Conodont Biostratigraphy Of The Sidazhai Formation And The Xinyuan Formation At Shaiwa Section, Ziyun, South Guizhou

The Shaiwa Setion is northwest to the Sidazhai village, Ziyun County, Anshun, Guizhou province. Exposes along the round hill road, the stratigraphy of Shaiwa Section is successive and belong to the Permian and the Triassic. Lithostratigraphic units divided in ascending order are:the Sidazhai Formation, the Linghao Formation of the Permian and the Luolou Formation, the Ziyun Formation, the Xinyuan Formation and the Bianyang Formation of the Trassic.The Shaiwa Section represents the slope setting between the deeper-water intraplatform basin and the platform during Permian and consists of claystone, carbonates of gravity flow, siliceous banded limestone and deposits of terrigenous elastics. Most of the carbonates are within the Sidazhai Formation, which provides a basis for the study of conodont biostratigraphy.Depositions of middle Triassic at Shaiwa Section formed in a geological setting of peripheral foreland basin, the middle Triassic are mainly composed of sediments of turbidite flow, content of carbonates of deep-water is less and limestones exposes restricted in the upper part of Xinyuan Formation.Former studies on conodont biostratigraphy at Shaiwa Section focus mainly on the Linghao Formation, there are sporadic conodonts reported from the upper part (equals to bed 12 in this study) of the Sidazhai Formation. Therefore, detailed conodont biostratigraphy is necessary and will be helpful for further research. Studies of conodont biostratigraphy on basinal middle Triassic of South Guizhou are poor as so far, conodont biostratigraphy of the Xinyuan Formation at Shaiwa Section is only reported from its base, and conodonts from the upper part of the unit are reported for the first in this study. It is helpful to enhance the precision of division and correlation of basinal middle Triassic of South Guizhou as well as provides a timescale for the discussion of recovery of ecosystem after the mass extinction during P/T transition.A total of 39 conodont samples are collected from the Sidazhai Formation and 1158 conodont specimens are obtained,16 species belong to 7 genus are identified from 608 P1 elements. Seven conodont zones based on distribution of stratigraphical conodont elements are established in ascending order as:Sweetognathus aff. whitei Zone, S. guizhouensis Zone, S. subsymmetricus Zone, Mesogondolella nashuiensis Zone, M. idahoensis lamberti Zone, J. aserrata Zone and J. postserrata Zone. Among these zones, the S. guizhouensis Zone, S. subsymmetricus Zone and Mesogondolella nashuiensis Zone are newly established in this study. Indicating a time interval from Artinskian of the Early Permian to early Capitanian of the Middle Permian by correlating with conodont zonations in typical area.We didn’t collect any samples from the upper part of bed 10 and bed 11 except SDC-032 and SDC-033, because there are many cherty beds and post-dolomitization are popular in this interval, and what a result we failed to observed J. nankingensis, the index element for the Cisuralian-Guadalupian boundary. But what can be comfirmed is that the C/G boundary is within the sample barren interval base on the conodont sequence and the fusulinids. The base of the Kungurian stage is tentatively defined by the first occurrence of S. guizhouensis,174.5 m above the base of the Sidazhai Formation.A total of 1436 conodont specimens are obtained from the Xinyuan Formation, from which 632 P1 elements are distinguished and 34 species of 12 genus are identified based on the analysis of Pi elements. Four conodont zones are established in ascending order as:Nicoraella kockeli Zone, Paragondolella bifurcata Zone, Neogondolella constricta Zone and N. constricta cornuta Zone, these zones can be correlated with conodont zones in other sections of the adjacent area and suggests Pelsonian substage and Illyrian substage of the Anisian stage of middle Triassic. Conodont characteristics shows no element of Ladinian, however, elements obtained from upper intervals (bed36-38) of the limestone Member of the Xinyuan Formation indicates that the top of the Xinyuan Formation is closed to the Anisian/Ladinian boundary. The base of the Pelsonian substage is defined at 2.30m above the base of bed 1 according to the first occurrence of Ni. kockeli. Based on the first occurrence of P. bifurcata, boundary between the Pelsonian substage and the Illyrian substage is defined at 37.80m of bed 9.The world-famous Luoping Biota, Yunnan and Panxian Fauna, Guizhou were reported with advanced preservation in horizons within the Ni. kockeli Zone, which can be correlated to a great extent with the same biozone at Shaiwa section. Conodont zonation of the Xinyuan formation at Shaiwa section can do the service of improving correlations of Middle Triassic of deeper-water basinal facies in Guizhou, as well as providing the framework of discussing the relationship between death of marine reptile in Luoping, Yunnan and Panxian, Guizhou and the environmental change, and the mechanism of environmental change in deeper-water district.