Thrombolite reefs with archaeocyaths from the Xiannüdong Formation (Cambrian Series 2), Sichuan, China: implications for early Paleozoic bioconstruction

The early Cambrian incorporation of organisms with calcareous skeletons into microbial reefs initiated the metazoan bioconstructions of the Phanerozoic. Microbial reefs containing archaeocyaths from the middle early Cambrian Xiannüdong Formation of the South China Block are investigated. The Xiannüdong thrombolitic frameworks are composed primarily of Girvanella clumps and crusts, micritic clumps, and subordinate Epiphyton bundles. Amalgamated microbial frameworks contain sparse and rare (<5%) archaeocyaths, with irregular archaeocyaths dominating the regular archaeocyaths by 6:1, and enclosed by Girvanella and other microbial elements. These Xiannüdong thrombolitic reefs are broadly similar to other lower Cambrian thrombolitic reefs containing archaeocyaths, developed around shoals and lagoons. Similar thrombolitic reefs of the middle Cambrian from the Sino- Korean Block and Australia show reduced and increased contributions of Girvanella and Epiphyton, respectively, and the incorporation of lithistid and heteractinide sponges instead of archaeocyaths. These data suggest that the late early Cambrian decline of the archaeocyaths and their nearly instantaneous replacement by other metazoans allowed the continuance of microbial reefs with rare metazoans until the late Middle Ordovician shift in reef construction to metazoan reefs.