The oncoids genesis in the Middle Member of the Guanzhuang Formation of Eocene in Pingyi Basin
平邑盆地下第三系官中段核形石成因分析

Many types of oncoids are well developed in the Middle Member of the Guanzhuang Formation of Eocene in Pingyi Basin. By the research of the internal structure, six types of the fundamental lamina are classified, which are controlled mostly by the developed degree of the algae, water dynamics and species of algae. The different lamina grouping types reflect sedimentary environment. It is discovered that the algal core coherents detritus only on the top and the bottom of the core forms regular thin melalamina. Consequently, two genetic types of the oncoids (shallow water-high energy and deep water-low energy oncoids) are classified according to the grain size, the types of the lamina groups, the amount of the lamina and the filling material. The genetic analysis and classification of the oncoids are helpful for the indication of the sedimentary environment.