贵州海相拉丁阶-卡尼阶界线研究展望

Research Prospects on Marine Ladinian-Carnian Boundary in Guizhou Province

全球海相拉丁阶-卡尼阶界线(Ladinian-Carnian Boundary,LCB)层型已经确定意大利的Prati Di Stuores/Stuores Wiesen剖面,菊石Daxatina canadensis(Whiteaves)的首现位置标志着卡尼阶的底界.该界线通常被认为与牙形石Quadralella polygnathiformis或者Budurovignathus diebeli的首现接近,然而该界线在我国的确定却存在一定争议.一方面,由于主要的标志菊石Daxatina canadensis缺乏;另一方面,菊石(如Trachyceras)生物地层确定的界线比起牙形石生物地层确定的界线要高许多.如在我国华南海相LCB地层发育最好的贵州西南地区,依据菊石一般将LCB置于竹杆坡组顶部附近,而依据牙形石生物地层,则又置LCB于竹杆坡组底部附近.贵州贞丰龙场大坪子剖面杨柳井组上部发育有牙形石Quadralella polygnathiformis和Budurovignathus属分子共生的过渡层,其时代可能归入卡尼阶的第一个亚阶Cordevolian.是以贵州贞丰龙场大坪子剖面的LCB可能置于杨柳井组上部.但是,确定我国海相LCB仍需综合牙形石、菊石等生物地层,辅以磁性地层以及绝对年龄等多手段的研究来互相印证.

The Global Boundary Stratotype Section and Point （GSSP） for Ladinian-Carnian Boundary （LCB） was defined at Prati Di Stuores/Stuores Wiesen Section in Italy at a level coinciding with the lowest occurrence of the ammonoid Daxatina canadensis. This GSSP level is commonly considered to near the lowest occurrence of the conodonts Quadralella polygnathiforrnis or Budurovignathus diebeli. However, the placement of the LCB within sections of South China is controversial, because the primary ammonoid marker for LCB （D. canadensis） hasn＇t been reported in South China. The working placement of LCB based on ammonoid biostratigraphy （e.g., Trachyceras） is considerably higher than the levels where the conodont biostratigraphy would suggest its position. For example, ammonoids in the southwest Guizhou region with the best marine record in South Chi- na suggest the LCB should be assigned near the top of Zhuganpo Formation, whereas conodont biostratigraphy suggests that the LCB should be at the base of Zhuganpo Fm.. The Dapingzi Section in Zhenfeng County of Guizhou Province, yielded elements of the conodonts Quadralella polygnathiformis and Budurovignathus sp. from the transitional bed near the top of Yangliujing Formation, which suggests an assignment to the lowermost Carnian （the ＂Cordevolian＂ substage, as used by some authors）. Therefore, this conodont assemblage would imply that the LCB in the Dapingzi Section should be placed within the upper part of Yangliujing Formation, below the base of the Zhuganpo Formation. However, to accurately assign the LCB within the marine deposits of South China requires integrating the ammonoid and conodont biostratigraphy with additional stratigraphic correlation methods, such as magnetostratigraphy and U/Pb ages.