中上扬子地区下寒武统筇竹寺组结核体发育特征及沉积环境意义

以四川盆地及周缘露头和钻井资料为基础，系统研究了下寒武统筇竹寺组结核体发育特征及沉积环境意义，得到3点认识：①筇竹寺组结核体产出层位与岩相特征区域差异大。在川东—鄂西—湘黔和川南坳陷区，结核体主要发育于SQ1段，并以钙质硅质混合页岩相、硅质页岩相、云质页岩相和白云岩相为主，黏土含量普遍在25%以下且明显低于围岩，TOC含量为下部核体高、上部核体低。在川中—川北坳陷区，结核体在SQ1段和SQ2段均发育，且以钙质页岩相和泥灰岩相为主，钙质明显高于围岩，黏土质较围岩显著降低，TOC含量普遍在1.50%以下。结核体内岩石矿物成分差异明显，钙质、硅质等脆性矿物主要富集于结核体中心区，黏土质和有机质则富集于结核体边缘，GR值大多表现为在核体中央区为相对低值响应、在核体边缘响应值明显升高的显著特征。②结核层普遍赋存于TOC含量在0.7%以上的炭质页岩、粉砂质页岩和硅质页岩中，在SQ1段TOC>2.0%的炭质页岩和硅质页岩中尤其发育，在TOC>0.2%深灰色粉砂质页岩中也偶尔产出，显示结核体与深色页岩（尤其优质页岩）具有共生关系。③结核体为扬子海盆裂陷活动的重要沉积响应，根据筇竹寺组结核体分布特征及其与黑色页岩的配置关系，将筇竹寺期扬子海盆的裂陷活动划分为发展期（SQ1时期）、调整期（SQ2时期）和萎缩期（SQ3时期）3个期次。受SQ1时期大规模裂陷作用和上升洋流活动共同控制，扬子海盆结核体和富有机质页岩广泛发育于筇竹寺组下部。在裂陷调整期和萎缩期，随着裂陷活动减弱和萎缩，构造活动以区域抬升为主，水体变浅，洋流活动向东南方向退却，结核体和富有机质页岩仅在少部分裂陷区呈小规模沉积。

Characteristics and environmental significance of concretion in the Lower Cambrian Qiongzhusi Formation in the Middle-Upper Yangtze area

Based on the outcrop and drilling data of the Sichuan Basin and its periphery， this paper reveals the concretion developmental characteristics of the Lower Cambrian Qiongzhusi Formation and its significance to the environment. Three preliminary conclusions are drawn： （1） The regional difference between the exposure layer and the lithofacies characteristics of the concretion in the Qiongzhusi Formation is large. In the eastern Sichuan， western Hubei， western Hunan， northern Guozhou and southern Sichuan Depression area， the concretion develops in the section of SQ1， and is mainly composed of calcareous siliceous mixed shale facies， siliceous shale facies， dolomitic shale facies and dolomitite facies. The clay content is generally less than 25% and is significantly less than the surrounding rock， and the TOC is generally higher in the lower concretion layers and less in the upper concretion layers. In the central Sichuan and northern Sichuan depression areas， the concretion appears in the sections of SQ1and SQ2， and is dominated by calcareous shale facies and marlstone facies. The calcic content is obviously higher than that of surrounding rock， and the clay content is less than that of surrounding rock， and the TOC is less than 1.50%. Because mineral composition difference is high， most of the GR curves of the concretions showed trough response characteristics， reflecting the central region enriched with calcium or siliceous， and the edge enriched with clay and organic matter. （2） The concretion layer generally occurs in carbonaceous shale， silty shale and siliceous shale with a TOC over 0.7%， and especially develops in carbonaceous shale and siliceous shale of SQ1， and occasionally appears in grey silty shale with a TOC over 0.2%， showing that the concretion layer is basically symbiotic with grey shale （especially with high-quality shale）. （3） The concretion is a kind of important sedimentary response of faulted depression in marine basin. According to the distribution characteristics of concretion and black shale in the Qiongzhusi Formation， the depression process of Yangtze platform can be divided into three periods including development period（SQ1）， adjustment period（SQ2） and abortion period（SQ3）. In the period of SQ1， controlled by massive rifting and upwelling in Yangtze platform， the concretion and organic rich shale widely develop in the Lower Qiongzhusi Formation. In the periods of SQ1 and SQ2， while rifting activity of Yangtze platform decreases and shrinks， and while sea level falls and upwelling recedes to the southeast， the concretion and organic rich shale just appear in the local depression area.