Abstract:

Oncolite, a main kind of microbialite, has long attracted much scientific attention. Until now, uncertainties remain on the distribution model, sedimentary environment, microbial characteristics and formation mechanisms of oncolite. In the western suburbs of Beijing, the Dingjiatan section contains well-preserved Cambrian sequences. Controlled by the 4^th order sea-level change and hydrodynamic characteristics, the EHST of Changping Formation, Cambrian deposits in subtidal-lagoon environments, including three sets of oncolites with different shapes. The first set of mm-scale oncolite is scattered in the lower part of EHST, whose envelope contains plenty of mud debris, sand-/bio-clastics. The second set of (sub-)cm-scale oncolite concentrates in the middle part of EHST. Axiolitic oncolite has obvious core, and its envelope includes light/bacterial/bioclastic/coated lamina. The third set of cm-scale oncolite is distributed in the upper central part of EHST, with obvious core and envelope of many light lamina and few bacterial lamina. Microscopic identification results show that the oncolite envelope consists of light and dark lamina. Their different ultra-/micro-structures stand for various forms of interactions between microbial mats and sedimentary environment. Through studying the formation environment, this microbial set is related to the biological extinction, although that may not have been decisive.

Key words: oncolite, Changping Formation, Series 2 of Cambrian, Beijing