Abstract:

There are a great many oncoids in the limestone of Pingyi basin. By means of field inspection and microscope observations, the host rocks as well as associated fossils and sedimentary structures were investigated. Some of the chemical compositions were also measured to discuss the environmental features. Oncoids were found intensively distributed in the environment which was characterized by shallow water, medium or mediumlow energy. Slight amount terrigenous influx was discovered to contribute to the growth of oncoids, while superfluous influx was found adverse. Oncoids were divided into 3 types: regular micrite laminaes oncoids (type A), regular microspar laminaes oncoids (type B), irregular micrite laminaes oncoids (type C). Type A, which was mainly represented by macrosize, multinucleus, regular multilaminaes, could be considered as the mark of shallow water, mediumenergy, slight amount influx. Type B, which was characterized by light color, wellsorted microsize, microspar laminaes, could represent shallow water, relatively highenergy environment. Type C was identified by irregular laminaes, and could represent the low energy condition. Type A and B indicated lowporosity rocks, while type C indicated comparatively favorable pore space.

Key words: Pingyi basin, Paleogene, lacustrine limestone, oncoid, sedimentary environment