山西兴县关家崖剖面马五_5亚段核形石灰岩特征及成因

在观察兴县关家崖野外露头剖面马五_5亚段核形石灰岩岩性特征的基础上,进行了薄片镜下鉴定,开展了全岩X衍射,主、微量元素,锶和碳、氧稳定同位素及有机碳分析。研究结果表明,核形石灰岩形成于开阔海环境,准同生阶段核形石发生重结晶作用,浅埋藏期压实作用使巨晶方解石呈镶嵌接触并产生压力双晶纹;马五_5亚段底部核形石灰岩形成于海平面上升过程,未受早期淡水溶蚀;马五_5亚段顶部核形石灰岩形成于海平面下降过程,明显受早期大气淡水淋滤与溶蚀作用影响;在中—深埋藏阶段受地下热液影响,在水-岩作用过程中发生了同位素交换及有机质成熟及排烃作用。该套核形石灰岩的发现,明确了马五_5亚段形成于开阔海环境,这对于马家沟组烃源岩的研究具有参考意义。

Characteristics and Genesis of Middle Ordovician Majiagou Submember-55 Oncolitic Limestone at Guanjiaya Outcrop in Xingxian,Shanxi

Several layers of oncolitic limestone that are subordinate to Middle Ordovician Majiagou Submember-55 are discovered at Guanjiaya outcrop in Xingxian area, Ordos Basin. Based on the outcrop observation and microscopic thin-section identification to lithologic characteristics of the oncolitic limestone, some experimental analysis of outcrop samples were carried out, including whole rock X-ray diffraction, major and trace elements, C, O and Sr stable isotopes and organic carbon. It is indicated that the oncolitic limestone formed in tidal flat environment in open sea, and recrystal ization of oncolite took place in the penecontemporaneous stage. Megacrystal ine calcites presented mosaic contact under compaction and pressuring twin-crystal lamellae formed in shallow burial stage. The oncolitic limestone at the bot om of Submember-55, which formed during sea level rising, was not affected by early freshwater leaching and corrosion. Otherwise, the oncolitic limestone on the uppermost, which formed during sea level fal ing, was evidently affected by the leaching and corrosion. Led by underground hydrotherm, isotope exchange, organic maturation and hydrocarbon expulsion took place in water-rock interaction during the moderate-deep burial stage. The discovery of this set of oncolitic limestone makes it clear that the Majiagou Submember-55 limestone formed in the environment of open sea, which can provide important reference for the research of Ordovician Majiagou source rock.