Tectonic Evolution Of The Paleozoic To Mesozoic And Its Control On The Development Of The Buried Hills In Huanghua Depression

Huanghua depression,located in the centeral Bohai Bay Basin,had experienced multiple stages and different properties tectonic evolution during Paleozoic and Mesozoic periods.The present tectonic framework reveals the overprint of the original tectonic frameworks of different evolution period and subsequent superimposed reconstruction.Therefore,it is of great theoretical and practical importance for the Prepaleogene buried hills hydrocarbon exploration in both Huanghua depression and Bohaibay Basin.Based on the new released exploration data as well as the analyses on the spatio-temporal distribution of strata in different layers,combing with the analyses of the geometry characterization and evolution process,the tectonic division effect and the differential tectonic deformation process of the Mesozoic-Paleozoic were clarified.Then,the tectonic evolution model since the Paleozoic was established,and the control of spatial and temporal tectonic evolution on the development of buried hills in Huanghua depression was discussed.The results show that:(1)The Mesozoic-Paleozoic synthem in Huanghua depression can be divided into 5subsynthems,namely Cambrian to Lower and Middle Ordovician,Upper Carboniferous to Permian,Lower and Middle Triassic,Lower and Middle Jurassic,Lower Cretaceous from bottom to up.There are significant differences in the distribution of the residual strata,denuded strata and original strata of each subsynthems.Among the Cambrian to Lower and Middle Ordovician,Upper Carboniferous to Permian,and Lower and Middle Triassic subsynthems,each has a parallel unconformable contact relationship with the upper layer.Their deposition is characterized by sediments of cratonic basin.The strata erosion was mainly controlled by Indosinian tectonic movement,and it is characterized by a decreased trend of the erosion range bottom up,and erosion intensity of the central-north area is greater than in the south part.The original thickness of Lower and Middle Jurassic is larger in the south part than that in the middle and north part.And subsequent erosion also showed obvious variations,leading to larger residual thickness in south part.The zircon U-Pb dating shows that the previously called “Upper Jurassic-Lower Cretaceous” strata in Huanghua depression should be divided into Lower Cretaceous.The original deposits mainly distributed in central-north area.Affected by erosion in late Yanshanian and Himalayan epoch,the residual strata are missing in the south area,and remain thick in the central area and thin in the north area.(2)The fault system cutting Mesozoic-Paleozoic in Huanghua depression is composed of four sets of faults,which have a strike of NNE(nearly SN)、NE、NW(W)、EW,separately.Various structural styles can develop in study area such as compression,extension,strike-slip and inversion structure.The present tectonic framework is characterized by north-south parts division and east-west belts division.Bounded by Haihe-Xingang fault and Koucunnan fault,Huanghua depression can be divided into three parts.The north part is dominated by NEoriented extensional faults with negative inversion structure and compressional structure developing near the boundary.The upper Paleozoic is relatively thin in the north area whereas lower-middle Triassic is generally missing,with sporadic distribution of lower-middle Jurassic and widespread distribution of Lower Cretaceous.In the central area,the main faults are characterized by multi-direction and multi-property.Compressional,extensional,strike-slip and inversion structures coexist in this area.The Paleozoic underwent intensive erosion,whereas Lower-Middle Triassic and Lower-Middle Jurassic residues locally in the east of the coastal strike-slip zone and the Lower Cretaceous develops extensively in west of the coastal strikeslip zone.The faults in the south zone are mainly distributed in NNE oriented,with negative inversion structure and compressional structure dominating in the west and large amount of extensional structure and a few strike-slip structures in the east.Besides,Qijiawu fault strike slip zone and coastal strike-slip zone divided the Huanghua depression into three belts.The west belt is dominated by NNE oriented compressional and negative inversion structure,while the middle belt is a superimposed belt of extensional and strike-slip structures,with NNE(near SN)strike-slip faults and extensional faults superimposed by strike-slip faults.Meanwhile,EW extensional faults and strike-step structural style predominate in the east belt.(3)Based on the comprehensive analyses of fault intensity,balanced cross section profiles of typical seismic lines,as well as apatite fission track evolution history of key areas,the Mesozoic-Paleozoic of Huanghua depression experienced a multi-stage and multi-property tectonic evolution history.The depression was on the stage of stable craton deposition from the Paleozoic to Middle Triassic.From Late Triassic,the basin suffered compression and then uplifted,with the development of NW(W)oriented reverse faults.The framework is characterized by south-north division,where the anticline was distributed in the north and central parts,while syncline in the south part.During the Jurassic,the basin experienced the transition from the differential erosion to stable uplifting.The middle part suffered sustained erosion,and the south part suffered blanker deposition.Besides,the NEE-trending strike-slip zone formed in local part of the south and middle part.During the Early Cretaceous,the basin suffered differential subsidence,and the middle part deposited thick strata with intensive volcanism,while the strata locally distributed in the south part.During the Late Cretaceous,the basin suffered denuded under the background of regional uplift.While the Paleogene basin is characterized by intensive rifting.The Mesozoic active faults like the Cangdong fault sustained the extensional activity.Meanwhile,a large number of newborn NE and NEE-trending faults cut through the underlying strata,causing the local denuded of the strata.(4)The tectonic evolution of Mesozoic-Paleozoic in Huanghua depression since Indosinian period had only provided chances for the development buried hills,but also controlled the evolution process,mrphological structure,types and the plane distribution of the buried hills.According to the matching relationship between the formation of buried hills and the burial process,the buried hills in study area can be divided into four types based the structures and genetic,namely “single-stage uplift and single-stage buried”,“single-stage uplift and multistage buried”,“multi-stage uplift and single-stage buried”,“multi-stage uplift and multi-stage buried”.Differential tectonic evolution and vertical superimposition since Indosinian period provide conditions for the development of buried hills.The one formed in Mesozoic and subsided in the Cenozoic and the one uplift in the Mesozoic and subsided in the Cenozoic are beneficial for the development of buried hills.The structural division of the Huanghua depression that is characterized by the “south to north division,and the east to west division”controlled the planar distribution of different kinds of buried hills.