Late Ordovician stromatoporoids from the Xiazhen Formation of South China: Paleoecological and paleogeographical implications

摘要

The Upper Ordovician Xiazhen Formation, which crops out in Zhuzhai, near the Jiangshan-Changshan-Yushan region of southeast China, consists of a variety of carbonate-siliciclastic lithofacies representing a shallow-water platform environment. A total of eleven stromatoporoid genera in two orders, comprising three clathrodictyid genera (Clathrodictyon, Ecclimadictyon, and Labyrinthodictyon) and eight labechiid genera (Cystostroma, Rosenella, Pseudostylodictyon, Aulacera, Stylostroma, Pachystylostroma, Labechia, and Labechiella), are recognized from 18 stratigraphic units representing a wide range of depositional environments in the formation. Clathrodictyon is the most abundant stromatoporoid genus, occupying a long stratigraphic range and a wide range of lithofacies. It also occurs as a major component of patch reefs throughout the formation. The labechiids, in contrast, are highly diverse but most genera have restricted stratigraphic distributions, occurring in a narrow range of depositional environments. The size and growth form of stromatoporoids in the formation are considered to be primarily intrinsic characteristics of each genus but in some cases also seem to be related to lithofacies: clathrodictyids exhibit a wide spectrum of growth forms and have broader ecological plasticity and more flexible growth strategies than do labechiids. The Late Ordovician stromatoporoids of the Xiazhen Formation exhibit close biogeographic affinities to those of central New South Wales and the Tasmanian shelf of Australia. The discovery herein of Labyrinthodictyon and Stylostroma from the Upper Ordovician of South China further supports this close relationship. The stromatoporoids from South China exhibited one of the highest diversities of all paleocontinents during the Late Ordovician, suggesting that South China, together with Australia, was a locus of Late Ordovician stromatoporoid diversification.