Paleomagnetic Study Of Eocene Dalianhe Formation In Yilan Basin And Its Geological Significance

In this paper,the Eocene Dalianhe Formation in Yilan Basin is taken as the research object.On the basis of field profile measurement,systematic paleomagnetic sample collection test analysis and organic geochemical analysis,the petromagnetism,paleomagnetism and paleoclimate evolution characteristics of the Dalianhe Formation are studied in detail.The paleomagnetic research results show that the magnetic-bearing minerals of the Dalianhe Formation are magnetite with low coercivity,and the rock magnetic fabric is the sedimentary fabric under the action of typical flowing water.Palaeomagnetic alternating demagnetization measurements distinguish obvious positive polarity and polarity reversal.In the geographical coordinates,the average direction is Dg=357.1°,I=59.2°,k=13.1,?95=6.6°.In the stratigraphic coordinates,Ds=1.9°,I=56.2°,k=11.3,?95=5.7°.The characteristic residual magnetic component of the Dalianhe Formation passed the C-level inversion test at 95% confidence level.The paleomagnetic pole position is: ?p=80.4°N,?p=300.2°E,dp/dm=5.9/8.2;The paleolatitude is ?=36.8±7.0°.The magnetic inclination angle is shallowed,and the position of paleomagnetic pole after correction is: ?p=83.0°N,?p=138.8°E,dp/dm=8.3/9.7;The paleolatitude is ?=53.1±9.0°,which is consistent with the present latitude at 95% confidence level.The Yilan Basin has not experienced obvious latitude movement since the formation of the Dalianhe Formation.On the basis of regional stratigraphic correlation,the paleomagnetic polarity inversion sequence of the Dalianhe Formation is compared with the international stratigraphic age.The results show that the lower coal-oil shale section corresponds to the international formation Ypresian,with an age of 56.0Ma-47.8Ma.The middle oil shale section corresponds to the international formation Luteian,with an age of 47.8Ma-41.2Ma.The upper sand shale section corresponds to the international formation Bartonian,with an age of 41.2Ma-37.8Ma.The results of organic geochemistry and isotope show that the climate evolution of Yilan Basin in Eocene presents a warm and humid climate in the early Eocene,and a cooling trend in the middle and late Eocene,which is consistent with the climate evolution characteristics of the transition from greenhouse to glacial chamber from the early Eocene to the middle and late Eocene.At the same time,climate change in different stages of Yilan Basin has a certain impact on its sedimentary environment and sedimentary mineral formation.The lower coal-bearing-oil shale section(Early Eocene)was formed under warm and humid climatic conditions.The sediments showed the characteristics of high TOC(average of 39.44%),high TOC/TS(average of 53.03),negative partial organic carbon isotope(average of-26.57‰)and high TOC/TN(average of 35.15).Warm and humid climate and organic source of terrestrial higher plants were important factors for the formation of thick coal in the lower coal-bearing-oil shale section.The periodic rise and fall of base level lead to the transformation of sedimentary environment between lakes and peat swamps.Under the influence of organic matter input of submerged,emergent plants and plankton,the source of mixed organic matter with terrestrial higher plants is the key factor for the formation of oil shale and mudstone.The warm and humid climate of the early Eocene is still maintained in the early sedimentary period(early Miocene)of the middle oil shale section.The sediments are characterized by high TOC(average 4.97 %),high TOC/TS(average 15.00),relatively negative organic carbon isotope(averag-25.58‰)and low TOC/TN(average 3.40‰).Warm and humid climate and organic matter sources dominated by aquatic plants and algae are important conditions for the formation of oil shale.During the middle and late Eocene,the climate continued to cool down,and the lake basin was uplifted under the action of regional compression stress.Sediments accumulated into the lake center,forming the upper sand shale section.