龙门山地区泥盆纪层序地层及海平面变化——以四川北川桂溪剖面为例

志留纪末期的加里东运动使扬子板块与华夏板块基本上拼接联合为统一的华南板块,由于古特提斯洋北支向东扩张,泥盆纪时龙门山地区形成华南板块西北缘的被动大陆边缘.在这个比较稳定的构造背景下,以桂溪剖面为代表的稳定类型泥盆系大致可以划分为18个三级沉积层序(DS1-DS18),进一步可以归并为2个二级构造层序,对应于两次海侵高潮.层序地层划分表明龙门山地区在泥盆纪经历了一个从海侵作用初期的海侵碎屑岩沉积(DS1-DS9)到高水位碎屑岩和碳酸盐岩混合沉积(DS10-DS12),再到海退期清水碳酸盐岩沉积(DS13-DS18)的有规律的沉积演变过程,从而形成了一个复杂而有序的层序地层序列.这种沉积演变规律及海平面变化与隔陆相望的滇黔桂地区的泥盆系具有相似性.但龙门山地区的泥盆系比滇黔桂地区发育齐全,厚度更大,所以后者的泥盆系只能划分出13个三级层序.

Devonian Sequence Stratigraphy and Sea-Level Change in Longmenshan Mountains -An Example from the Guixi Section in Beichuan County, Sichuan Province

The Caledonian orogeny that occurred in the transitional period from Silurian to Devonian led to convergence of the Yangtze Plate and the Huaxia Plate and formed the uniform South China Plate. Because of the eastward expanding of the northern branch of the Paleotethys,during Devonian,the Longmenshan area had become the passive margin of the northwestern South China Plate. On this relatively tectonically stable background,the stable Devonian Guixi section could be approximately subdivided into 18 third-order sedimentary sequences (DS1-DS18). The sequence stratigraphic subdivision indicates the Longmenshan area underwent an evolutionary process of sedimentation from siliciclastic sediments of transgression (DS1-DS9) to the mixed carbonate-siliciclastic sediments of highstand period (DS10-DS12),and then to the sedimentation of ramp carbonate of regression period (DS13-DS18),which therefore formed a complex but regular sequence stratigraphic succession. For the Guixi section,18 third-order sequences can further be grouped into 2 second-order tectonic sequences corresponding to two highstands of transgression. The characteristics of the sedimentary succession and sea-level changes could be correlated with the Devonian Dianqiangui area. But the Devonian Guixi section is much thicker and the number of sequences is more than that in the Dianqiangui area in which 13 third-order sequences can only be discerned. Variable facies fabrics of different stages are considered to be in response to the different patterns of sea-level changes and the paleogeographic backgrounds.