ONCOLITES FROM THE LIANGLITAG FORMATION(KAITIAN,UPPER ORDOVICIAN),TAZHONG,TARIM BLOCK,NW CHINA

Morphological features of the coating laminae are of the main parameters for the classification of the oncolites. The shapes of the coated lamination are somewhat controlled by the energy condition. Therefore,oncolites morphology is a proxy for reconstruction of the palaeoenvironment. Oncolites occur abundantly in the limestone of the Lianglitag Formation(Kaitian,Upper Ordovician) in Tazhong,Tarim Block,NW China. The rounded or elliptical onco-lites are favorable in grainstone and packstone members,indicating relatively mid-high energy settings. The normal ones often occur in high-energy environments,while the superficial oncolites occur in lower-energy environments. Once the oncolites were agitated by weak currents,they rested or turned over intermittently and resulted in inconsecu-tive lamina. The sub-spherical and compound shape oncolites were formed in the low-energy environment with weak agitation. The sub-spherical oncolites were formed in wackestones,indicating a relatively lower-energy environment. The compound oncolites possess the characteristics of the sub-spherical as well as the superficial cases and could be the indicator of the alternated mid-high energy environment.