Cambrian Series 2 calcimicrobial crust–cement boundstone in the Yangtze Block, China: A distinctive bioconstruction as a legacy of Precambrian reef evolution

Hybrid microbial, skeletal, and abiotic carbonates have changed over time and space on various scales, and are key to understanding sedimentological and palaeoecological records. Although lower Palaeozoic calcimicrobial crust–cement reefs were a characteristic style of bioconstruction that contain abundant marine cement and lack metazoans, their developmental processes remain poorly understood. This study investigated Cambrian Series 2 Girvanella crust–cement reefs in Sichuan Province, China, to elucidate how and why they developed. They comprise a reef complex along with thrombolitic reefs and peloid–intraclast packstone to grainstone containing abundant fragments of these crusts. Girvanella crust–cement reefs are centimetre-scale structures that consist of thin Girvanella crusts and fibrous cement, with rare internal sediments. The thrombolitic reefs consist of mesoclots and subordinate archaeocyaths, with internal sediments infilling nearly all the pore space. The lack of internal sediments and archaeocyaths in these cement-rich reefs, along with the abundance of fragmented crusts in surrounding sediments, indicate that the easily broken and semi-closed calcimicrobial frameworks developed in shallow subtidal conditions. These characteristics may have led to instability of the substrate for settlement of open-surface metazoans and difficulties for larval invasion of cryptic metazoans. This implies that the interplay of the architectural characteristics of these reefs and the environmental setting resulted in the development of lower Palaeozoic microbial–abiotic reefs. These features suggest that lower Palaeozoic calcimicrobial crust–cement reefs, which represent a unique style of Cambrian reef development, reflect aspects of the Cambrian Explosion, rather than an anachronistic Archaean to Palaeoproterozoic reef-like hybrid carbonates.