华北地台北部奥陶纪露头层序地层

Ordovician Outcrop Sequence-Stratigraphyin Northern North China Platform

本项研究以河北唐山、北京西山和河北曲阳剖面为重点，首次动用运用多学科的方法对华北地台北部奥陶纪露头层序地层进行综合研究。完善了研究程序，建立了层序地层划分对比标准和区内相分布格架。研究中将副层序直接用于野外分层；将副层序归纳为代表６种沉积相的２１个模型；识别了三类层序界面，即ＳＢ１面、间断型ＳＢ２面和相转换型ＳＢ２面；进行地层多级沉积旋回或分，包括２个二级旋回，６－７个三级旋回（每个旋回时限约６７Ｍａ），２２—２６个四级旋回（每个旋回时限约１５—１８Ｍａ）及１５５—１８５个五级旋回（每个旋回时限约０２１－０２６Ｍａ）；取得多项系统成果。区域相分布和相对海平面变化研究表明，区内奥陶纪台地环境的演变分为３个阶段：藻礁缓坡－台盆阶段，潮滩缓坡－台盆阶段和泻湖潮滩－缓坡阶段。

Achievements on Ordovician outcrop sequence-stratigraphy,about sequence division and correlation ,facies distribution,relative sea-level elevation and basinal evolution ,have been made ofr the first time,based on the study of some sections Tangshan,West-hills of Beijing and Quyang etc.in northern North China Platform.The schemes of the sequence division and the relative sea-level variation have been used as a corerelating standard in the neighbour areas. Parasequence,whichauthors consider it the most improtant unit for facies analysis and making up sedimentary system and sequence,has been used to divid the strata straightly in our fieldwork.21 parasequence moulds are abstracted representing 6 facies—basin,deep ramp,intermediate ra,p,shallow ramp,tidal flat and lagoon.Thre kinds of sequence-interfaces wrer distinguished, a kind of SB1 and two kinds of SB2.They can be found in the whole area. Ordovician strata here,only the Lower and the basal Middle Ordovician preserved ,can be divided into multi-grade sedimentary cycles in each section ,including 6(7 in the western area third-grade cycles(sequenses)),22-26 fourth-grade cycles and 155-185 fifth-grade cycles (parasequences).Range of the cycle n each grade in successively about 6.7Ma,1.5～1.8Ma and 0.21～0.26Ma. Those third-grade cycles can be corrrelated separatively by sequence-interfaces and lithologic characters. A facies-distribution pattern and some curves of relative sea-level elevations have been reconstructed on the basis of biostratigraphic isochronous frame.The pattern shows that the working area was characterstic of widspread ramps.This area suffered 6-7 large transgression-regression cycles.Among these,two regressions are remarkable which caused logn-period denudations.The framse is erected mainly on the study of conodont,cephalopid and trilobite stratigraphy. Three evolutionary stages have been divided for the platform sedimentary environment variation,i.e.the reef-ramp stage ,the reef-ramp-shoal-ramp stage and the shoal-ramp stage.Due to the influence of the 'Huaiyuan Movemnet”,the surface relief underwent a variation of lower eastwards in the early period and lower westwards in the later period.Relative sea-level elevation curves have been erected in the working area,which provide a standard for correlations of sequence stratigraphy and relative sea-level elevations.