Cambrian oncoids and other microbial-related grains on the North China Platform

Abstract

This study illustrates features of the Cambrian oncoids and provides a comparison with other microbial-related carbonate grains found in the Cambrian succession of the North China epeiric platform. Based on cortex structures, four types of oncoids were distinguished: thin-cortex (superficial) oncoids, laminated-cortex oncoids, clotted-cortex oncoids, and full-cortex (without nucleus) oncoids. Thin- and clotted-cortex oncoids are often associated with oolites, laminated-cortex oncoids are present within oolitic-bioclastic grainstones, and full-cortex oncoids occur in bioturbated wackestones. The oncoids with nucleus？cortex structures are easily distinguished from other carbonate grains due to the lack of nucleus？cortex structures, and from microbial-related ooids which have more circular shape and more continuous cortex than oncoids. Oncoids without nucleus and with only crudely laminated cortex (i.e., full-cortex oncoids) can be differentiated from microbialite intraclasts and microbial lumps by the following evidences: (1) microbialite intraclasts, either rounded or angular, are characterized by margins that sharply truncate the included calcified microbes or carbonate grains and, in addition, intraclast-bearing conglomerates commonly show clear sedimentary structures such as cross-stratification and normal grading;(2) microbial aggregates have irregular but smooth margins, and rather chaotic inner structures.