渝东北地区观音桥段沉积与有机碳同位素特征及其区域对比

Sedimentary and Organic Carbon Isotopic Characteristics of the Kuanyinchiao Member in Northeastern Chongqing and Its Regional Correlation

渝东北地区五峰组—龙马溪组间观音桥段厚度不一、沉积特征各异,自西向东对庙坝渝参1井(YC1)、田坝剖面(TBP)、白鹿剖面(BLP)、九湖四方洞剖面(SFP)和巴东两河口(LHP)剖面的分析表明,观音桥段以发育浅灰色中—薄层状泥质灰岩为主,局部为含介壳泥灰岩层,且强烈风化后常呈褐黄色、土黄色。研究区内两河口剖面为灰黑色泥质灰岩,也发育介壳化石,四方洞剖面观音桥段表现为粉砂质含量的明显增多。仅田坝剖面和两河口剖面见赫南特贝化石印模。各剖面有机碳同位素垂向演化特征相似,具有区域及全球对比性;在赫南特期观音桥段均表现出明显的碳同位素正漂以及峰值,可能受局部环境影响,其正漂幅度不尽相同;五峰组至观音桥段碳同位素呈震荡性升高,可出现短暂的小型峰值;观音桥段至龙马溪组,碳同位素呈现持续稳定的下跌,下跌速度取决于龙马溪组厚度。

The thickness and sedimentary features of the Kuanyinchiao Member of Wufeng Formation and Longmaxi Formation in northeastern Chongqing vary greatly. Analysis of five profiles, Well YC1, Tianba profile （TBP）, Bailu profile （BLP）, Sifangdong profile （SFD） and Lianghekou profile （LHP） from west to east, shows that the Kuanyinchiao Member is dominated by grayish medium-thin bedded argillaceous limestone, locally containing shelly fauna （the Hirnantia fauna）. It is common that argillaceous limestone is intensively weathered into brownish-yellow. The Kuanyinchiao Member is characterized by grayish- black argillaceous limestone and hosting shelly fauna in LHP, and increasing content of silty or sandy grains apparently in SFP. The characteristics of organic carbon isotopic distribution in these profiles show many resemblances, and can be correlated regionally and even globally. The Kuanyinchiao Member of Hirnantian Stage shows positive δ13 Corg excursion with a roughly similar amplitude and peak value probably due to local environment perturbation. The δ13Corg value increases pulsatively from the Wufeng Formation to Kuanyinchiao Member, with a few peaks. On the contrary, δ13Corg shows continuous, steady fall from Guangyinqiao Member to the Longmaxi Formation, with drop rate depending on the thickness of the Longmaxi Formation.