四川盐边稗子田志留系与泥盆系的界线划分及地质意义

The Silurian Devonian boundary in Baizitian, Yanbian, Sichuan and its geological implications

四川省盐边县北部稗子田地区的志留系发育良好 ,牙形刺化石极为丰富 ,与上覆泥盆系连续沉积 ,并呈整合接触。该剖面为我国扬子区较为理想的志留纪地层剖面 ,其代表和反映了扬子区西部一种新的断陷盆地碳酸盐岩沉积类型 ,丰富了区内志留系的研究内容。笔者详细记述了稗子田志留系至下泥盆统下部剖面的岩性特征和生物化石的垂直分布状况 ,在此基础上建立和完善区内新的地层系统 ,提出地层划分的对比依据和建议。新方案除对志留系底的黑色硅质碳质页岩沿用龙马溪组这一岩石地层单位名称外 ,其上的碳酸盐岩均重新划分和建立了新的岩石地层单位 ,并确定其涵义和时限标准。新的地层划分和对比方案 ,下志留统自下而上包括龙马溪组、国胜组和乡涧组 ;中志留统下部为下稗子田组 ,中志留统上部至上志留统下部为上稗子田组 ;上志留统上部为沟口组 ;顶志留统为岩口组 ;下泥盆统自下而上为榕树组和小梁子组。

The Silurian strata containing abundant conodont fossils are well developed and comformable with the overlying Devonian strata in the Baizitian region, northern Yanbian, Sichuan. The Baizitian Silurian section is believed to be an idealized section and represents a new type of carbonate sediments in rift basins within the realm of the Yangtze area in China. The authors have proposed a new classification scheme for the division and correlation of the stratigraphic systems on the basis of lithology and vertical fossil distribution in the Silurian and Lower Devonian strata in the study area. According to this scheme, the Longmaxi Formation is only referred to as the black siliceous and carbonaceous shales at the base of the Silurian strata. All the carbonate rocks above it have been redelineated and redivided into new lithostratigraphic units. The strata in this region consist, from bottom to top, of the Lower Silurian Longmaxi, Guosheng and Xiangjian Formations; Middle Silurian Lower Baizitian Formation and Middle and Upper Silurian Upper Baizitian Formation; Upper Silurian Goukou Formation; latest Silurian Yankou Formation, and Lower Devonian Rongshu and Xiaoliangzi Formations.