奥陶系宝塔灰岩网纹构造成因研究进展及新发现

综述了扬子地区分布广泛的奥陶系宝塔灰岩网纹构造成因的研究进展,整合了多位学者的主要观点及认识,结合四川旺苍唐家河等地奥陶系剖面的考察与镜下薄片观察,对网纹构造及其成因进行了深入研究并提出未来研究方向的思考与建议。学界目前普遍认为这种网纹构造并非暴露干裂成因,生物遗迹、海底硬地、准同生变形构造等成因解释也缺乏足够的证据支持,但关于水下胶缩、成岩作用以及构造作用等解释仍在热烈讨论中;对四川广元旺苍唐家河剖面宝塔灰岩的野外考察过程中发现了一些新的现象,比如在灰色宝塔灰岩中观察到了非沥青充填的“黑色细纹”,网纹构造的形态、泥质含量等也随沉积旋回而呈现出规律性变化。进一步研究这些新的现象或许能为后期确定网纹构造的成因提供新的证据。

Research of the Origin of Network Structure in the Ordovician Pagoda Limestone:Advances and New Discovery

Pagoda limestone,which is widely distributed in Yangtze area,refers to a set of micritic limestone or bioclastic micritic limestone in Ordovician Pagoda Formation.The unique network structure is developed pervasively throughout the entire Pagoda Formation in bedding-plan view,and these networks are similar to turtle shell or horseshoe,so it is also known as "turtle-shell cracks" or "horseshoe cracks".In this study,combining with the field survey in Tangjiahe section of Wangcang in Sichuan province,we systematically reviewed the literature on the issue of the unique network structure,and summarized the main views about the explanations.Up to now,agreements have been achieved that the cracks should not be considered as an evidence of desiccation during surface exposure;biogliph,hardground and other origins are lack of evidence;while syneresis,diagenesis and other causes are still in heated discussion.In addition,some new phenomena were observed on Tangjiahe section,for example,we found the black fine lines on the bedding surface of gray Pagoda limestone.Both the existing problems and the new findings may boost the study on the origin of the special network structure in Pagoda Formation.They can also help improve our understanding of the mechanisms of the special networks.