Characteristics and genesis of the oncolites from the Jurassic strata in the Qiangtang Basin

Unlike common oncolites,no laminated structures and no algal organisms are observed in the coatings of the oncolites from the Jurassic strata in the Qiangtang Basin.The cores generally contain bioclastics of varying sizes.Both the oncolites and their interstitial materials consist of micritic calcite.The oncolites occur mainly in the lower-energy lagoon and restricted shelf environments.