| In the context of special tectonic setting,the Pre-Mesozoic basement of Hefei basin is characterized by special character,severe deformation and metamorphism,complex composition and little exposure on the surface,which is relatively difficul to study.However,the delimitation of basement attributes and recoverying the formation history are significant for researching the evolution of basin,which has long been one of the most important discussions about Hefei basin.The research work of this article is mainly centered on the only well,the An'can-1 in the basin which drilled Pre-Jurassic basement strata more than 1000 meters.The primary lithology is dark mudstone without coal streak,coal seam or limestone.Its age assignment will provide an important basis for identification and division of basement stratum and sequence interface.In the paper,cores and debris sample,the natural potential data,the cycle curve processed by INPEFA to the GR and the 3 seismic profiles are main research objects.The target is further clarifying the depositional age of Pre-Mesozoic basement stratum and identifying its sequence interface.A large number of spro-pollen samples in well An'can-1(4160-5152m)were studied,and correlation of these sporo-pollens and assemblages with other areas in North China to ascertain its geological age.This paper introduces and discusses the principle of INPEFA and the analytical method of recognizing the geologic meaning of INPEFA curve.On the basis of clarifying the age of sporo-pollen assemblages,the depth of burial Pre-Jurassic top interface and other sequence interface was identified by using the INPEFA cycle curve(3500?5200m)of the well An'can-1,to divide the stratigraphic sequence of three seismic profiles within basin.And combined with the plane distribution characteristics of the Pre-Mesozoic,seismic profile interpretation results and the tectonic environment,to devide the secondary structural units of the Pre-Mesozoic.68 cores and debris samples in well An'can-1(4160?5152m)were studied,23 genera,51 species and 647 sporo-pollen fossils well-preserved in 5 samples have been found by analysis and identification.According to the genera and content change and vertical variation of sporo-pollen fossils,the sporo-pollen can be distinguished into 2 assemblages as follows(in descending order):1.Densosporites reynoldburgensis-Laevigatosporites perminutus,?.Triquitrites-Macrotorispora media.The above 2 sporo-pollen assemblages are similar to the sporo-pollen in the Taiyuan and the Upper Shihezi Formation of late Paleozoic in the north of China respectively,keep consistent respectively with Early Cathaysia flora and Middle-Late Cathaysia Flora.The geological time of the sporo-pollen assemblages should be the late Late Carboniferous to early Early Permian and the Late Permian respectively.They are direct evidences for the existence of Carboniferous-Permian in Pre-Jurassic basement strata in Well An' can-1.The GR INPEFA curve was used for verification,the depth of the Pre-Mesozoic top surface should be in the depth of 4006m.Based on this,the author divided the stratigraphic sequence of three seismic profiles within basin and found the Pre-Mesozoic lack of Siluric,Devonian and Upper Ordovician,the other stratums under the Indosinian unconformity formed different thrust block.Combined with the tectonic background before the Jurassics the evolution history of the North China plate,the Yangtze plate,and Southern North China basin,the secondary strcuctural units of the Pre-Mesozoic basement in Hefei basin can be devided into 5:Thrust belt of basement in North Huaiyang,Lu,an-Hefei posterior imbricate thrusting zone,Huoqiu-Daqiao front imbricate thrusting zone,Mutaizi uplift belt of crystal basement,Huainan front synclinore zone... |
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