Abstract: The neotectonic movement in the middle reaches of the Jinsha River is active and the earthquakes occur frequently. Lacustrine sediments are commonly distributed on both sides of the river with stable sedimentary environment, good horizontal continuity and relatively developed stratification, which are good carriers for recording paleo-seismic events. In this study, a large number of soft sedimentary deformation structures are found in the riverside lacustrine sediments in the Taoyuan Town area in the middle reaches of Jinsha River, with strong deformation and large scale. We focus on the comprehensive analysis of four soft-sedimentary deformation profiles. In which the profiled strata are mainly medium-fine sand and clay. And the soft sedimentary deformation structures mainly include sand liquefaction, rootless faults, clay lumps and folds.
Causes analysis: In the profiles of soft sedimentary deformation structures, there are medium and fine sand layers whose thickness is from thick to super thick. Sedimentary bedding has not been observed in the sand layer; and a large number of clay debris or lumps are involved in the sand layer, which are often filled between the adjacent clay lumps; and there are quicksand channels in the sand layer. All the features indicate that the sand layer in the study profiles has been liquefied. In the study profile, we found that the soft sedimentary deformation structure has the following characteristics: The faults found in the study profile extend downward and terminate in the lower liquefied sand layer and a large number of clay lumps. There are clay lumps in the place where the clay fold structure develops, and a large number of liquefied sand bodies are filled between the fold structures. The deformation structures in the profiles are not contrastive in terms of extension, chaotic deformation characteristics and obvious stress direction. Based on the characteristics of sand liquefaction and clay deformation in the above profile, it is inferred that the deformation structure in the profile is mainly due to sand liquefaction. The liquefaction strength of sand layer determines the deformation degree of clay layer.
Trigger factors analysis: There are many factors that can trigger the liquefaction deformation of the unconsolidated sediment, such as flood, freeze-thaw, collapse and earthquake, which can cause the liquefaction deformation of the sediment under certain conditions. In this paper, the possible trigger factors are analyzed based on the combination of the structural characteristics of soft sedimentary deformation, sedimentary environment and geological background of the area. First the stratigraphic characteristics also reflect the hydrostatic sedimentary environment at that time. The soft sedimentary deformation on such a large scale could not be mainly caused by the disturbance of lake waves. The research profiles are located at a sheltered bay with weak hydrodynamics, and no alluvial strata have been found in the upper part of the soft sedimentary deformation stratum. Moreover, the soft sedimentary deformation structure caused by flooding is often a small-scale curly layered structure, which has a large difference with the deformation structure and scale in the study profiles. This suggests that alluvial and diluvial events are not the main triggering factors of the deformation. Although the landslide is likely to occur near the study area, no trace of bedrock landslide is found near the study profiles. Therefore, the invasion of bedrock landslide into the sedimentary layer cannot be the triggering factor. Moreover, the occurrence of lacustrine sedimentary layer is nearly horizontal, which is a relatively stable sedimentary state, and it is impossible to form such a large-scale slump structure due to its own gravity effect. And we don't find any sliding surface in the profiles. Therefore, the collapse is ruled out. According to the geological background and geological survey of the study area, this area does not have the conditions triggered by volcanism, glaciation and freeze-thaw. Because of the active neotectonic movement and frequent earthquakes in the study area, and seismic actions are the main trigger factors for liquefaction. So it is considered that seismic action may be the main trigger factor for the strong liquefaction deformation in the study area. According to the previous studies, the relationship between the soft sedimentary deformation structure, the liquefaction thickness and the seismic strength is discussed, the magnitude of this ancient seismic event probably reached 7 or higher.
There are sand layers in the section of “soft sedimentary deformation structure” caused by earthquake, the lower stratum is sand layer and the upper stratum is clay layer. The thickness and deformation strength of the lower sand layer determine the strength of the deformation structure of the overlying clay layer. The upper and lower surface of the sand layer are undulating, and there are clay lumps in the sand layer. The deformation structure of clay layer is complex and there is no obvious deformation rule.

Key words: Xigeda Formation, soft-sediment deformation, palaeoearthquake, middle reaches of the Jinsha River

摘要： 在一定的地质环境中, 地震可引起大范围的砂土液化和软沉积变形, 所造成的破坏可能远大于地震震动直接造成的破坏。 金沙江中游的永胜县涛源镇沿江分布有大面积的昔格达组湖相沉积物。 同时, 活动性强、 地震频发的程海断裂带正好从永胜县涛源镇一带通过。 在昔格达组未固结时, 可能发生过1次强烈的地震触发了昔格达组的变形。 基于研究剖面的变形特征分析认为: 地震触发的大规模软沉积变形构造剖面中需存在砂层, 下伏砂层、 上覆黏土层的 “二元结构”是地震触发软沉积变形构造的理想地层结构, 且下伏砂层的厚度和变形强度决定了上覆黏土层变形强度; 砂层中含有上覆黏土挤落的团块等非原层物质, 砂层的上、 下层面高低起伏不平; 黏土层中的主要变形构造有地层错位、 无根断层、 砂层挤入、 砂层管道、 黏土层复杂褶曲、 负载构造、 火焰构造和肿缩构造等; 变形构造样式复杂, 无明显规律, 且可形成大规模的强烈地层变形。

关键词: 昔格达组, 软沉积变形, 古地震, 金沙江中游