The Sea Level Change And The Significance Of The Paleoenvironment Of The Carbonatic Rocks During The Early-middle Triassic Period In The West Yangtze Region

This paper’s research area is in Xianfeng and lichuan city, which located inthe western Yangtze plate, and takes the early Triassic epoch JialingjiangFormation and the middle Triassic epoch Badong Formation as research object.Mainly the research methods of carbon, oxygen and strontium isotope, it aims tofind out the paleoenvironment significance and the sea level change situation ofthe Jialingjiang formation and the Badong formation in the early-middle Triassicperiod. In order to establish more detailed sequence of the transgression andregression.According to previous research results, the main indexes of judge whetherdiagenetic alteration happens are (1)Generally consider that if Mn/Sr <10, thenthe carbonate rocks kept the original carbon isotopic composition (2)when thecarbonate rocks’δ18O<-5‰, then may have encountered alteration, and if thecarbonate rocks’δ18O<-10‰,it would be better don’t use data (3)Often think thatif the δ13C and δ18O does not have obvious correlation (discrete), it reflected themarine carbonate rocks preserved the original carbon and oxygen isotopiccompositions. All the89marine carbonate rocks have not been serious diageneticalteration, δ18O were greater than-10‰, the mean value of Mn/Sr ratio is0.75inthe Nashuixi village, lichuan city, the mean value of Mn/Sr ratio is0.06in theTaiPingGou village, Xianfeng city. And from the covariant relationship of tehcarbon and oxygen isotope, R2is0.0566in Jialingjiang formation and0.1462inthe Badong formation. All have good representativeness for the ancient seawater information. In the early Triassic period, Jialingjiang formation inTaipinggou village Xianfeng city, the δ13C value range from2.19‰to4.63‰and the mean value is3.73‰, the δ18Orange from-3.69‰—-6.64‰，and themean value is-5.48‰，the87Sr/86Sr range from0.707082-0.707552, and themean value is0.707209. In the middle Triassic period, Badong formation inNashuixi village Lichuan city, the δ13C value range from-0.34‰-3.07‰and themean value is1.80‰, the δ18Orange from-6.86‰—-0.82‰，and the mean value is-5.42‰，the87Sr/86Sr range from0.707788-0.708686, and the mean value is0.708027.All the experimental dates showed that all the formations have marinedepositional environments and cyclicity. Predecessors divided the Jialingjiangformation which in the early Triassic period in the middle Yangtze region into2times transgression-regression cycles. This paper divided into six and seventimes. Every marine transgression-regression contains several small sea-levelfluctuation cycles. The dates of δ13C and T in the Jialingjiang formation in the earlyTriassic period are greater than Badong formation. So we think that the jialingjiangformation is further off the coast then the Badong formation.