Fabric Characteristics and Genesis of Oncoids in a Low-Energy Environment:A case study from the Upper Ordovician Lianglitage Formation in well Zhong-15
[Objective]A global biological radiation occurred during the Ordovician period,which could be poten-tially revealed by analyzing the oncolites formed during this period.[Methods]Taking the oncoids of well Zhong 15 in Tazhong area of Tarim Basin as the research object,through detailed core observation,thin section and scanning electron microscope identification,the macroscopic morphology and microscopic fabric characteristics of oncoids were counted and analyzed,and their formation environment,genesis and geological significance were discussed.[Results]Further confirming that the argillaceous banded limestone member of the Lianglitage Formation in well Zhong-15,Tazhong area,Tarim Basin represents open-platform deposition in an intra-platform depression.A set of dark gray and bioclastic argillaceous banded oncolitic limestones with a thickness of approximately 12.3 m are devel-oped in the middle and upper parts of this member,with a large particle size and high abundance of oncoids.The core consists of the early platform-margin bioclasts and intraclasts.The oncoid cortex is thick and rich in bacterial lamina.The matrix is primarily filled with plaster and fine biological fossil fragments,with commonly seen argillaceous bands.It represents a sedimentary origin of in-situ or little transportation with weak water agitation.The oncoids can be divided into four types based on shape:spherical,dome-like,strip-shaped,and ellipsoidal,in which dome-like and spherical are dominant.Vertically,there are three sequences of oncolitic development,within each of which the content and particle size of oncoids tend to increase from the bottom to the top.[Conclusions]Spherical and dome-like oncoids are formed in the environment where the water body is in an alternating state of agitation and stasis,while strip-shaped oncoids are formed in the low-energy environment with weak agitation.The oncoids of well Zhong-15 result from various factors,such as global sea level rise in Ordovician,biological radiation attributed to organic coupling of rich core material sources(bioclasts and intraclasts in the platform-margin),highly developed bacterial adhesion and growth,and stable or weakly fluctuating sedimentary environment.
oncoidsformation environmentwell Zhong-15Lianglitage FormationUpper Ordovicianwestern Tazhong area
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