冀北滦平盆地陆相侏罗系—白垩系界线候选层型剖面初步研究

Terrestrial Jurassic-Cretaceous boundary stratotype candidate in Luanping basin, northern Hebei

在发现陆相侏罗系—白垩系界线地层连续剖面的基础上,补充大北沟组和大店子组的含义,将2组系统分为7个岩段,命名4个标志层;确定年代地层层序,完善大北沟阶和义县阶定义内容;研究重点门类化石分布和演化,建立系统生物地层分带序列,包括7个介形虫带和6个叶肢介带;划分陆相生物群发展阶段,将侏罗系—白垩系界线置于“早期热河生物群”阶段的底部;开展旋回地层和层序地层学研究,建立地层等时格架,识别盆地演化的3个阶段和5个构造期;测得界线层型剖面点存在O同位素高值异常;锆石206Pb/238U测得张家口组顶部年龄为135.4Ma±1.6Ma,大北沟组中部为133.9Ma±2.5Ma和130.1Ma±2.5Ma。综合多学科研究成果,将界线层型剖面点定在张家沟剖面第9层底,以介形虫化石Cyprideastenolonga的首现为标志。

Based on the discovery of the successive sections of Jurassic-Cretaceous transitional strata, a boundary stratotype candidate has been preliminarily erected in the Luanping basin of northern Hebei. The Dabeigou and Dadianzi formations are divided to 7 members, in which 4 marker beds are distinguished and named. A chronostratigraphic sequence has been determined and the definitions of the Dabeigouan and Yixianian stages improved. A systematic biostratigraphic zone sequence, including seven ostracod zones and 6 conchostracan zones, has been distinguished according to the occurrence and evolution of fossils. The faunal developmental stages have been defined and the J-K boundary is placed at the base of the 'earlier Jehol (Rehe) fauna stage' (earlier Yixianian). Cyclostratigraphy and sequence-stratigraphy have been studied and an isochrononic framework of strata has been constructed. Three stages and five tectonic phases for basin have been recognized. An O-isotope high-value anomaly appears at the boundary stratotype. The zircon 206Pb/238U dating gave a age of 135.4 Ma±1.6 Ma for the top of the Zhangjiakou Formation and ages of 133.9 Ma±2.5 Ma and 130.1 Ma±2.5 Ma for the middle part of the Dabeigou Formation. According to an integration of multidisciplinary research results, the boundary stratotype point is placed at the bottom of bed 9 of the Zhangjiagou section and marked by the first-occurrence of the ostracod Cypridea stenolanga.