Research On Stratigraphic Framework-structural Characteristics And Evolution Processes Of Buried Hills In Dawangzhuang-yihezhuang Area

Dawangzhuang-Yihezhuang area is located in the northeast of the Chezhen Sag.Since Mesozoic,it has gone through multiple tectonic movements,such as Indosinian,Yanshanian and Himalayan,and suffered from cut and transformation of multiple faults and structural superposition evolution processes,forming various types of buried hills.The study taked the buried hills in Dawangzhuang-Yihezhuang area as research objects,analyzed their stratigraphic framework and the combination relationship,structural styles of their main controlling faults and secondary faults,and reveals spatial-temporal differences of the buried hills’.Through the analysis of the evolution and the control effect of superimposed relationship on the development of buried hills in each stage since the Indosinian,the controlling effect of tectonic evolution on the development of buried hills was analyzed,combined with the “mountain formation and burial” of buried hills,the “genesis-structure” type of buried hills was classified,in addition,the distribution rules of different types of buried hills were clarified.The following conclusions have been obtained:The Mesozoic and Palaeozoic in Dawangzhuang-Yihezhuang area can be divided into four structural layers: Cambrian-Ordovician,Carboniferous-Permian,Lower-Middle Jurassic,lower Cretaceous.As a whole,in the study area the top of the Paleozoic is entirely covered by the lower-middle Jurassic.Affected by factors such as the late Cretaceous strata denudation differences and pre-existing faults in the Cenozoic,there are differences in stratigraphic framework of the buried hills in the study area.the superimposing relationship of formation at the key interface is complicated.NW-trending,NNE-trending and NEE-trending faults are mainly developed in Dawangzhuang-Yihezhuang area,the NW-trending eastern Chengnan Fault and the Luoxi Fault are the main controlling faults.NW-trending and NNE-trending inner faults are mainly developed in the study area,the development of the buried-hill inner faults leads to the development of complex structures inside the hill.Under the influence of main control faults and inner faults in different directions,there are four types of structural styles:including the extensional,extension-strike-slip,compression and inversion structures.The formation and evolution of buried hills in Dawangzhuang-Yihezhuang area is controlled by three groups of faults: NW-trending,NNE-trending and NEE-trending.The late Triassic was influenced by compressive stress regionally,the NW-trending thrust faults developed and formed the NW-trending reverse thrust mountain system,which was denuded generally.In the early-middle Jurassic,the study area was continuously under the structural background of the late Triassic,while overall subsiding and covered with sediment.Late Jurassic strata were uplifted and eroded generally.In the early Cretaceous,the NW-trending faults turned into normal,and the NW-trending mountain system formed,meanwhile,the northern segment of which is divided into three sections under the influence of the new NNEtrending fault strike-slip activity: west,middle and east.In the late Cretaceous strata were compressed and uplifted as a whole and eroded generally.In the Paleogene,the eastern Chengnan Fault and the Luoxi-1 Fault continued its movement,leading to inherited development of the southern segment of the mountain system.During the Cenozoic,the NEEtrending Dagu-60-Nan Fault and Yigu-26-Nan Fault were active and cut to form Dagu 61 buried hill and Shaogu 3 buried hill.In the Neogene,the buried hill was buried as a whole under the regional depression background.The tectonic evolution of Mesozoic-Paleozoic in Dawangzhuang-Yihezhuang area since Indosinian period had only provided chances for the development buried hills,but also controlled the "mountain formation and burial ",types and the planar distribution of the buried hills.Buried hills’ formation periods in Dawangzhuang-Yihezhuang area included late Indosinian,Yanshanian(middle and late),and Himalayan periods.Burial periods included early Yanshanian and Himalayan periods,according to the formation period,compared with the specific period of the burial period,the buried hills in the study area have generally experienced the Indosinian period,the middle and late Yanshanian mountain-forming periods,and the early Yanshanian burial period.However,the formation and burial periods of the Himalayan period are different;according to the nature of main controlling faults and the matching relationship between the mountain formation and burial period,this paper proposed a "genesis-structure" type classification scheme for buried hills,and divides the buried hills in the study area into inherited buried hills,modified buried hills,and inherited-modified buried hills;the difference in the tectonic evolution of the Mesozoic and Paleozoic in the study area controlled the planar distribution of different types of buried hills.