Sedimentary Features Resulting From Seismic Events Of The Early Cambrian In The Western Zhejiang Province And Its Geological Significance

The Palaeozoic strata in the western Zhejiang Province is relatively complete and has received in-depth research,among which the Duibian section of Jiangshan is a GSSP of international Cambrian Jiangshan Stage.A large number of Seismic soft-sediment deformations,tsunamite and Seismoturbidites have been found in the carbonate rocks of Kaihua and Changshan under the Cambrian Dachenling Formation in Zhejiang Province.By means of the knowledge of petrology,geochemistry,microscopetin,electron microprobe,scanning electron microscope?SEM?,mass spectrometer?MS?and other technical means,the authors have carried out the microscopic identification of seismic event sedimentary layers,seismic liquefaction veins and their parent rocks.Meanwhile,the studies on trace elements,rare earth elements,carbon and oxygen isotope analysis are conducted.Based on the sedimentary sequence of seismic events in early Cambrian in western Zhejiang Province,the spatial distribution law and activity period of seismic activity are determined.It is considered that the ancient seismic deposition was related to the Kaihua-Chun'an fault activity.Based on these studies,the origin of the material and migration direction of the liquefied veins are revealed,the unique minerals and geochemical anomalies at different stages of the seismic liquefaction are determined,the formation pattern of the liquefied vein is established,the causes of the land-phase geochemical characteristics of the marine seismic liquefaction vein are explained and the theory of seismic event deposition is perfected.In terms of the sedimentary facies of Dachenling formation of Lower Cambrian in western Zhejiang,there have been two views of deep-water and shallow-water facies.Through the analysis of tsunami deposition and carbon isotope anomaly,it is concluded that the tsunami process made the source of different water depth form a mixed layer,resulting in abrupt changes of sedimentary environment.The scientific explanation of the cause of the sedimentary anomaly in Dachenling period is in order to study the mixed sedimentary layer and to provided examples for the study of mixed sediments and sedimentary anomalies.Based on the author's systemic investigation in the west of Zhejiang province,it is found that the Cambrian carbonate of Dachenling Formation,outcropping in Peijiaoling area in Kaihua county of Zhejiang province,contains typical sedimentary marks of tsunami such as abundant liquified micrite vein,soft-sediment deformation,breccia and associates turbidite.According to the seismites' characteristics and their distribution on the profile,seismic activities that happened in Peijiaoling area are divided into two stages: the first stage is characterized by A—B—C—D sedimentary sequence,which comprises seismites?Unit Aa,including liquefied micrite vein dolomitic limestone?,tsunamigentic rocks?Unit B,intra^dolomitic limestone with mound bedding or parallel bedding?,seismoturbidites?Unit C,dolomitic calcirudite or psammitic dolomitic limestone with graded bedding or Bouma sequence and having flute cast on the bedding surface?and background sediments?Unit D?;While the second stage is characterized by A—D sedimentary sequence,that is seismites?Unit A,including liquified micrite vein dolomitic limestone,seismic corrugated lamination and autoclast breccia?and background sediments?Unit D?.The above features are not obvious in other places such as Baishi and Fangcun in Changshan county western Zhejiang province,where only liquified deformation structure may locally be observed.However in Duibian area,Jiangshan city,seismites' characteristics disappear.Based on the seismites' distribution,seismic activities in Peijiaoling western Zhejiang province were not controlled by Jiangshan—shaoxing deep fault zone but possibly related to the formation of Kaihua—Chun'an major deslocation.So it is considered as an evidence for the formation of Kaihua—Chun'an major deslocation in Eopaleozoic.The research on the paleoseismic magnitude and rhythm of the two Early Cambrian seismic events that happened in the western Zhejiang province has revealed that the magnitude of the seismicity was between 6 and 7.6,and the rhythm was of both events less than 5 Ma,much lower than that reported by other researchers.This maybe results from the frequent activities of the Kaihua—Chun'an fault in its early stage of formation.As compared with the geotectonic environments in Kiamusze,Himalaya,Gangdisi,Qiangtang and China—Burma blocks,and based on the analysis of the Cambrian lithofacies and paleogeogrophy in the western Zhejiang province,a conclusion has been drawn that the seismic events that happened in the western Zhejiang province were only a part of the Sinian tectonic activities or the Pan-African tectonic activities.The carbonate rocks dealt with in this paper were collected from the classic section of the Early Cambrian at Duibian.Their carbon and oxygen isotopes composition was determined.Variation range and average of ?¹³C and ?18O are respectively-1.9‰—0.6‰,-12.5‰—7.9‰ and-0.08‰?-10‰?The shape of the strata curve standing for C & O isotope differs from the evolution curve of isotopeaging obtained by Veizer et al.,and also disagrees with characteristics of local lithofacies and paleogeography.In the shallow basin area,there is a positive shift of?¹³C,and Arthricocephalus foissl often occurring in deep water is found in thess area.The positive-shifting of ?¹³C and deep water fossils is should corresponding to seismic and tsunami sedimentation events,and combined with the research results of the area,it is considered that the rapid change of the environments might be caused by tsunami triggered by seiemic event,and mixed sediments derived from different depth of water level are the response in geochemistry and biology.Liquified vein in carbonate rocks is an important clue to identify the earthquake-triggered sedimentation,though there exists different genetic interpretation on the similar inharmonic veins.Several methods for analyzing the vein and its adjacent rocks,such as identification under the microscope,energy spectrum analysis and macroelement's quantitative analysis have been made.By means,of above research,it is found under the microscope that the carbon and silica contents in the vein are obviously higher thore in country rocks.While by means of linear and planet analysis,it is found that contents of Ca,Sc in country rocks are respectively higher than those in the vein,and the K,Al,Si,Mg contents are lower than those in the vein.There is almost no chemical change in calcite when it is tested using the electron probe.This result is different from that obtained by other researchers by analyzing the molar structure.In general,the lithological and geochemical analysis of the vein and its country rocks show,that the inharmonic vein in the carbonate in the western Zhejiang is the vein originated from earthquakes.Microscopic geochemical behavior was less been studied.Trace element analysis of veins and vein-bearing carbonate rocks was been done on twenty-one samples from three beds of seismic veins of two seismic events deposition at the Peilingjiao section.The experiment result shows that the values of Cs,Ba,Th,Zr,Hf,Cr,Rb and Nb of liquified veins are obviously higher than vein-bearing carbonate rocks,which reflect terrigenous origin.In contrast,the values of Sr,Y and Ni are more abundant in vein-bearing carbonate rocks,which reflect the features of marine sediments.Based on the research of enrichment of V and Mo and characteristic parameters of trace elements including Sr/Ba and Th/U,liquified veins are interpreted to be oxidizing environment,and vein-bearing carbonate rocks be reducing environment.In addition,the veins and vein-bearing carbonate rocks are characterised by freshwater sedimentation.The result shows that there was complement of terrigenous material from erosion source areas during the deposition process of dolomitic limestone in the Early Cambrian.Under thesudden vibration force of earthquakes,the water-rich soft sedimentary rock layers were liquefied and water escaped.Because of properties differences between chemical and clastic sediments,the plastic carbonate sediments were partially lost with water,leading to the lower content of the carbonate sediments per unit area than the surrounding carboante rocks and more terrigenous material in veins.Soft sedimentary beds are of supratidal sediments,which were exposed during the late stage of liquefaction draining to diagenetic stage.Effected by the late terrigenous resupply and atmospheric precipitation leaching alteration effects,leading to abnormal paleoclimate and salinity of the soft sediments,which provide geochemical evidence for shallow water sedimentation of Dachenling Formation in Western Zhejiang.The research on the veins and C-O-Sr isotopes of the vein-bearing rocks were carried out.Firstly,the results show that ?¹³C_PDB-w of the two stages of veins is+0.01^-1.09‰,and ?¹³C_PDB-v is-0.07^-1.51‰,both of which belong to the normal marine carbonate sedimentary zone.However,the isotope values of each sample are featured by ?¹³C_PDB-w > ?¹³C_PDB-v,and carbon isotope of the veins show obviously negative shift compared with the surrounding rocks.The first average negative shift is0.3‰ and the second is 0.5‰,which was interpreted to be caused by isotope fractionation,late-stage alteration and ?12C prior migration over ?¹³C during water-escape process during seismic events.Secondly,the value of oxygen isotope for the two stages of veins and surrounding rocks is obviously different.The ?18OPDB of the first stage is-9.9 ^-11.6‰,which is similar with veins and surrounding rocks and shows slight negative drift,reflecting the original sedimentary environment.The?18OPDB of the second stage is-12.7^-18.6‰,which appears to be strongly influenced by aleration.The research shows that the first earthquake event was accompanied with strong tsunami and deposited in deep water environment,and the liquefied veins and surrounding rocks were covered by overlying sediments after earthquake,so the original sedimentary environment was retained.However,the second earthquake occured in very shallow water environment,so water-escape structures were partially and transient thermal altered during formation process,which were easily affected by precipitation and late hydrothermal alteration due to unsubstantial tectonic surfaces after lithification.Thirdly,strontium isotope data of the frst stage is little,so it is not analysed here.The average Sr⁸⁷/Sr⁸⁶ w of the second surrounding rocks is 0.712635,which is higher than the value reported at home and abroad.The average Sr⁸⁷/Sr⁸⁶ of the liquefied veins is 0.733804 and even go up to 0.794550,which is higher than the surrounding rocks.This value is different with marine carbonate rocks of commonmarine,terrestrial and mantle source,but is quite the same with continental deposits,granite and metamorphic rocks.Based on field obvervations and microscopic analysis,the liquefied veins don't have the composition of granite,which indicates that when the earthquake happened,the soft sediments were near or exposed to the surface.The ground fissures were formed by seismic liquefaction and water-escape process,which were filled with late continental sediments and cemented by late carbonate.Therefore,the Sr⁸⁷/Sr⁸⁶ value of the liquefied veins is characterised by continental sedimentation,and the water escape structures are accompanied by short-time thermal alteration,which are also suggested for strontium isotope and consistent with results of carbon and oxygen isotopes.The shape of the seismic liquefaction veins is restricted by the earthquake intensity.The seismic intensity is strong,which can form the upper and lower liquefied veins,or the liquidfied sand mound and the sandstone wall.If the earthquake intensity is strong,the veins are spindle-like and do not penetrate the layer.If the earthquake intensity is moderate and the veins are stratified with occasional flaming structures.Seismic liquefied veins were formed on the surface.The geochemical behaviors of the seismic liquefied veins vary at four stages from Earthquakes event,Seismic soft-sedimentary deformation layer stabilization,Diagenesis consolidation and the late diagenesis.In the period of earthquake occurrence,K,Na and other elements in the parent rock migrated to the veins;In the stable period of the deformation from soft sedimentary to soft sedimentary deposit,the carbon and oxygen strontium isotope fractionated;After the period of diagenesis,the geochemical behavior of veins was characterized by the further fractionation of oxygen isotopes,the abnormality of trace elements such as Sr,Ba and Gd.On the basis of these variation,a model of seismic liquefaction filling mechanism that combined with the dynamic mechanism of earthquake liquefaction vein formation was established.