| Weixi'nan Sag is located in the Northern Depression of Beibuwan Basin.It experienced two stages of rifting during Paleogene,when the distribution of major faults and the depocenters changed markedly.The Paleogene stratum overall appears as doublelayer structure,which is controlled by two fault systems including the early basement involved fault system(T83-T90)and the later overburden fault system(T60-T83).In order to determine the genesis and evolution of the double-layer structure and its effect on sequence stratigraphy as well as the distribution of sedimentary facies,geological and geophysical data is used to reconstruct the temporal and spatial evolution of major faults(No.1 Fault and No.2 Fault).Furthermore,the sequence stratigraphy,depositional systems and evolution of the two structural layers have been studied in detail.The results show that the evolution of No.1 Fault is consistent with the “isolated fault model”,which initially consisted of 5 individual fault segments during the third member of Liushagang Formation.Subsequently,No.1 Fault experienced the following 3 stages,i.e.fault interaction and linkage stage(the lower second member of Liushagang Formation),the through-going fault stage(the upper second member of Liushagang Formation)and the fault death stage.The evolution of No.1 Fault controlled the distribution of depocenter and the expansion of the lake within the lower structural layer.The evolution of No.2 Fault is consistent with the “constant length fault model”,which initially consisted of a dozen fault segments that were aligned in right-step en-echelon,and the final fault lengths were established relatively early in the displacement history.The fault segments maintained soft linkage in the following evolution history.Under this condition,the hangingwall developed a series of discrete depocenter.The evolution of No.2 Fault controlled the development of the upper structural layer,and the depocenter migrated from the hangingwall of No.1 Fault to the hangingwall of No2.Fault.Overall,the subsidence rate was relatively smaller during the upper layer than that of the lower layer.Thus the upper structural layer was characterized by shallow water depositional systems.Combining the analysis of regional tectonic setting and the change of stress field,it is found that No.1 Fault developed under the control of regional tensile stress striking NWSE during early to middle Eocene.The lower structural layer was under the condition of rapid expansion,and the lake experienced the process from expansion to climax,and finally depauperization.The regional tensile stress rotated clockwise since late Eocene,and the upper structural layer was under the condition of transtension during late Eocene and Oligocene when No.2 Fault and other overburden faults were activated,controlling the basin fill of the upper structural layer.With the constraint of the evolution of double-layer structure,the sequence architecture,distribution of sediment source and depositional systems developed in each structural layer and their differences were investigated.The results show that each thirdorder sequence consists of transgressive systems tract and regressive systems tract(T-R circle),and two types of T-R circle was recognized including symmetric and unsymmetric type.Overall,the unsymmetric type mainly developed in the lower structural layer(e.g.SQ 1?SQ3),while the symmetric type mainly developed in the upper structural layer(e.g.SQ4?SQ7),which shows an obvious correspondence between tectonic evolution and the development of sequence stratigraphy.Deep and semi-deep lake facies were well developed in the lower structural and favorable sands mainly occurred at the bottom and the top of the sequence.However,deltaic-shallow lake facies were well developed in the upper structural layer,and the vertical filling sequence was featured by sandstone interbedded with mudstone or shale.According to the recognition of fault related structures,such as fault slope belt,multilevel fault terrace belt,fault transfer zone and transverse anticline,the effect of the structures on sequence architecture was revealed.We discerned five types of sequence architecture,including fault slope controlled sequence,transverse anticline controlled sequence,relay ramp controlled sequence,multi-level fault terrace controlled sequence and flexural slope controlled sequence.Distribution of sediment source and depositional systems varied with the evolution of double-layer structure during Paleogene.The lower structural layer was characterized by well developed transverse depositional systems,with the sediments deriving from steep slope or gentle slope and the sedimentary bodies stretching in a N-S direction.While the upper structural layer was characterized by the well developed axial depositional systems,with the sediments deriving from the axial ends of the basin.Meandering river deltaic systems sourced from both ends of axial direction continued to prograde into the basin,forming large-scale axial depositional systems.Finally,combining the evolution of tectono-stratigraphy,the sedimentary bodies such as steep slope fan,axial deltaic system and sub-lacustrine fan within Sub-sag B,were depicted in detail and the dispersive patterns of favorable sandstones were uncovered.It shows that large-scale steep slope fan was well developed in early to middle SQ4 and distribution features of steep slope fanvaried along the NO.1 Fault: In western part of steep slope belt,large-scale incised valleys developed,producing large-scale sub-lacustrine fans,which are characterized by thick beds of fan deltaic deposits;While in middle part,smallscale channelized turbidite sub-lacustrine fans dominated;In eastern part,a series of small-scale symmetrical fans located along the border fault,with sparse valleys presented.Fault related structures(such as transverse anticline and transfer zone)and paleogeomorphic difference induced by activity of No.1 Fault were the key factors controlling the lateral variation of steep slope fan.Axial depositional system dominated in the upper structural layer.For SQ4,Based on the analysis of seismic reflection termination relationship,shoreline migration trajectories and the constraint of some boreholes,the axial system can be divided into six parasequence sets which are recognised as aggradational(type A)or progradational(type P)clinothem.During the development of type A,rate of accommodation is high,and the clinothem is characterized by its S foreset,with relatively thicker topsets and more elongated bottomsets.Furthermore,it has ascending shoreline trajectories.While type P,which has flat shoreline trajectories,is characterized by foreset with high angle,and with its maximum thickness at the center.These two types of clinothem occur alternately and overlay by each other,filling the depocenter vertically and laterally.The spatial distribution of axial deltaic system is mainly controlled by tectonic activity,paleogeomorphology and the cyclical relative lake level changes,which are induced by differential subsidence along the No.2 fault system.Furthermore,in upper structural layer,sub-lacustrine fan were well developed in Sub-sag B.The sediments mainly sourced from the western axial deltaic system and the northwestern deltaic system,and sediments from latter were controlled by transfer zone between segments of No.2 Fault.Thus the sediments entered into Sub-sag B,forming large-scale sandstone dominated sub-lacustrine fan. |
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