Allogenic succession in Late Ordovician reefs from southeast China: a response to the Cathaysian orogeny

摘要

>Several Late Ordovician (late Katian) reef complexes are known from the border area of Jiangxi and Zhejiang provinces in southeast China. We studied two coral-stromatoporoid reefs exposed in the Xiazhen Formation at Zhuzhai (Yushan, Jiangxi). The reefs have a combined thickness of 7.4 m and are metazoan-dominated with most reef-builders in growth position. Stromatoporoids and tabulate corals constitute the framework of the reefs. Stromatoporoids >(mostly Clathrodictyon) dominate the first unit and show a vertical increase in proportion and dominance from the middle part to the top of the unit, whereas tabulate corals (dominated by Catenipora and Agetolites) are the main reef-builders in the second unit where stromatoporoids are rare. We attribute this change to a greater tolerance of tabulate corals to turbidity, allowing them to thrive in the muddy facies of the upper unit. This facies change is probably related to the increasing terrestrial input from the northwestward expansion of the Cathaysian Land during the late Katian. The Cathaysian orogeny also led to a short-term exposure of the sea floor in the study area, which terminated the reef growth.