陕西镇安西口石炭系—二叠系界线剖面综合地层学研究

Integrated stratigraphy of the Carboniferous-Permian boundary section at Xikou, Zhen'an County, Shaanxi Province, Northwest China

对中国海相石炭—二叠系界线典型剖面陕西镇安西口剖面进行了生物地层、层序地层、磁性地层、事件地层多学科综合研究。在石炭—二叠系界线附近建立了4个牙形石生物带,自下而上依次为Streptogna-thoduselegantulus带,S·elongatus带,S·gracilis带和S·isolatus带。将研究区石炭—二叠系界线置于S·isolatus带的底界,较以带Pseudofusulinakrotowisphaeroidea-Dunbarinella(PD)带的底界为标志确定的石炭—二叠系界线层位低3·3m。在西口剖面上石炭统逍遥阶至下二叠统隆林阶,以初始海泛面作为层序界面,识别出12个四级层序(大体相当于副层序组),构成5个Ⅱ型三级层序。这5个三级层序及其对应的海平面变化与贵州独山、罗甸纳水、紫云扁平剖面同期地层中三级层序及海平面变化旋回之间显示出较好的对应关系。除下杨家河阶外,其余各阶的底界,包括逍遥阶、上杨家河阶、范家河阶、垭口阶及隆林阶的底界,基本上都位于沉积相转换点或其附近,即层序地层及海平面变化旋回的关键界面或其附近。表明这5个三级层序的关键界面与年代地层界线的关系相当密切。碳酸盐岩磁化率大小与碳酸盐岩微相存在一定的对应关系,显示相对海平面变化是控制碳酸盐岩磁化率大小的重要因素。另外,在重要地层界线附近常出现磁化率异常高值,说明碳酸盐岩磁化率可能作为地层划分对比的重要依据。西口地区晚石炭世逍遥期至早二叠世隆林期地层中可识别出两次明显的类辐射事件。第一次类辐射事件发生在Occidentoschwagerinaalpina-O·postgallowayi(OS)组合带底部,即下杨家河阶的底部。第二次辐射事件发生在Mccloudiaregularis-Par-aschwagerinafragosa-Robustoschwagerinaxiaodushanica(MPR)组合带底部,即范家河阶的底部。具有三级和三级以上隔壁的四射珊瑚动物群突然大量繁盛,代表四射珊瑚演化进程中一次重大变革。在陕西镇安石炭—二叠系界线附近,这个生物事件首现层位相当于类Pseudofusulinaurdalensi(PU)带下部,高于该带底界不到12m,以Xikouphyllum-Shaannanophyllum-Szechuanophyllum-Wentzellophyllum组合带为代表。在这个层位牙形石也发生了重要变化。各类地质事件记录出现的层位关系密切。在垭口阶底界各类地质事件记录吻合最好,隆林阶底界次之,然后是范家河阶底界。与上述3条界线相比,在镇安西口剖面上,以牙形石Streptognathodusisolatus的首现位置确定的石炭—二叠系界线事件地层特征不明显,既不是一个生物辐射演化面,碳酸盐岩磁化率变化也不太明显,在实际工作中不易使用。

An integrated study has been carried out on hiostratigraphy, sequence stratigraphy, rock magnetic stratigraphy, and event stratigraphy of the Carboniferous-Permian boundary section at Xikou, Zhen＇an County, Shaanxi Province, which represents a typical section of marine deposits in China. Four conodont biozones were established around the Carboniferous-Permian boundary. In ascending order they are Streptognathodus elegantulus Zone, S. elongatus Zone, S. gracilis Zone, and S. isolatus Zone . Consequently the Carboniferous- Permian boundary is set at the base of the S. isolatus Zone defined by the first appearance of S. isolatus, which is 3.3 m lower than the boundary defined by the base of fusulinid assemblage of Pseudofusulina krotowi sphaeroidea-Dunbarinella Zone. Twelve 4th-order sequences, approximately corresponding to the parasequence sets, are recognized in the deposits ranging from the Upper Carboniferous Xiaoyao＇an to the Lower Permian Longlinian, which constitute 5 3rd-order sequences whose boundaries are the first flooding surfaces. In a detailed biostratigraphic framework, these depositional sequences are on the whole well correlated with those recognized in the synchronous deposits in southern Guizhou. Except for the base of Xiayangjiahe＇an, all bases of the stages, including the bases of Xiaoyao＇an, Shangyangjiahe＇an, Fanjiahe＇an, Yakou＇an, and Longlinian, are located either on the key surfaces of the sequences and sea-level changes, or around them, which usually are facies-change surfaces. This fact implies a close relationship between the key surface of the 3rd-order sequence and the chronostratigrapy boundary. A statistically evident correlation between magnetic susceptibility and carbonate microfacies as well as sequence stratigraphy, indicates that sea-level change is one of the principal factors controlling the magnetic susceptibility variations in carbonates. In addition, almost all elevated magnetic susceptibility values occur around stratigraphic boundaries, which suggests that the magnetic susceptibility may be considered as one of the important indices for stratigraphic classification and correlation. Two distinct radiation events of fusulinid fauna are recognized in the Xikou area from the Upper Carboniferous to the Lower Permian. The first took place at the base of fusulinid assemblage of Occidentoschwagerina alpina- O. postgallowayi Zone, i. e. the base of Xiayangjiahe＇an, and the second at the base of Mccloudia regularis- Paraschzvagerina fragosa-Robustoschwagerina aiaodushanica Zone, i. e. the base of Fanjiahe＇an. The sudden appearance and rapid propagation of Rugose corals with third order or higher than third order septa in the Late Carboniferous-Early Permian is an important bioevent in the evolution of rugose corals. This bioevent occurred in the lower part of fusulinid Pseudofusulina urdalensi Zone, and is characterized by coral assemblage of Xikouphyllum-Shaannanophyllum-Szechuanophyllum-Wentzellophyllurns, and its first appearance datum is about 12 m higher than the base of the fusulinid zone. Similarly, important evolutionary events also took place in conodont fauna at the almost same time. An integrated approach with biostratigraphy, sequence stratigraphy, rock magnetic stratigraphy, and event stratigraphy of the Carboniferous-Permian boundary section at Xikou reveals that the records of most geological events occur on relatively concordant horizons. The records of all kinds of geological events exhibit the best accordance first at the base of the Yakou＇an, and secondly at the bases of Longlinian and Fanjiahe＇an. Unlike the three boundaries mentioned above, the record of the Carboniferous-Permian boundary defined by the first appearance of the S. isolatus is not easily recognized by other means, because neither is it an evolutionary surface nor is there an obvious magnetic susceptibility variation or sea-level change. Therefore, the Carboniferous-Permian boundary defined by the first appearance of the S. isolatus could not be used easily in this work.