Reconstruction of karst palaeogeomorphology for the Ordovician Yingshan formation in the central Tarim basin

As part the central uplift of the Tarim basin, the Tazhong (central Tarim) II area is a low swell of the middle section of this uplift belt. In this area, before sedimentation of the Lianglitage formation, the strata of the Tumuxiuke, Yijianfang and Yingshan formations had experienced erosion of different degrees, leading to destruction of the original landforms. To delineate the karst paleomorphology of the top Yingshan formation is of great importance for modeling of karst reservoirs and forecasting favorable blocks in exploration. This work compares methods for paleogeomorphology reconstruction. The impression method can well describe micro morphology, but its resulting regional tectonic trends do not accord with the real ones. While the residual thickness and residual thickness trend surface method is able to build the palaeostructure at that time, but cannot depict the micro morphology. To solve this problem, this study proposes a combination approach of “residual thickness of trend surface and impression residuals” to recover the ancient karst landform. This new method not only facilitates the paleogeomorphology restoration of the Tazhong area, but also helps computer modeling. The research suggests that the Ordovician Yingshan formation of the Tazhong II area can be divided into three kinds of second-order geomorphic units， karst platform, karst basin, karst gentle slope, and gives the index system for subdivision of these landform units.