The Study Of Magnetic Properties Of Late Cenozoic Sediments On South And North Flanks Of Yangtze Delta

The research on geomorphological evolution of the Yangtze River would not only shed light on tectonic,climatic and sea-level change of eastern Asia during the Cenozoic Era,but also provide scientific references for more practical significance,including water utilization and flood regulation in the Yangtze catchment.To resolve the question of how and when the Yangtze River connected to the sea has become the key subject for earth scientifis since>100 years ago.However,owing to the intricate geological differences in such a large-scale river basin,a wide range of chronologies of the Yangtze's connection to sea has been concluded,maily from ca.23?0.1 Ma.Having received a huge amount of detrital material from the river basin,the Yangtze delta has archived>400-500 meters of Late Cenozoic sediment.This has no-wonder been an idea study area for investigating the geomorphologial development of the river.Of note,many previous studies were concentrated on the south flank of the delta,rarely on the north flank in context of source to sink process.Also noted is that those studies mainly focusd on the nature of sediment source,while less was given to compare sediment signatures relating to specific potential mother rock sources in the Yangtze River basin.Geologically,the Yangtze basin is comprised of complicated source rocks varying from Archean metamorphic and metasedimentary rocks to Quaternary clastic sediment.A large-scale Mesozoic basaltic outcrop,the E'mei Basalt block(2.5 x 105 km2),occurs mainly in the upper basin(above Three-Gorges valley).Compared with other source rocks in the basin,the E'mei Basalt has a much higher magnetic susceptibility.So a switch in sediment magnetic properties found in the Plio-Quaternary strata probably indicates the arrival of the surficial sediments from upper basin above the Three Gorges valley and completion of the connection of Yangtze River to the sea.Therefore,the distinct magnetic properties of sediments in association with the E'mei Basalt could be used to trace the evolution of the Yangtze River.In this study,two continuous cores(LQ11,301.5 m in length;LQ19,318 m in length)penetrating the Plio-Quaternary strata to bedrock on the southern Yangtze delta plain and one core(ZKJ39,320 m in length)on the northern part were taken for a detailed magnetic investigation.Firstly,based on the paleomagnetic results,the chronological framework was constructed.Secondly,with the analyses of the magnetic parameters in association with grain size,total organic matter and foraminifera,we discussed the factors affecting the magnetic properties of core sediment including particle grain size,diagenesis and pedogenesis.Then we examined the spatio-temporal distribution of magnetic properties of both cores and surficial sediments from the catchment,in order to identify the use diagnostic magnetic properties as sediment tracer.Finally,we determined,by magnetostratigraphy,the timing of the Yangtze's eastward connection to the sea.The conclusions are drawn as follows:1.For most samples,the remanence intensities decrease to 10%of the natural remanent magnetization(NRM)after a stepwise heating to 585?,indicating the magnetite is the main carrier of the NRM.For some other samples,the remanence intensities decrease to 20?30%at 585? and 10%at 680?,suggesting that both magnetite and hematite are ChRM carriers.Compared with previous paleomagnetic results using alternating field(AF)demagnetization test,progressive thermal demagnetization can isolate ChRM components more successfully for the samples from the Yangtze delta.All the three sequences have recorded the Brunhes,Matuyama,and Gauss Chrons.2.Temporal variations of magnetic susceptibility(?lf),together with some other magnetic parameters of core LQ11,divide the Plio-Quaternary core sediment into two major parts.Extremely low and unvarying ?lf values mostly less than 15*10-8m3/kg,occur in the lower part of the core(301?145 m).This low magnetic content is associated with low SIRM and S-ratio,and high ?ARM/?lf and ?ARN/SIRM.In marked contrast,high ?lf,SIRM and S-ratio values,and ?ARM/?lf low and ?ARM/SIRM are displayed in the upper part of the core(145?0 m).The sharp change of magnetic in the upper part indicates high ferrimagnetic content and coarse ferrimagnetic grain size,whilst the lower part suggests much lower ferrimagnetic content,as characterised by finer magnetic grain size,and higher concentrations of hematite.The magnetic properties of core LQ19 from south flank are similar with those of core LQ11.Magnetic parameters of the lower part(324?235 m)of core ZKJ39 from north flank displays lower ferrimagnetic content and finer magnetic grain size.The ferrimagnetic content of the middle part(235?135 m)increases remarkably,and the magnetic grain size gets coarser.The upper part(135?0 m)displays lower ferrimagnetic content with decreasing grain size of sediments.3.The correlation between grain size and magnetic parameters of core sediments is generally poor.It displays little evidence of post depositional reduction.Total organic matter content is generally low throughout the entire sequence,likely minimizing sediment vulnerability to reductive dissolution.The lower content of sulfates may be another factor constraining the process of diagenesis of sediments in the lower part of sediment cores.Repeated redox fluctuations caused by the rapid wetting-drying cycles could have promoted the formation of hematite through the pathway of ferrihydrite-maghemite-hematite in the upper part(135?0 m)of the core ZKJ39.So the magnetic properties of fine sediments in the upper portion(135?0 m)may be changed by the formation of hematite to a certain extent.Therefore,sediment provenance is apparently responsible for magnetic propery changes throughout both the lower and upper core sections.4.The surficial river sediment in the tributaries of upper Yangtze catchment show remarkably high ?lf,low ?ARM/?lf and ?ARM/SIRM,slightly higher S-ratio than that of the tributaries from the mid-lower Yangtze catchment,indicating high ferrimagnetic content and coarse ferrimagnetic grain size in the upper tributaries.The?lf values of surficial river sediment in the tributaries matches well with the types of bedrock of the catchment.Among the tributaries,the Yalong River,which flows across the E'mei Basalt block in the upper Yangtze basin,shows the highest value of ?lf(272*10-8 m3/kg)and make a large contribution to the ?lf of surficial river sediments from the mainstream.Distinct magnetic properties of E'mei Basalt and its detrital materials could be used to trace the evolution of the Yangtze River.5.The distinctive magnetic properties(high ?lf,SIRM and S-ratio,and-low?ARM/?lf and ?ARM/SIRM)in the upper part of core sediment are comparable with those of the modern river sediment samples.A strong match is found with the highly magnetic surficial sediment of the upper Yangtze catchment,especially with those of the Yalong River.In contrast,the low ?lf,SIRM and S-ratio,and high ?ARM/?lf and?ARM/SIRM values of Plio/early Quaternary sediment in the lower part of core appear similar to those of the surficial river sediment of the mid-lower Yangtze catchment.It is then confirmed that the obvious change of magnetic properties from lower to upper core sediment imply the arrival of clastic material in the upper basin,especially those from the E'mei Basalt area to the river mouth area.The appearance of this new,distinctive sediment source in the Yangtze delta sediment marks the Yangtze's eastward extension and connection to the sea..The palaeomagnetic chronology identifies that the major change in sediment provenance in the proto-Yangtze drainage basin in the north and south flanks of delta occurred at?2.0 and 1.2?1.0 Ma respectively.It probably implies that the time of Yangtze's connection to the sea appeared at no later than 2.0 Ma,and the river channel shifted southward since then.