| 华北地台北部奥陶纪露头层序地层 |
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| 关键词:outcrop sequence-stratigraphy Ordovician northern North China Platform |
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| Ordovician Outcrop Sequence-Stratigraphyin Northern North China Platform |
| 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. |
| TIAN Shu-gang,ZHANG Yu-xu.1997.Ordovician Outcrop Sequence-Stratigraphyin Northern North China Platform[J].Acta Geoscientica Sinica,18(1):87-97. |
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