四川盆地下志留统龙马溪组结核体发育特征及其沉积环境意义

Development characteristics of concretions in the Longmaxi Formation of Lower Silurian in the Sichuan Basin and the indicating significance of their depositional environment

目前,对于四川盆地下志留统龙马溪组结核体发育特征及其对黑色页岩段沉积环境指示意义的研究甚少。为此,以该盆地川南—川东坳陷及周缘的露头和钻井资料为基础,通过分析自然伽马、岩石矿物、薄片、地球化学等资料,并结合围岩岩相、地球化学、电性等地质特征的研究结果,探究龙马溪组结核体的发育特征,揭示其对沉积环境的指示意义。研究结果表明:①龙马溪组结核体产出层位与岩相特征区域差异大,结核体层在川南—川东南坳陷区发育于Coronograptus cyphus、Demirastrites triangulatus笔石带中,以钙质硅质混合页岩相、钙质黏土质混合页岩相、黏土质硅质混合页岩相和钙质页岩相为主,在川东—鄂西坳陷区发育于Demirastrites triangulatus、Lituigrapatus convolutus笔石带中,以黏土质硅质混合页岩相、硅质页岩相和钙质硅质混合页岩相为主;②结核体层自然伽马曲线呈低谷响应特征,核体中心富集钙质或硅质,边缘富集黏土质和有机质;③结核体层多赋存于TOC介于0.8%~2.4%、黏土含量超过40%、脆性指数低于50%的碳质页岩和黏土质页岩中,与优质页岩不共生;④鲁丹阶结核体层可作为川南坳陷腹部Coronograptus cyphus带中段对比标志层,埃隆阶结核体层不能单独作为区域对比标志层;⑤结核体主要形成于同沉积—早期成岩阶段,发育于前陆挠曲初期和前陆挠曲发展期,是前陆期深水—半深水陆棚相快速沉积的产物,结核体层的空间分布特征反映出前陆挠曲坳陷自南向北迁移。结论认为,结核体层与笔石、斑脱岩密集段相结合,对于笔石带分层和优质页岩分布研究具有重要的参考价值。

So far,the development characteristics of concretions in the Longmaxi Formation of Lower Silurian and the indicating significance of their depositional environment of black shale have been rarely studied.In this paper,the GR,rock mineral,thin section,organic geochemistry and other data were analyzed.Then,based on the outcrop and drilling data of the southern-eastern Sichuan Depression and its periphery,combined with the research results on the geological characteristics of the adjacent rocks(e.g.lithofacies,geochemistry and electrical property),the development characteristics of concretions in the Longmaxi Formation were explored and the indicating significance of their depositional environment was revealed.And the following research results were obtained.First,the occurrence horizons and lithofacies characteristics of concretions in the Longmaxi Formation are regionally different.In the southern and southeastern Sichuan Depression,the concretionary layers develop in the Coronograptus cyphus and Demirastrites triangulatus graptolite belts,and their lithofacies is dominated by calcareous-siliceous mixed shale facies,calcareous-argillaceous mixed shale facies,argillaceous-siliceous mixed shale facies and calcareous shale facies.In the Eastern Sichuan-Western Hubei Depression,the concretionary layers develop in the Demirastrites triangulatus and Lituigrapatus convolutus graptolite belts,and their lithofacies is dominated by argillaceous-siliceous mixed shale facies,siliceous shale facies and calcareous-siliceous mixed shale facies.Second,the GR curves of these concretionary layers present the characteristics of trough response.The centers of concretions are enriched with calcareous or siliceous,and the edges are enriched with clay and organic matter.Third,concretionary layers are mostly distributed in carbonaceous shale and argillaceous shale with TOC contents of 0.8-2.4%,clay content over 40%and brittleness index below 50%,and they are not associated with high-quality shale.Fourth,the Rhuddanian concretionary layer can serve as the marker bed for regional correlation in the middle segment of the Coronograptus cyphus belt in the central part of the Southern Sichuan Depression,while the Aeronian concretionary layer cannot independently serve as the marker bed for regional correlation.Fifth,the concretions are mainly formed in the stage of syndeposit-early diagenesis and developed in the initial stage-development stage of foreland flexure.They are the products of rapid sedimentation of deep water and semi-deep water continental shelf facies.The spatial distribution characteristics of concretionary layers reflect the migration of the foreland flexural depression from south to north.In conclusion,the combination of concretion layers with graptolites belts and bentonite dense layers is of a great reference value to study the layering of graptolites belts and the distribution of high-quality shale.