| The mass extinction near the Permian-Triassic Boundary (PTB) represents the largest mass extinction in the Phanerozoic record, during which90%-100%of marine invertebrate species included in7Phylums disappeared in the strata sequence of marine facies in South China. In recent years, a large quantity of researches has been done on the mass extinction event and environmental evolution in that period, but most of the work focuses on the interval near the PTB. In fact, the reef ecosystem of Late Permian flourishing widely in South China had disappeared long before the end-Permian mass extinction event. However, the relationship between the disappearance of Late Permian reef ecosystem and end-Permian mass extinction remains unknown.Reefs are one of the most diversified ecosystems in the earth. The numbers of species and biomass in the reef communities can be comparable with those in modern tropical rainforest. Reef is an ecosystem that is highly sensitive to any ecological changes in the environment. Meanwhile, most of organisms in reefs are preserved in situ and can fully reflect the mutual relationships between organisms and environment. Therefore, reefs are ideal place to carry out research in paleoecology.The Changhsingian reefs are widely distributed in the Sichuan basin, middle lower reaches of Yangtze River, Dian-Qian-Gui region. Detailed study has been done on the reef facies, stratigraphic sequence and reef-building organisms. However, most work was carried out in the shallow-water carbonate platform and the relationships between the shallow-water platform reefs and deep-water shelf ecosystem are scarcely concerned. This study is based on the discovery of deep-water shelf environment spongia at Hubei Jingshan in Changhsingian and selected a series of Changhsingian sections in different depths from inshore coral reef, marginal platform sponge reef to deep-water shelf sponge biostrome, trying to gain a clear idea of the evolution history of reefs and the ecological migration of the key reef-building organisms in Changhsingian at Late Permian.Inshore coral reefs developed at Hunan Cili Gaya Section in early Changhsingian, in which reef-building organisms just were compound coral--waagenophyllum. Comprehensive analysis of paleogeography and sedimentary microfacies shows that Hunan Gaoya mainly represents shallow-water carbonate platform inshore build-up. Coral reefs at Gaoya section began to grow at the4-6meters thick of echinoderm-clastic bank in early Changhsingian. With the uplift of eastern marginal platform sponge reefs and the formation of semi-restricted lagoon, coral reef disappeared soon. After that, the coral reef was overlaid by the reef flank facies composed of echinoderm-clastic dolomitic limestone, lagoon facies of dark algae foraminifers’ limestone, and at last by the light grey open platform facies of bioclastic limestone deposition. We get a speculation that it is possible that coral reefs have appeared in the north or west of Gaoya at this period and then coral reef spread on a flat surface to the surrounding forming a reasonably sized coral reefs core facies. Evolution from reef core facies to reef flank facies actually have begun on the top of reef core facies, mainly manifest isolated distribution of compound coral on the top of reef core facies while bottom is closely arranged coral framestone. The further weakening Hydrodynamic force from reef flank facies to lagoon facies form relatively static water environment. Diversity is very low in lagoon, biological assembly characterized by Gymnocodium and foraminifers and also see some gastropod fossils. The depth of the water becomes shallow and the water body becomes unstable from lagoon facies to open platform facies bioclastic limestone. The top of open platform facies bioclastic limestone is microbialites which appearance indicates the beginning of end-Permian.The marginal platform reef facies was built up in the Early-Middle Changhsingian in the Daluokeng section of Hunan Province. The reef-building organisms in sponge reefs mainly include Sphinctozoa, Inozoa, hydrozoan, bryozoan and so on. On the top of the reefs is Waagenophyllum, and the reef is about60m thick. During the earliest Changhsingian, Daluokeng section deposited a few meters strata of tiny crinoids clastic limestone, then record sponge reefs deposition, and coral reefs appeared on the top of sponge reefs. Disappearance of coral reef indicates the collapsion of reef ecosystem, and then deposition on reef flank with echinoderms clastic dolimitic limestone. Then it recorded back-reef facies with lagoon and deposited dark algae foraminifera limestone, and the upper strata with white bioclastic limestone belonged to shallow water facies. Similar to Gaoya section, on the top of bioclastic limestone appeared microbialites, which indicates the beginning of the End-Permian.Gaoya section and Daluokeng section, in Cili, are located at the margin of the North Yangtze platform, and Jingshan section is located in the siliceous shelf along the north side of the platform. The Changhsingian strata in this area were mainly silicalite interbedded with limestone, belonging to the Dalong Formation. Lower part of the Dalong Formation was dominated by the shallow continental shelf facies, partly by the upper shelf facies. The lower strata of Dalong Formation were lower shelf deep-water sedimentation, consist of siliceous and mudstone. The upper strata of Dalong Formation were dominated by carbonate deposition, and the top bed belonged to deep-water siliceous. And sedimentary facies changes correspond to Jingshan section, the fossil fauna also evolved regularity. In this study, five fossiles faunas were recognised in the end-Permian. There were ammonoid-brachiopods fauna, foraminifera gastropoda fauna, radiolarian-sponge spicules fauna, calcareous sponges fauna and sponge spicules-radiolarian fauna. According to the ecological characteristics, the sea-level of the end-Permian changed occurred several times, that is, from rising early to middle-last fall, and then to the lastest period to rise.The discovery of calcareous sponges in Late Permian period of Jingshan section is of great importance for this research. Most mutual adhesion sponges formed the obviously biological skeleton, but the framestones did not make the landform uplift apparently. Therefore, it is called biostrome. Sponge biostrome is about0.45m thick, and the frame-building organisms are quite single, which only exist Peronidella of Inozoa sponges of calcareous sponges. Calcareous sponges were the main reef-building organisms at Late Permian. The appearance of calcareous sponges in Jingshan section on one hand reflects that the sea-level is becoming shallow. On the other hand, it provides important information focusing on the disappearing of the organic reef or biostrome in different water depths, and the ecological migration and ecological adaptation of reef-building sponges.Based on the research of reef-building organism and reef facies stratigraphic sequence in different depths of the water environment, this research comes to the following conclusions:1) According to the research, during the Changhsingian age of Late Permian, the evolutionary sequences of organic reef and biostrome in different paleogeographic background are different. In shallow platform environment, reef-building organisms appear firstly. It forms fringe reef or platform margin reef in the early Changhsingian. While, sponge biostrome only appears in middle-late Changhsingian in deep-water shelf environment.2) Although the reef stratigraphic evolution in different environments exist distinction, organic reef or biostrome ecosystem in different depth of the water environment disappears obviously earlier than that of the end-Permian mass extinction.3) According to the research, the reason of disappearance about coral reef at Gaoya in early Changhsingian is uplift caused by the sponge reef at Daluokeng section. The back-reef lagoon is formed behind the sponge reef uplift, and then makes the coral reef ecological environment worse.4) According to the research, the water depth at platform margin in Daluokeng section becomes shallow, and then coral reef that grows better in shallow water appears. At the same time, it reflects that the coral makes a ecological migration while the sea level becoming shallow.5) The appearance of sponge biostrome in Jingshan deep-water shelf environment is caused by the decline of sea level in middle-late Changhsingian. The decline of sea level makes the shallow water platform no longer fit for the growth of reef-building organism. Instead, the shallow deep shelf becomes a favorable habitat for the ecological migration of reef-building sponge.6) According to the research, only Inozoa participates in frame-building in Jingshan section biostrome, indicating that Inozoa sponges are more adaptable to deep-water shelf environment than Sphinctozoa.7) The research finds the volcano quartz in the algal and foraminifera limestone overlying the reef in the Gaya section. There are also several volcanic clays above the sponge biostrome. The reason of the reef disappear could be related to the volcanism. |
