Most of karst landforms observed in Tibet plateau originated mainly from the Tertiary underground karst, which were exhumed out with the uplift of the plateau through a long period. The karst processes continued at least in the southern part of the plateau during Pleistocene interglaciations when the climate was relative more humid and warmer. However, some small-scale karst caves were formed by the melting water of the Pleistocene glaciers when climate was relative drier and cooler. The parallel karrens on limestone surface, which depths are not more than 20cm, stemmed from the corrosion in Holocene. The relative deposits of palaeokarst, i.e., red weathering crust, distributed in a remnant form on the main planation surface.

The speleothems found in relict caves reflected the underground cave circumstances of the Tertiary karst process, and twenty fission tracks data of the speleothem calcite demonstrated that the palaeokarst forming period, i.e., the main planation surface forming period, was during 19~7 Ma.B.P.

The relative deposits of the Tertiary palaeokarst, i.e., limestone residual red earth, distribute widely on the main planation surface of the plateau. We analyzed this red earth systematically of its chemical and clay mineral compositions. SiO₂, Al₂O₃ and Fe₂O₃ are the three main components of the red earth. The clay mineral compositions mainly belong to "ilite-kaolinite" type, a few samples are the "kolinite-illite" type. Judged by the silica-allumina ratio and the clay mineral composition, the residual red earth reflected a kind of preliminary laterization process. However, considering the vertical distribution of the chemical weathering degree in the weathering section, the red earth still reflected the humid and warm climate, i.e., a kind of tropical or subtropical climate.

Surface textures of quartz grains in red earth were studied with scanning electrical microscope (SEM). The eroded and deposited features on quartz grain were mainly chemically produced, the mechanically eroded features were secondary. All of those reflected the humid-tropical environment when the red weathering crust and the palaeokarst were formed. The mechanically eroded features are in the majority in surface textures of some samples taken place remarkable movement by various geomorphologic processes which were driven by the uplift of the plateau, those samples such as slope wash and alluvial deposit derived from red earth. As for the evolution pattern in time of the surface textures, much display that the chemically produced features were overlaid by mechanically ones. On the other hand, a few samples display that the mechanically produced features were overlaid by chemically ones. All of those reflect the property of polycycle and complex of the Tibetan environmental evolution since Ternary period.

Acknowledgement: This project was supported financially by the NSF (No. 49371011 and No.49901002).