低能环境下核形石组构特征及成因 --以中15 井上奥陶统良里塔格组核形石为例 (Chinese)

Acta Sedimentologica Sinica; Feb2024, Vol. 42 Issue 1, p84-94, 11p

[Objective] A global biological radiation occurred during the Ordovician period, which could be potentially 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 developed 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 domelike 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. [ABSTRACT FROM AUTHOR]

【目的】奥陶纪时期全球发生了一次生物大辐射, 核形石研究对于揭示该期古环境特征及演化具有重要意义。 【方法】以塔里木盆地塔中地区中15井核形石为研究对象, 通过详细地岩心观察和室内薄片、扫描电镜鉴定, 统计、分析核形石 宏观形态、微观组构特征, 探讨了其形成环境、成因及地质意义。【结果】塔里木盆地塔中地区中15井上奥陶统良里塔格组泥质 条带灰岩段为开阔台地的台内洼地沉积, 中上部发育一套厚约12.3 m的深灰色含泥质条带核形石灰岩、含生屑泥晶核形石灰 岩, 核形石粒径大、丰度高。核心为早期台缘生屑、团块;包壳厚度大, 富含细菌纹层;基质中多充填灰泥、细生屑, 普遍发育泥 质条带;代表了一种水体搅动及搬运作用不强烈的原地或近距离搬运沉积成因。核形石可分为球状、帽状、板条状、椭球状等 四类, 以帽状、球状为主。垂向上为三个核形石发育段, 单个旋回含量、粒度呈变大趋势。【结论】球状、帽状核形石形成于水体 搅动和静止相交替状态的环境, 板条状核形石形成于弱搅动的低能环境。中15井核形石的产出受奥陶纪全球海平面上升以及 生物大辐射影响, 具体为丰富的核心物质来源（相邻台缘带生屑、团块）、发育程度高的细菌黏附生长、稳定或弱波动的沉积环 境等因素的有机耦合。. [ABSTRACT FROM AUTHOR]

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