川西类前陆盆地晚三叠世须家河期构造演化及层序充填样式

Tectonic evolution and sequence filling patterns in the western Sichuan foreland-like basin in the Upper Triassic Xujiahean period

晚三叠世须家河期.龙门山构造活动呈现明显的阶段性。须二段和须四段沉积期为龙门山逆冲推覆活动期,川西类前陆盆地沉积、沉降中心位于靠近盆缘冲断带一侧的前渊坳陷内,盆地两侧以储层发育和沉积极不对称为特征。在盆缘冲断带一侧,随着龙门山造山带构造活动的增强,下切剥蚀力也不断增强,低位体系域相对发育.而高位体系域则因后期暴露剥蚀而保存不完整;在前陆隆起斜坡带一侧,表现为稳定的缓慢隆升,下切侵蚀作用较弱。故低位体系域缺失或不发育,而高位体系域则因后期剥蚀较弱而相对发育。须三段和须五段沉积期为龙门山逆冲推覆休眠期.川西类前陆盆地的沉积、沉降中心向前陆隆起方向迁移,盆地两侧沉积的不对称性也逐渐减弱.以发育区域性盖层和生油层为特征。其层序充填样式与活动期相似。但造山带前缘因构造剥蚀相对减弱而使沉积层序.尤其是高位体系域保存相对完整。而前陆隆起区则主要因沉积物供应不足而使层序发育相对不完整.只保留最大洪泛面附近的沉积。

The tectonic activity in the Longmen Mountains exhibited notable phased changes in the Upper Triassic Xujiahean period. The deposition stages of the second and fourth members of the Xujiahe Formation coincided with the stages of thrusting in the Longmen Mountains. The depocenter and subsidence center of the western Sichuan foreland-like basin are located in the foredeep near the basin-margin thrust belt, and the reservoirs on both sides of the basin are well developed and their deposits are very asymmetric. As the downcutting erosion increased with enhancing tectonic activities in the Longmen Mountains orogen, the lowstand systems tract on the side of the basin-margin thrust is better developed, while the highstand systems tract is preserved incompletely because of later subaerial exposure and erosion. On the other hand, the side of the forebulge slope was marked by steady slow uplift and weak downcutting erosion, and so the lowstand systems tract is absent or not well developed and the highstand systems tract is better developed due to weaker erosion in the late stage. The deposition stages of the third and fifth members of the Xujiahe Formation coincided with the repose stage of thrusting in the Longmen Mountains. The depocenter and subsidence center of the western Sichuan foreland-like basin moved toward the forebulge;the deposits on both sides of the basin are less asymmetric, which are characterized by the development of regional caps and source rocks. The sequence filling patterns of the two stages are similar to those of the thrusting stages. However, their depositional sequences, especially the highstand systems tract, in the front of the orogen are completely preserved because of weaker tectonic erosion; whereas in the forebulge the sequences are relatively incomplete mainly owing to lack of supply of sediments and thus only the sediments near the maximum flooding surface are preserved.