旋回层序模式。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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