| Terrestrial red bed is one of the important targets in paleomagnetic studies.Previous research has focused mainly on paleomagnetism of lacustrine-fluvial red beds,while paleomagnetism of aeolian red beds has been rarely investigated.As a result,the reliability of paleomagnetic data from aeolian red beds and the remanence acquisition in different depositional processes remain poorly understood.In this study,we carried out a paleomagnetic study of the Upper Cretaceous aeolian red beds from Chishan Formation?Fm?in Jurong Basin,Jiangsu Province and the Upper Cretaceous aeolian red beds from the Tangbian Fm and the lacustrine-fluvial red beds from Hekou Fm of the Guifeng Group in Xinjiang Basin,Jiangxi Province,South China.The purposes of this study are to investigate the reliability of the paleomagnetic records of aeolian red beds and the influence of different depositional processes on magnetic remanence acquisition by comparing the paleomagnetic data of aeolian red beds and those of lacustrine-fluvial red bedsThe samples of aeolian red beds and lacustrine-fluvial red beds were investigated by the stepwise thermal demagnetization,rock magnetic experiment,thin section observation,chromaticity experiment.Based on the results of the samples,the following conclusions are drawn.?1?The mean low temperature component?LTC?of Chishan Fm is Dg=10.5°,Ig=45.8°?n=154,?=7.5,?95=4.4°?in geographic coordinates and Ds=4.0°,Is=40.7°?n=154,K=7.1,?95=4.60?in stratigraphic coordinates,which is the viscous remanence of present geomagnetic field.Most aeolian red bed samples from the Chishan Fm failed to yield stable characteristic remanent magnetizations?ChRMs?.Rock magnetic results of the Chishan Fm samples show that the magnetic fabric is sedimentary fabric and remanence are carried by hematite and magnetite.?2?The mean LTC of Tangbian Fm is Dg=1.2°,Ig=43.1°?n=149,K=317,a95=2.1 °?in geographic coordinates and Ds=12.4°,Is=29.5°?n=149,k=30.3,a95=2.1°?in stratigraphic coordinates,which is the viscous remanence of present geomagnetic field.Only 19%of the aeolian red beds samples of Tangbain Fm yield stable ChRMs.The convex shape of the intensity decay curves and the observation of detrital hematite under the microscope from samples of Tangbian Fm suggest a primary origin of remanence carried by detrital hematite grains.The mean ChRM of Tangbian Fm is Dg=5.7°,Ig=43.7°(n=25,K=12.8,?195=8.4°?in geographic coordinates and Ds=15.60,Is=28.9°?n=25,k=13.0,a95=8.3°?after tilt correction,corresponding to a Late Cretaceous paleopole at Latitude=70.7°,Longitude=245.6°,A95=6.8°.Rock magnetic results of Tangbian Fm samples show that the magnetic fabric is sedimentary fabric and remanence are carried by hematite and magnetite?3?The mean LTC of Hekou Fm is Dg=358.8°,Ig=49.0°?n=66,k=43.8,a95,=2.7°?in geographic coordinates and Ds=1.0°,Is=21.7°?n=66,k=43.8,a95=2.7°?in stratigraphic coordinates,which is the viscous remanence of present geomagnetic field.Most lacustrine-fluvial red bed samples from the Hekou Fm failed to yield stable ChRMs.Rock magnetic results of Hekou Fm samples show that the magnetic fabric is sedimentary fabric and remanence are carried by hematite and magnetite?4?The new paleopole of the Tangbian Fm is indistinguishable with the coeval paleopole from lacustrine-fluvial red beds of the Ganzhou Fm and the recently updated Late Cretaceous paleopole of South China,suggesting that the aeolian red beds of the Tangbian Fm can carry reliable paleomagnetic records and different depositional processes have no significant effect on remanence recording in terrestrial red beds.In addition,further investigations of the Chishan Fm and the Tangbain Fm aeolian red beds with thin section observations and redness analysis indicate that the chemical remanence magnetism carried by the pigmentary hematite obscures the detrital remanence magnetism carried by the detrital hematite in the Chishan Fm,and degree of cementation may affect the stability of remanent magnetization of Tangbian Fm aeolian red beds as well. |
PDF Full Text Request