Abstract:

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.

Key words: Xigeda Formation, fluviallacustrine deposit, contemporaneous structure, geological structure, landslide damming, landslidedammed lake, sag pond, uplift of QinghaiTibet plateau