Baise Basin, Paleogene High-resolution Sequence Stratigraphy

Sedimentolgy, tectonics, basin analyses and high-resolution sequence stratigraphy are applied in this paper. On basis of many data of drillings, outcrops, observation well and seism,we study paleogene sequence by the high- resolution sequence stratigraphy in the Bai Se basin. Some new opinions and results obtained as follow:(1) Basing on the mid-term and long-term datum level analysis and linking data of the interface and the age of some squences, Paleogene sequence delimiting and contrast is verified in the Bai Se basin. On basis of predecessor study, stratigraphic delimiting which accord with sequence division and evolution rule of sedimentary filling is put forward. Each formation age assignment in turn: Liu Niu formation belonging to Paleocene, Dong Jun formation belonging to early Eocene, Na Du formation belonging to mid-late Eocene, Bai Gang formation belonging to Oligocene, Fu Ping and Jian Dou Ling formation overlaying Bai Gang formation.(2) On basis of sedimentary facies analysis, Paleogene sequence stratigraphic feature is particularly studied by theories and methods of high-resolution sequence stratigraphy which is used firstly. Main evolvations: ①identified four grades datum level cycle sequence, such as super-long term (SLSC cycle), long term(LSC cycle), mid-term(MSC cycle) and short term (SSC cycle). Mid-term, long term and super-long term cycle sequence interface and isochronic correlation,significance and sign of flooding surface in the basin are made certain. ② Paleogene is divided 4 super-long term cycle sequences which correspond stratigraphic formations in the Bai Se basin. Establishing relation between super-long term cycle sequence and every phase tectonic activity, unconformable contactformed by tectonic stress field transition is key which control super-long term cycle sequence growth. So super-long term cycle interface denotes stress field transition of different tectonic evolution phase. Namely equence is result of different tectonic evolution phase;③Liu Niu formation is divided into 3 long term cycle sequences(LSCl-LSC3). Dong Jun formation dividing 1 long term cycle sequences(LSC4), Na Du formation dividing 3 long term cycle sequences(LSC5-LSC7), Bai Gang formation dividing 3 long term cycle sequences(LSC8-LSC10). Relation between long term cycle and lacustrine transgressive regressive cycle is confirmed. Factors of long term cycle growth is connected with variety of the periodical tectonic subsidence activity in the same stress field. Thus long term cycle interface denotes tectonic change surface of periodical tectonic surface in the same tectonic evolution phase; ④Liu Niu formation is farther divided into 3 mid-term cycle(MSCl), 2 mid-term cycle Dong Jun formation (MSC4-MSC5), 12 mid-term cycle in Na Du(MSC6-MSC17), 7 mid-term cycle in Na Du(MSC18-MSC24). Secondary lacustrine transgressive regressive action of mid-term datum level cycle bears on provenance supply and climate change; ⑤On basis of many age data and sedimentary evolution feature of different grade datum level cycle, the age of super-long term, mid-term and short term cycle areconfirmed. Super-long term age is 10-13Ma; long term cycle age is 3-5Ma; mid-term cycle age is 0.8-2.5Ma; short term cycle which correspond parasequence or least stratigraphic unit is tens thousand. Age confirmation of different grades datum level cycle is basis for age contrast precision and resolution of isochronic stratigraphic unit.(3) Analyzing structure type of short term datum level cycle, the authors identified three main types which are up-deeping asymmetry(A), up-shallowing asymmetry(B) and symmetry from up-deeping to up-shallowing(C). Further in the light of the high or low accommodation, seven subtypes of structure are distinguished and they are up-deeping asymmetry of low accommodation (Type Al) or high accommodation (Type A2), up-shallowing asymmetry of low accommodation (Type Bl) or high accommodation (Type B2), complete-nearly complete symmetry (Type Cl), incomplete symmetry with majority of uprising semi-cycle (Type C2) or subsiding semi-cycle (Type C3). It is mainly discussed that sedimentary kinetic feature during every type short term cycle stratigraphy and stacking patterns of mid-short term datum level cycle during mid-term cycle.(4) According to above research, in 2 secondary pull-apart basin, Nadu and Baigang formation of I and II grade tectonic units makes regional correlation by means of super-long term cycle as frame, long term cycle as grain and mid-term cycle as isochronic stratigraphyic unit. The author establish high-resolution sequence stratigraphy of the I grade tectonic unit and discuss datum level cycle structure type, stacking patterns, distribution rule of the different tectonic unit in stratigraphic frame.(5) Applying research achievement of high-resolution sequence stratigraphy, basing on the choice sequence interface SB5,SB6,SB7,SB8 as isochronic contrast sign, the same sequence thickness and facies interpretation of hundreds drilling well as control point(facies type of control point decided by optimum method), LSC5 uprising semi-cycle, LSC6 uprising semi-cycle, LSC7 subsiding semi-cycle (Nayi up subsegment), LSC8 uprising semi-cycle (low Bai three segment) as mapping unit, we selectively make four sequence lithofacies-paleographic map which objectively reflect paleogene lithofacies-paleogeographic frame, development and evolution in the Baise basin.(6) On basis of the study of the sedimentary system, sequence stratigraphy and lithofacies-paleogeography, linking paleogene feature of tectonic stress field and tectonic type in the Baise basin, Baise basin is defined small strike-slip pull-apart basin which develops on the young craton folded basement in the Hercynian-Indosinian. Form, development and evolution history of Basin divide 3 periods and 7 phases: (Drelief basin period of the LSC1-LSC3 period(Liuniu sedimentary period); (2)LSC4(Dongjun formation period) early rift-subsidence phase, LSC5-LSC6(Nadu early-middle period—E2n3-E2n2) strong rift-subsidence phase, and LSC7(Nadu late period—E2n') stable rift-subsidence phase; (3)LSC8(Baigang early period— E3b3) early depression phase, LSC9(Baigang middle—E3b2) strong depression phase and...