PROTEROZOIC STRATA AND CHARACTERISTICS OF THE EVOLUTION OF THE SEDIMENTARY BASIN IN GUANGXI

The Precambrian in Guangxi may be divided into the middle proterozoic Jixian System（Sibao Group）, the upper Proterozoic Qingbaikou System（Danzhou Group） and the Sinian System. The ages of their formation are 1400-1000Ma, 1000-800Ma and 800-600Ma respectively. These systems （groups） may be further divided into eleven stratigraphical units（formations）. According to their features, ophiolites in various systems were formed in the extentional zones in the extentional zones in the stages when very intense subsidence took placein the seaimentary basins（marginal seas）of various periods. Their volcanic rocks are of in-situ eruptive-sedi-mentary origin. The process of formation of the sedimentary basin in each phase is as follows: in the zones of weakness of the crust on the continental margins, rifts expanded gradually and sank to form marginal seas and receive depostiss, and then the zones gradually uplifted and closed to form land or mountains. The evolution of the sedimentary basins of various phases is as follows（from old to young）: from west to east the subsidence center of the basin gradually shifted and the continental crust gradually accreted and thickened, thus changing from oceanic crust and transitional crust→transitional crust and continental→contine ntal crust.