The Geological Settings Of Potash-forming And Depositional Model Of Lanping-Simao Basin, Western Yunnan Province

Lanping-Simao basin from Western Yunnan Province is one of the most essential potash-forming basins in China. To sniff out the potash depositional potential of Lanping-Simao basin and fully understand the depositional mechanism of potash deposit and the salt-forming age, we have undertaken comprehensive analysis regarding regional tectonic evolution, paleogeographic and paleoclimatic conditions, distribution of salt deposits and geochemical characteristics of Br and Sr. Our results demonstrate that this basin hold favorable marine environment for potash-forming deposits and excellent exploration potential. This basin preserves three horizons of potash deposit which are distributed in Late-Triassic, Middle-late Jurassic and Paleocene strata respectively. The Late-Triassic and Middle-late Jurassic salt layers produced with in marine environment whilst Paleocene salt layer favors continental environment, more precisely, small lake basin. In addition, geochemical evidences from Br concentration as well as Sr isotope from Mengyejing show that salt components primarily derived from ancient seawater.Based on comparative analysis of salt deposits and geological settings within Lanping-Simao basin, we propose that Late Triassic and Jurassic era are major salt-forming period of Lanping-Simao basin and a thick layer of source salt was produced at the bottom of the basin.According to the characteristics of structural compression and horizontal distribution of salt layer in downhole, we deduce that salt layer had suffered differential loading and compression that originated from regional overthrust after era of salt-forming in Mengyejing potash deposits, which flow plastically to relatively shallow parts form deep parts, and thus enriched and mineralized in the strata of Paleocene Mengyejing formation (E1m) via intruding. From the analysis results of geophysical data of EH4 and MES in 2011, we deduce that there perhaps exists evaporite sediment within Mesozoic strata, which is roughly 2000-2130m at depth in the basin. Thus, we undertaken a "stratigraphic well" drilling project at Baozang county, Jiangcheng in July,2011, from which we developed a 67.82m thick salt layer(2254-2321.86m) in the MK-1# well. After analyzing the characteristics of drilling core and stratigraphic column of the mining area, we believe that the pathway of salt flowing do exist in the deep basin. In addition, this essay set up a depositional model of Mengyejing potash deposit, which is the salt deposited in the deep basin, migrated through tectonic compression, and finally placed at the shallow position. This model is of vital importance for further studying potassium forming conditions and direction of potassium hunting in western Yunnan Province. We suggest confidently that more attentions should be paid to the deep buried Mesozoic evaporate strata and structural conditions in the further exploration of potash in SW Yunnan.