龙门山平驿铺组沉积体系及旋回层序研究

Depositional Systems and Cyclic Sequences of Ping Yipu Formation Longmenshan Area

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。

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 order T-R cyclic sequences have been distinguished from the formation. In this region, the paleogeographic pattern along the ancient continental margin of the Yangtz Plate, was formed of the transitional facies zone alternating with littoral facies zone. In the vertical sections, the third order cyclic sequences were made of different depositional systerms sparately in the two facies zones, which the sequences of foreshore (or tidal)→estuary→inner outer shelf→delta were developed in the transitional facies zone, and the other sequences of foreshore→nearshore→inner shelf (yes or no)→foreshorebackshore were developed in the littoral facies zone. The sedimentary evolutional characteristics of the two facies zones were different and controlled by different geological conditions, including tectonic activity intensity, rate of tectonic descent and deposition, paleogeographic seat and world sealevel changes during Early Devonian. Becouse of the world sea level changes was the main and primary factor to control 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 the South China and the world. According to the evidences above mention, two cyclic sequence models of ancient continental margin of the Yangtz Plate have been advenced in this paper, which were of the active or relatively stable tectonic descent setting.