昔格达组地层研究中需要注意的若干关键问题

Some problems in the study of the genesis of Xigeda Formation

昔格达组与下伏、上覆冲积卵石层之间的垂向叠置关系具有重要的沉积环境指示意义。主要由水平纹层状杂色细粒沉积构成的昔格达组常夹有数十厘米厚的冲积砂卵石层,而粉细砂层中还常见交错纹理和交错层理,成因研究应充分考虑其冲湖积特征和湖底底流;不同地区昔格达组细粒沉积的矿物成分和卵石层的岩石类型各异,基本无可比性。有些地区昔格达组地层中发育有同生变形构造,有些还见有较多的地质构造形迹。昔格达组地层露头区的平面离散性很大,但对现今河流体系高度依赖,要么追踪不同序次干流及支流形成树枝状结构,要么仅沿干流分布、不受支流影响,形成棒状结构,表明昔格达组地层形成于现今河流之后,而且与河流密切相关;垂向上,不同昔格达组地层露头区之间的最大高差达2 290m,不同露头之间常存在标高突变。昔格达组地层的沉积环境主要是不同序次河流岸坡失稳形成的滑坡坝堰塞湖,其次为沿断裂带发育的河流局部下陷演变而来的过水断陷湖泊。

The vertical superimposition relationship between the Xigeda Formation and its overlying,underlying fluvial gravel layers is of great significance for indicating sedimentary environment.The Xigeda Formation,which is mainly made of horizontally laminated mottled fine-grained deposits often containing sandwiches of tens of centimeters of fluvial gravel layers,and the silt layers frequently contain cross-bedding and cross-lamination.The fluvial-lacustrine characteristics and the bottom currents in Xigeda lakes should be taken into account in the study of the genesis of the formation.The mineral components of Xigeda fine-grained deposits and the rock types of gravel layers within Xigeda Formation change with the geographic locations of outcrops.Contemporaneous deformation structures and faults develop in some Xigeda Formation distribution regions.Xigeda outcrops are scattered at the south-east margin of Qinghai-Tibet plateau,but highly depend on the current river systems,either tracking some master stream and its tributaries to form branchy structures or only distributing along the master stream to form rod-like structures,which suggests that Xigeda Formation deposited after current river system had arisen and has a genetic relation with the river systems.The highest height difference among Xigeda outcrops is up to 2290 m and there are often outcrop elevation jumps among different Xigeda Formation distribution regions.Xigeda Formation deposited primarily in landslide-dammed lakes;secondly in sag ponds derived from local subsidence of rivers,which developed along the fault zone.