Distribution of the archaeocyath-calcimicrobial bioconstructions on the Early Cambrian shelves

Abstract

The differences and variety of structural, depositional, and compositional features observed in the Early Cambrian microbial-archaeocyath buildups preserved in the present-day continents, suggest a direct correlation between the physico-chemical factors of deposition and the structural architecture of the buildups. This can be explained in terms of their palaeogeographic collocation on the shelves (depth, energy), and hence of the areal distribution of epeiric basins and perioceanic/pericontinental platforms. Data on the analysis of biohermal communities and their architectures indicate that the relative development of the main reef-building components, as well as their evolution within the reef communities, reflects the dominant physico-chemical factors, mainly temperature and nutrient availability, and the physiography of the primary depositional setting. The bioaccumulations show different reef building styles, defined by the types of associated facies and by the early diagenetic features. They are represented by (i) mud-supported simple ( Kalyptrae) to compound mounds locally with stromatactis-like cavities; (ii) dendrolitic bioherms and crustose buildups with large shelter cavities and low synoptic relief; (iii) cement-supported skeletal reefs with wave resistant frameworks often associated with oolitic shoals, and (iv) bioclastic sands, developed at photic and shallow sub-photic depths on low-angle/low-energy ramps (i–ii) or on high-energy conditions on platforms distally rimmed (iii) or occasionally swept by storm currents (iv). The results of the analysis provide information on the spatial conditions of the primary depositional settings of the first metazoan involved in reef building in the history of the Earth, and suggest that the architecture of the bioconstructions was controlled not only by the physiography of the depositional setting and global/astronomic climate but also by local climatic conditions constrained by the latitudinal distribution of the Early Cambrian continental blocks.