Lopingian (Late Permian) foraminiferal faunal succession of a Paleo-Tethyan mid-oceanic carbonate buildup: Shifodong Formation in the Changning–Menglian Belt, West Yunnan, Southwest China



This paper deals with a Lopingian (Late Permian) foraminiferal faunal succession of the Shifodong Formation in the Changning–Menglian Belt, West Yunnan, Southwest China, which has been geologically interpreted as one of the closed remnants in East Asia of the Paleo-Tethys Ocean. The Shifodong Formation is the uppermost stratigraphic unit in thick Carboniferous–Permian carbonates of the belt. These carbonates rest upon bases consisting of oceanic island basalt and are widely accepted as having a Paleo-Tethyan mid-oceanic (seamount- or oceanic plateau-top) origin. Sixteen taxa of fusuline foraminifers and 37 taxa of smaller (non-fusuline) foraminifers are recognized from the type section of the Shifodong Formation located in the Gengma area of the northern part of the Changning–Menglian Belt. Based on their stratigraphic distribution, three fusuline zones can be established in this section: they are, in ascending order, the Codonofusiella cf. C. kwangsiana Zone, Palaeofusulina minima Zone, and Palaeofusulina sinensis Zone. These three biozones are respectively referable to the Wuchiapingian, early Changhsingian, and late Changhsingian, of which the Wuchiapingian is first recognized in this study in the Changning–Menglian mid-oceanic carbonates. The present study clearly demonstrates that the foraminiferal fauna in a Paleo-Tethyan pelagic shallow-marine environment still maintained high faunal diversity throughout the almost entire Lopingian, although the very latest Permian fauna in the upper part of the Palaeofusulina sinensis Zone of the Shifodong section records a sudden decrease in both faunal diversity and abundance. Moreover, the Shifodong faunas are comparable in diversity with those observed in circum-Tethyan shelves such as South China. The present Paleo-Tethyan mid-oceanic foraminiferal faunas are definitely more diversified than coeval mid-oceanic Panthalassan faunas, which are typically represented by those from the Kamura Limestone in a Jurassic accretionary complex of Southwest Japan. It is suggestive that the Paleo-Tethyan mid-oceanic buildups presumably supplied a peculiarly hospitable habitat for foraminiferal faunal development in a pelagic paleo-equatorial condition.