Constructing and reworking of the marine proto-Sichuan Basin
Wang Qingchen1 Cai Liguo2
1.State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029;2.Petroleum Exploration and Production Research Institute, SinoPec,Beijing 100083
Evolution of a sedimentary basin could be divided into two stages：constructing stage and reworking stage.To restore the prototype basin has to recurrence the basin characters(original boundary，sedimentary environment，tectonic setting，etc.)in the constructing stage and to reveal tectonic evolution process in the reworking stage.The seismic topographic images revealed that the original boundary of the crystalline basement of protoSichuan Basin went beneath the QinlingDabieshan orogenic belt in the north，beneath the SongpanGanzi orogenic belt in the northwest，along the eastern margin of the Threeriver orogenic belt in the west and southwest，and along the WuchuanSihuiChenzhouJiangshanShaoxing fault in the Huanan orogenic belt in the south east.
The constructing stage of the marine protoSichuan Basin could be divided into three substages：crustforming substage(Archean to middle Neoproterozoic)，rifting substage(late Neoproterozoic)and basin enlarging substage(Ordovician to Middle Permian).The reworking stage of the marine protoSichuan Basin could be divided into two substages：intracontinental rifting substage(Middle to Late Permian)and intracontinental compressing substage(Triassic to Cretaceous).The protoSichuan Basin was reworked at margins by several orogenic belts in the Mesozoic.The Basinward deformation propagated from those orogenic belts in different periods and impacted basin in different regions.The deformation extent is the greatest where crystalline basement was involved into thrust，and is the smallest in the basin center where developed only gentle fold.
The constructing substage provided favorable condition for marine source deposits.For example，submarine volcanism provided abundant nutrition favoring marine productivity，and anoxic environment favoring accumulation and preservation of source rock.The reworking substage affected the source rocks both positively and negatively.For example，rifting enhanced the submarine relief and caused anoxic environment favoring source rock accumulation and preservation.Uplift during thrusting and folding could cause erosion of source rocks，while gentle folding or less deformed area could preserve source rocks.Suitable bury depth in foreland basin could bring source rock into oil window，while ultradeep burning could bring source rock into postmature condition.