| The Songliao Basin that located in Northeastern China is a large Meso-Cenozoic continental sedimentary basin,and one of the major oil-gas producing areas in China.Two set of strata fills in this basin,the oil-bearing shallow strata of depression stage and coal-gas bearing deep strata of fault depression stage.The strata of depression stage are key layers of oil-gas exploration and scientific research in the past.With the characteristics of widely distributed and continuous deposition,the division and correlation of strata in depression stage are relative easy.So,they are well studied in the past decades.However,the strata of fault depression stage,also known as rift sequences,composed of Huoshiling Formation,Shahezi Formation and Yingcheng Formation,that a set of volcanicsedimentary strata,scattered in many independent fault deprssions,are low studied in the past,because of deeper burying and less drilling.With the breakthrough of natural gas exploration of deep layers in Songliao Basin,The study on the rift sequences has attracted more and more attention from geologists.In Northern Songliao Basin,the exploration area of Daqing Oilfield,a large number of geochronology research on volcanic rocks has been done,and a set of continuous palynological assemblages found mainly by researchers from Daqing Oilfield.However,the research of rift sequences in Southern Songliao Basin,especially in Eastern Fault Depression Belt,is relatively backward.Less research on volcanic geochronology has been done,and continuous palynological assemblages has not been set in this area.So,much more basic geological research work on rift sequences in Eastern Fault Depression Belt of Southern SongliaoBasin is desideratedMainly based on the research methods of lithostratigraphy,biostratigraphy and volcanic chronology,assisted by geophysical data,the rift sequences have been studied systematically in this paper.The study area can be divided into two parts: one part is the southeast margin area of Songliao Basin,that the location of stratotype and outcrop of rift sequences;another part is the Eastern Fault Depression Belt of Southern Songliao Basin,composed of Wangfu Fault Depression,Dehui Fault Depression,Yushu Fault Depression and Shuangliao Fault Depression.Geographically and tectonically,the latter is close to the stratotype area,also would be the key area to study the rift sequences inner Songliao Basin.According to the research area,the work of this paper can be divided into two parts.Research works in the margin area includes: restudy of stratotype,outcrops and exploration data of coal mines in the southeastern margin area,to make clear the rock association and sequence of each stratigraphic unit;first analysising the palynological samples from Shahezi Coal Mine,that the first named site of Shahezi Formation;first dating the volcanic rock samples from outcrops of Huoshiling Formation in stratotype area;first dating the volcanic rock samples from outcrops of Yingcheng Formation in Siping-Changtu area.Research works in the Eastern Fault Depression Belt includes: established the rock association and sequence of each stratigraphic unit basing on the study of typical drill bore;palynological study of fault depression stage is carried out in Yushu Fault Depression for the first time;palynological analysising work has been supplemented in Wangfu Fault Depression,Dehui Fault Depression,and Shuangliao Fault Depression;combined with the previous palynological data analysising by Research Institute of Jilin Oilfield,the most complete palynological assemblages of fault depression stage at present has been recognized in each Fault Deprssion,respectively;zircon U-Pb dating of volcanic rocks was first done in Wangfu Fault Depression and Yushu Fault Depression;zircon U-Pb dating of volcanic rocks has been supplemented in Dehui Fault Depression.Achievements of this study are embodied in three aspects,lithostratigraphy,biostratigraphy and chronostratigraphy,as follows:1.Basing on the study of lithostratigraphy from stratotype,outcrops,major coal mines in the southeastern margin area and drill bores from four fault depressions,established the rock association and sequence of each stratigraphic unit.The Huoshiling Formation can be divided into three members in order: Member 1 is characterized by intermediate-basic volcanic rocks;Member 2 is characterizedby clastic rocks;Member 3 is characterized by intermediate-basic volcanic rocks.The Shahezi Formation is a set of clastic rocks on the whole,can be divided into three members in order: Member 1 is characterized by fine grained clastic rocks;Member 2 is characterized by tuff and conglomerate;Member 3 is characterized by fine grained clastic rocks.The Yingcheng Formation can be divided into three members in order: Member 1 is characterized by intermediate-acidic volcanic rocks;Member 2 is characterized by clastic rocks;Member 3 is characterized by clastic rocks,and volcanic rocks interbedded with clastic rocks in the margin area.2.According to the system research of palynological data in this paper,the palynoflora in the study area is thought to be consistent with the Disacciatrileti-Cicatricosisporites Palynoflora that built by previous researchers,four palynological assemblages have been recognized in order.Palynological assemblage from the Huoshiling Formation is named as Piceaepollenites-Cicatricosisporites;the assemblage is characterized by an extremely high content of Piceaepollenites.Palynological assemblage from the Shahezi Formation is named as Perinopollenites-TaxodiaceaepollenitesAbietineaepollenites;the assemblage is characterized by high content of Perinopollenites,bisaccate pollen and pollen without sacci are both prosperous.Palynological assemblage from the Member 1 and Member 2 of Yingcheng Formation is named as Paleoconiferus-Protoconiferus-Pinuspollenites;the assemblage is characterized by an extremely high content of Paleoconifer pollen.Palynological assemblage from the Member 3 of Yingcheng Formation is named as Pinuspollenites-CyathiditesConcavissimisporites-Cicatricosisporites;the content of fern spores is higher than previous three assemblages.In addition,on the basis of palynological data,palaeovegetation and palaeoclimate is of the study area are briefly discussed in this paper.In the fault depression stage of Early Cretaceous,palaeovegetation and palaeoclimate changed little,the palaeovegetation was characterized by Filicales and Conifers,followed by Lycopodiales and Cycadales,the palaeoclimate was consistent with subtropical climate of today,and the palaeoenvironment was humid-subhumid,seasonal arid occured occasionally.3.By newly high precision volcanic zircon U-Pb dating,some important zircon U-Pb age of fault deprssion stage volcanic rocks are obtained: three sample from outcrops of stratotype area range from 130.9Ma to 130.5Ma,and one sample from Wangfu Fault Depression show 129.8±5.1Ma;four sample from outcrops range from 111.9Ma to 108.9Ma,and eight sample from Eastern Fault Depression Beltrange from 118.2Ma to 107.7Ma.Combined with previous studies,the chronostratigraphy of rift sequences in Songliao Basin is discussed.The geological age of Huoshiling Formation is thought to be 133-124 Ma,belong to Hauterivian-Barremian in Early Cretaceous,can be correlated with Yixian Formation in Western Liaoning;the geological age of Shahezi Formation is thought to be 124-120 Ma,belong to Early Aptian in Early Cretaceous,can be correlated with Jiufotang Formation in Western Liaoning;The geological age of Yingcheng Formation is thought to be 120-100 Ma,belong to Middle Aptian-Albian in Early Cretaceous,can be correlated with Shahai Formation and Fuxin Formation in Western Liaoning. |
PDF Full Text Request