Age constraints of the Hungshihyen Formation (Early to Middle Ordovician) on the western margin of the Yangtze Platform, South China: New insights from chitinozoans

The Hungshihyen Formation represents the Lower to Middle Ordovician near-shore siliciclastic deposits on the western margin of the YangtzePlatform, South China. However, its age is still debated, largely because of insufficient studies. Here we documented the Bursachitina maotaiensis–B.qianbeiensis chitinozoan assemblage from the lower part of the Hungshihyen Formation at Songliang, Qiaojia County, eastern Yunnan Province.This assemblage is remarkably similar to an assemblage that roughly correlates to the middle part of the Corymobograptus deflexus graptoliteBiozone in northern Guizhou Province, indicating that the Hungshihyen Formation at Songliang is probably of late Floian age and may extendinto the early Dapingian. Additionally, a revision on the chitinozoans previously documented from the strata assigned to the same formation in theWuding area was made according to the latest classification. The revision allows the recognition of the Euconochitina symmetrica–Eremochitinabrevis assemblage, and therefore suggests a late Tremadocian age for this interval based on regional and global chitinozoan biostratigraphiccorrelations. Considering also its lithologic succession, we argue that the chitinozoan-bearing strata in the Wuding area previously assigned tothe Hungshihyen Formation correspond most likely to the uppermost part of the underlying Tangchi Formation or the lowermost part of theHungshihyen Formation. The base of the Hungshihyen Formation appears therefore to be diachronous in eastern Yunnan Province