四川盆地东部上奥陶统五峰组—下志留统龙马溪组斑脱岩发育特征及地质意义

Developmental characteristics and geological significance of the bentonite in the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation in eastern Sichuan Basin, SW China

基于重庆石柱漆辽上奥陶统五峰组—下志留统龙马溪组剖面,系统研究四川盆地东部斑脱岩发育特征,并分析其地质意义和科学价值。研究得到5点认识:①石柱漆辽剖面共发现6个斑脱岩密集段,主要赋存于凯迪阶、鲁丹阶和埃隆阶的6个笔石带,大部分斑脱岩密集段显示出黏土矿物明显增加、自然伽马曲线出现峰值响应、火山灰与有机碳含量关系不明显等典型特征;②龙马溪组斑脱岩密集段在川东及邻区广泛分布,自然伽马曲线普遍显示尖峰特征,可以成为川东—鄂西地区Coronograptus cyphus笔石带底界和鲁丹阶顶界划分的重要参考界面;③以斑脱岩密集段作为鲁丹阶和埃隆阶分层依据,以此确定鲁丹阶在川东坳陷区沉积厚度为10~40m,但在宜昌上升区的腹部仅沉积Coronograptus cyphus笔石带上段,厚度仅为3~7m;④在宜昌上升区的腹部,五峰组—鲁丹阶缺失至少5.5个笔石带,鲁丹阶沉积时间不足0.4Ma;⑤斑脱岩密集段是奥陶纪-志留纪之交扬子海盆强烈挠曲的重要沉积响应,反映川东坳陷存在拗陷初期、拗陷中晚期、前陆挠曲初期和前陆挠曲发展期等4个构造活动期次,优质页岩主要发育于拗陷初期—拗陷中晚期。

Based on the Qiliao section of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in Shizhu,Chongqing city,the development of bentonite in eastern Sichuan Basin was examined systematically,and its geological significance and scientific value were analyzed.The main understandings are as follows:(1) Six bentonite dense layers were found in the Qiliao section,mainly occurring in 6 graptolitic belts of the Katian,Rhuddanian and Aeronian.Most of the bentonite dense layers showed obvious increase in clay,peak response of GR curve,and indistinct relationship between volcanic ash and total organic carbon (TOC).(2) The bentonite dense layers of Longmaxi Formation were widely distributed in eastern Sichuan Basin and its periphery,and generally showed GR peak,which can be an important reference interface for dividing the bottom boundary of the Coronograptus cyphus belt and the top boundary of the Rhuddanian in eastern Sichuan Basin and western Hubei province.(3) Taking the bentonite dense layers as the stratification basis of the Rhuddanian and Aeronian,it was determined that the sediment thickness of the Rhuddanian in the eastern Sichuan depression was generally 10–40 m,but only the upper part of the Coronograptus cyphus belt was deposited in the hinderland of Yichang Uplift,and the sedimentary thickness was only 3–7 m.(4) In the hinderland of the Yichang Uplift,at least five and a half graptolitic belts were missing in Wufeng Formation–Rhuddanian,and the deposition time of Rhuddanian was less than 0.4 Ma.(5) The bentonite dense layers were important sedimentary responses to the strong deflection of the Yangtze basin at the turn of the Ordovician–Silurian,which suggested that four tectonic activity periods existed in the eastern Sichuan depression,including the early stage of the depression,the middle-late stage of the depression,the early stage of the foreland flexure and the development stage of the foreland flexure.The high-quality shale was mainly developed from the early stage to the middle-late stage in the depression.