Construction mode of a Middle Permian sponge reef in theChangning–MenglianBelt, western Yunnan, China

Abstract

The Changning–Menglian Belt in western Yunnan, China has long been acknowledged as a major suture zone of the Palaeo‐Tethys in south‐east Asia. Although bearing a wealth of environmental information for this vanished ocean, the Permo‐Carboniferous limestones of seamount resemblance in this belt have only been sketchily studied heretofore in terms of their sedimentary features. This study presents a detailed investigation on a Middle Permian Archaeolithoporella‐microbial‐sponge reef in the Damingshan Formation of these limestones. This reef could be ascendingly subdivided into five parts according to its petrographic and sedimentolotgic properties. The first, third, and fifth parts are characterized by the mixture of Archaeolithoporella‐sponge boundstone, sponge cementstone, microbial boundstone, and skeletal wackestone. They are interpreted to be constructed in a moderate‐energy environment through the following processes: sponges in clusters were partly encrusted by Archaeolithoporella, and mostly toppled over after death; whereafter, spaces within frameworks formed by sponges and/or Archaeolithoporella were occluded by syndepositional cements; during the processes, sponge clusters were bound together by microbial domes, and both acted as bafflers that trapped sediments. In contrast, the construction of the second and fourth parts occurred in a high‐energy environment and is almost devoid of microbial contribution: sponges that were encrusted by Archaeolithoporella toppled over posthumously, and then spaces among them were filled by syndepositional cements, ultimately forming Archaeolithoporella‐sponge boundstone. The construction mode of the studied reef, especially sponge toppling and microbial domal frameworks in‐between sponge clusters, differs from that of most coeval sponge reefs, in which the framework is characterized by erect sponges and microbial encrustations on Archaeolithoporella. Our work not only enriches the sedimentary data of limestones in the Changning–Menglian Belt, but also would hopefully deepen the understanding of the growth model of Permian sponge reefs.