华北板块奥陶纪牙形石生物地层研究回顾及在西北缘区新进展

牙形石在以碳酸盐岩为主的华北奥陶系划分对比中占有举足轻重的地位。针对华北奥陶纪牙形石的研究已持续近半个世纪,总体上可以分为两个阶段:第一阶段自20世纪70年代到21世纪初,第二阶段为最近10年(2010—2020)。第一阶段以建立牙形石生物地层序列为目标,主要为解决石油勘探过程中地层时代的确定和地层对比的需求;第二阶段的研究以修订化石带为主,目的是与国际地层研究接轨。近年来在华北板块西北缘的工作显示,该区奥陶纪牙形石在纵向上具有显著的生态变化,可分为达瑞威尔期中期、达瑞威尔期晚期—桑比期中期、桑比期晚期—凯迪期中期3个时段。第一时段以介于北美中大陆区和北大西洋区之间的热带台地边缘型牙形石为特征;第二时段以北大西洋型为主混有少量亚澳型牙形石为特征;第三时段以亚澳型和北美中大陆型牙形石混生为特征。在华北西北缘尽可能使用广布性标准牙形石属种进行化石带厘定,共识别牙形石带12个,自下而上分别是达瑞威尔期Histiodella cf. holodentata间隔带、Histiodella kristinae谱系带、Histiodella bellburnensis延限带、Dzikodus tablepointensis间隔带、Eoplacognathus suecicus间隔带、Pygodus serra间隔带和Pygodus anserinus(早期型)间隔亚带,桑比期Pygodus anserinus(晚期型)间隔亚带和Belodina compressa间隔带,凯迪期Belodina confluens间隔带、Yaoxianognathus neimengguensis间隔带和Yaoxianognathus yaoxianensis间隔带。由于部分化石带与国际同名带的对比还存在一些矛盾,尚需今后进一步解决。

A review on Ordovician conodont biostratigraphy of the North China Plate and new research advances on its northwestern margin

Conodonts are particularly important biostratigraphically for the subdivision and correlation of the carbonate-dominated Ordovician strata in North China. Studies of Ordovician conodonts from northern China have lasted for nearly half a century and can be distinguished into two stages. Researches in the first stage, from the 1970s to the beginning of the 21^st century, were aimed to establish a basic conodont biostratigraphic framework for the paleontological dating and stratigraphic correlation of Ordovician strata in petroleum exploration. Studies in the second stage, from 2010-present, have been focused on the improvements and/or revisions of the conodont zonation established in the first stage in order to connect with international stratigraphic research. Investigations conducted recently on Ordovician conodonts from the northwestern margin of the North China Paleoplate suggested that three stratigraphic intervals with ecological differentiation can be recognized. The first ecologically-specific stratigraphic interval, which covers the middle Darriwilian, is characterized by the presence of conodont taxa inhabiting the marginal areas of tropical carbonate platforms between the North American midcontinent and North Atlantic ecoprovinces. The second interval, with a stratigraphic range spanning from the late Darrwilian to middle Sandbian, features conodonts mainly of North Atlantic affinity and associated with some Australasian species. The third interval, ranging from the late Sandbian to middle Katian, possesses conodont taxa with a mixture of Australasia and North American midcontinent affinities. For an easy and reliable international correlation, recent biostratigraphic studies on Ordovician strata on the northwestern margin of the North China Paleoplate have concentrated on using widespread conodonts as zonal indices, and an updated conodont biozonal succession comprising 12 biozones has been established. They are, in ascending order, (Darriwilian) the Histiodella cf. holodentata Interval Zone, Histiodella kristinae Phylozone, Histiodella bellburnensis Range Zone, Dzikodus tablepointensis Interval Zone, Eoplacognathus suecicus Interval Zone, Pygodus serra Interval Zone, Pygodus anserinus (early form) Interval Subzone, (Sandbian) Pygodus anserinus (late form) Interval Subzone, Belodina compressa Interval Zone, (Katian) Belodina confluens Interval Zone, Yaoxianognathus neimengguensis Interval Zone, and Yaoxianognathus yaoxianensis Interval Zone. However, correlations of some conodont biozones with their international namesakes are still somewhat controversial, hence additional biostratigraphical work remains to be done.