Evolution pattern of Early Permian carbonate buildups: With reference to the carbonate mounds in eastern Inner Mongolia, North China
Introduction
Early Permian (Cisuralian) witnessed the end of maximum of Late Paleozoic glaciations, a major paleoclimatic change from an icehouse to a greenhouse (Fielding et al., 2008; Montañez and Poulsen, 2013). During this period, carbonate buildups (reefs and mounds) were mostly constructed by organisms, such as phylloid algae, Palaeoaplysina, Shamovella, sponges and bryozoans, in the tropical Tethyan and the subtropical Pangean domains (Wahlman, 2002). These constructers are commonly thought to have been replaced by sponges, corals and bryozoans in the Middle and Late Permian carbonate buildups (Weidlich, 2002; Nakazawa et al., 2011; Chen et al., 2019). However, some studies indicate that the actual situation is much more complicated than current estimates, e.g. the disappearance of Palaeoaplysina and phylloid algae in Artinskian carbonate buildups (Davies et al., 1989), much earlier than the Middle Permian, and the drastic decline of Kungurian carbonate buildups (Flügel and Kiessling, 2002). Therefore, the evolution pattern of Early Permian carbonate buildups remains to be further explored.
According to the paleogeographic map, North China was located in the transitional region between the tropical Tethyan and subtropical Pangean domains during the Early Permian (Scotese and McKerrow, 1990; Scotese, 2003). Carbonate buildups from North China is thus of great significance to fully understanding the evolution pattern of Early Permian carbonate buildups. However, so far, they are still poorly understood, although individual examples have been reported in the last decade (Tian et al., 2011; Yan et al., 2017). In this paper, we first describe in detail the Early Permian carbonate mounds from Sonid Right Banner, eastern Inner Mongolia, North China, and then focus on the origins of lime mud. Subsequently we integrate the observations and interpretations into a construction model for carbonate mounds. Finally, we discuss the evolution pattern of Early Permian carbonate buildups on a global scale, and explore its link with paleoclimatic changes.
Section snippets
Geological setting and stratigraphy
The Deyanqimiao section is located 20 km east to Zhurihe station of Sonid Right Banner, eastern Inner Mongolia (Fig. 1). Tectonically, the study area is in the middle Xing-Meng Orogenic Belt, which is resulted from the closure of the Paleo-Asian Ocean (e.g., Xiao et al., 2003; Xu et al., 2014; Zhu and Ren, 2017). The complicated evolution history of the Paleo-Asian Ocean has always been a topic of hot debate (e.g., Khain et al., 2002; Xiao et al., 2003; Windley et al., 2007; Jian et al., 2010;
Definitions and methods
Carbonate buildup in this paper follows the definition of Heckel (1974), i.e. a circumscribed carbonate mass with topographic relief above equivalent deposits and showing different nature in comparison with typically thinner sediment and surrounding and overlying rocks. Carbonate mounds were defined by James and Bourque (1992) as those structures which were built by smaller, commonly delicate and/or solitary elements in tranquil settings.
Carbonate mounds occur in Units II and III of the Fourth
Sedimentary characteristics of carbonate mounds
The Early Permian carbonate mounds at the Deyanqimiao section are mound or lentoid-shaped, and generally isolated (Fig. 5). Their thickness varies from 1.1 to 5 m, and lateral extent varies commonly from 2.6 to 11.6 m (maximum, 50 m) (Fig. 5). Several mounds are superposed upon each other in the upper part of the section, reaching 11 m in thickness (Fig. 5A). They developed on and are overlain by thin- to medium-bedded lime mudstone or thin- to thick-bedded skeletal wackestone that include
Lithofacies of non-mounds and interpretation of depositional environments
Seven lithofacies of non-mounds are distinguished in the Fourth Member of the Amushan Formation at the Deyanqimiao section: medium- to thick-bedded packstone, thin- to thick-bedded wackestone (phylloid algae wackestone, echinoderm wackestone and fusulinid wackestone), thin- to medium-bedded lime mudstone, sandy limestone, conglomerate, calcareous sandstone, and shale (Fig. 9, Fig. 10, Fig. 11). The details of these lithofacies are summarized in Table 2. They can be grouped into three
Model of carbonate mound construction
As mentioned above, carbonate mounds at the Deyanqimiao section were characterized by high content of lime mud, tubiform microbes, and different assemblages of skeletal grains in various environments: phylloid algae-echinoderm mound and fusulinid-echinoderm mound in mid-ramp, and echinoderm mound in outer ramp (Fig. 3, Fig. 12). Echinoderm is the common element in all three types of mounds, while phylloid algae and fusulinids occur in phylloid algae-echinoderm mound and fusulinid-echinoderm
Evolution pattern of Early Permian carbonate buildups
Wahlman (2002) classified Early Permian carbonate buildups into a tropical Tethyan domain and a subtropical Pangean domain. The former are characterized by phylloid algae buildups, sponge-phylloid algae buildups, sponge-bryozoans and/or Shamovella buildups, bryozoans and/or Shamovella-dominated buildups, and foraminifer-brachiopod buildups, while the latter are composed of Palaeoaplysina buildups, Palaeoaplysina-phylloid algae buildups, phylloid algae buildups, microbial buildups, and bryozoan
Conclusions
For the first time, we report the Sakmarian-Kungurian (Early Permian) carbonate mounds in the Amushan Formation at the Deyanqimiao section from eastern Inner Mongolia, North China. They are characterized by high content of lime mud, tubiform microbes, and various proportions of phylloid algae, echinoderms and fusulinids. Lime mud in mounds originated from the micritization of skeletal grains, microbial induced precipitation, and/or phylloid algal degradation.
We propose a model for the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to acknowledge Zhongqiang Chen, an anonymous reviewer and the editor for their comments and suggestions, which greatly improved this manuscript. We express our sincere thanks to Wen Guo (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences) for his help in the field work. We also thank Natsuko Adachi (Osaka City University) and Hao Huang (Institute of Geology, Chinese Academy of Geological Sciences) for their invaluable comments. This research was supported
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