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Middle Permian sponge–microencruster bioherms in the Akiyoshi Limestone, SW Japan: implications for Late Palaeozoic reef evolution on mid-Panthalassan atolls


T. Nakazawa, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba 305-8567, Japan.



Middle Permian lagoonal bioherms are described from mid-Panthalassan atoll carbonates (Akiyoshi Limestone) in the Akiyoshi accretionary complex, SW Japan. The bioherms are composed of frame-building sphinctozoan and inozoan sponges, together with encrusting organisms, including chaetetids, Shamovella, calcareous red algae, and other microbes. Of them, sphinctozoan sponges, Shamovella, and microbes (forming microbialites) are the most dominant. Chaetetids, which were predominant in the Early Pennsylvanian but rapidly decreased in the late Moscovian, are also common in these bioherms. Archaeolithoporella and bryozoans are rare, probably due to the low-energy lagoonal environment.

This sponge–microencruster community replaced the Gzhelian–Asselian Palaeoaplysina–microencruster community on the Akiyoshi atolls during the Sakmarian–Kubergandian interval. This drastic change in the reef-building community type almost coincides with the rapid deglaciation of the Gondwanan ice sheet and superplume activity beneath the mid-Panthalassan ocean. The prosperity of calcareous sponges was possibly controlled by environmental factors such as warming and eutrophication after the Gondwanan glaciation. Copyright © 2012 John Wiley & Sons, Ltd.