重庆綦江观音桥剖面观音桥层的岩性新认识*

重庆綦江观音桥剖面是上奥陶统观音桥层(组)的建层(组)剖面。该剖面上奥陶统和下志留统地层连续,在前人研究中被广泛提及。观音桥层是奥陶系顶部层位,含较多特征明显的赫南特贝化石,可能反映了赫南特冰期海平面下降。在前人相关文献中均提及观音桥剖面观音桥层的岩性为灰岩,但本次研究发现,对观音桥层岩石滴稀盐酸时一般起泡较微弱,显微薄片、扫描电镜、阴极发光测试均显示观音桥层白云石含量通常大于50%,呈微晶—粉晶结构,自形晶—半自形晶,方解石含量常小于15%,因此所观察到的观音桥剖面观音桥层的岩性是(灰质)白云岩。进一步分析表明: 在阴极发光下白云石不发光,方解石呈现较强的红色光; 扫描电镜测试显示白云石多为含铁白云石; 显微薄片显示白云岩中少量白云石发生去云化作用,被方解石交代; 碳、氧同位素测试显示白云石碳同位素较腕足类化石及方解石脉偏负,腕足类化石碳、氧同位素与同期海水不一致。上述分析表明观音桥层碳酸盐岩经历了较强的成岩作用影响,推测观音桥层白云石化主要发生于成岩期。这一认识与80多年来关于綦江观音桥剖面观音桥层岩性的认识不一致,对白云岩的识别有助于更深刻认识奥陶纪—志留纪界限地质事件及环境变化。

New lithological cognition of Guanyinqiao bed at Guanyinqiao section in Qijiang area,Chongqing,Southwest China

The Upper Ordovician Guanyinqiao bed was established based on the Guanyinqiao section located in Qijiang area,Chongqing. In this section,the successive sequence of the Upper Ordovician and Lower Silurian was well developed and widely studied. The Guanyinqiao bed belongs to the uppermost part of the Ordovician strata,with plenty of typical Hirnantia fossils developed,reflecting the decline of sea level in the Hirnantian glaciation. Previous studies believed that the lithology of Guanyinqiao bed is limestone. However,the faint bubbles occur after dripping the diluted hydrochloric acid on rocks. Thin sections,scanning electron microscope(SEM),and cathode luminescence tests show that the content of dolomite in the Guanyinqiao bed is larger than 50%,while the content of calcite is less than 15%. The dolomite shows the microcrystalline-powder crystal structure,euhedral to subhedral crystal. Therefore,the lithology of Guanyinqiao bed should be revised to lime dolostone. The dolomite is nonluminous under cathode luminescence,and calcite presents the strong red light. Most of the dolomite should be ferruginous dolomite according to the SEM tests. The observation of thin sections reveals that dedolomitization occurs for some dolomites,indicating that these dolomites were replaced by calcites. The carbon isotopes in dolomites have the larger negative anomaly compared with that of brachiopod fossils and calcite veins. Furthermore,the carbon and oxygen isotope in the brachiopod fossils is inconsistent with the sea during the same period. The above studies implied that strong dolomitization occurred for the carbonates of Guanyinqiao bed during the diagenetic stage. The results from this paper is inconsistent with the previous studies on the lithology of Guanyinqiao bed in Qijiang area. The identification of dolomite is helpful in understanding the evolution of environment and geological events during the transition interval between the Ordovician and Silurian.