A Katian microphytoplankton assemblage from Wanhe, Yongshan County, northeastern Yunnan Province, South China: stratigraphy, paleogeography and paleoenvironment

 

Authors: Yan K, Li J, Servais T

Published in: Bulletin of Geosciences, volume 101, issue 1; pages: 157 - 171; Received 22 September 2025; Accepted in revised form 10 April 2026; Online 3 May 2026

Keywords: acritarchs, Katian, Yunnan, South China, paleogeography, biostratigraphy,

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Abstract

This study describes a Katian acritarch assemblage from the Wanhe section, northeastern Yunnan, South China, and discusses its biostratigraphic, paleogeographic, and paleoenvironmental implications. Thirty-eight samples were collected from the Zhenxiong, Lianfeng, Daduhe, and Lungmachi formations, with 11 yielding acritarchs from the upper Lianfeng and lower Daduhe formations. A total of 26 species assigned to 20 genera were identified, including 12 species in open nomenclature. Biostratigraphically, the assemblage is constrained to the late Katian (Ka4) based on co-occurring graptolite biozones (Dicellograptus complanatus, D. complexus) and zircon U-Pb dates (444.65-442.99 Ma) from K-bentonites. Key taxa such as Baltisphaeridium aspersilumiferum, and Cheuletochroa homoia confirm a Late Ordovician age, with some taxa (e.g., Oppilatala sp.) suggesting Silurian affinities. Paleogeographically, the assemblage exhibits mixed affinities with Laurentian/Baltican (e.g., Baltisphaeridium, Hoegklintia) and Gondwanan/peri-Gondwanan (low diversity, limited Navifusa) assemblages, supporting close links between South China, North China, and Tarim during the Katian. In terms of paleoenvironment, the assemblage reflects a deep-water setting (BA4-BA5) with a shallowing trend from the Daduhe Formation to the Kuanyinchiao Bed. A late Katian transgression followed by a latest Katian-Hirnantian sea-level drop may have influenced acritarch distribution. The dominance of Evittia remota, a cosmopolitan taxon, aligns with the “equatorial cold-water tongue ” model, linking its occurrence to Late Ordovician climate fluctuations.