THE STUDY OF THE SEQUENCE STRATIGRAPHY OF THE DONGCHUAN GROUP DURING MESO PROTEROZOIC IN HUILI-HUIDONG AND DONGCHUAN AREAS

The Meso Proterozoic sedimentary basin in Huili Huidong and its adjacent areas is an inheritied rift basin, where the Dongchuan Group was deposited. The Dongchuan Group can be divided from bottom to top,into the Yinming, Louxue, Heishan and Qinglongshan Formations. Ten sedimentary facies in the Dongchuan Group have been distinguished in terms of palimpsest sedimentary structures, lithologic features and depositional cycles.They are the alluvial fan facies, littoral shallow sea facies, shallow shelf facies, tidal flat estuary facies, carbonate platform facies, shoal facies, biohermal facies, slope facies, shallow sea basin facies and abyssal basin facies. According to the features of sequence boundary characterized by the paleo weathering crust and paleo karst as well as condensed section consisted of the carbonaceous shale, carbonaceous siliceous rocks and siliceous rocks, 12 depositional sequences have been eastablished, 2 depotional sequences in Yinming Formation, 2 depotional sequences in Louxue Formation,4 depotional sequences in Heishan Formation and 4 depotional sequences in Qinglongshan Formation. The features of depositional sequences showed that: 1)the study area has experienced two periods of sedimentary evolution characterized by the variation from the terrigenous to carbonate sedimentation; 2) the copper bearing strata is at the bottom of the Louxue Formation in Dongchuan, Tong′an and at the top in Yimen area, reflecting the transgression from north to south;3) the extensional activity in study area during the Late Meso Proterozoic took place intensively toward south and in north direction;4) the Dongchuan tectonic movement and Sanfengkou movement of Manyingou movement are problematical. In fact, these structural boundaries named by former geologists were formed by the paleo exposure and paleo solution during the sea level falling, characterzied by the paleo weathering crust and paleo karst.