The Research Of Mesozoic Volcano-sedimentary Strata In The North Margin Of Pingzhuang Rifted Basin

Mesozoic tectonic basins were well developed at the western part of Liaoning province.These basins were filled by terrestrial facies volcano-sedimentary strata that were equipped with great lithofacies change,many gaps and uneven distribution ancient fossils.All these phenomena led to a difficult of regional stratigraphic correlation.The geotectonic position of Pingzhuang basin was located at conjunction of the north margin of North China Block and Xing'an Mongolian Complex Orogenic Belt.Pingzhuang basin was a rifted basin which extended along NNE orientation.The basin was boundary by eastern,northern and western edge faults.The research area lied in the north margin of Pingzhuang basin.There were complex geological structures in the research area and it had thicker Quaternary overburden layers.All geological bodies looked like discontinuous distribution as island.This work was done on the detailed collection of previous information and surveying geological sections and routes.All kinds of laboratory samples was collected systematically.Combination of geological survey and lab analyses was applied.Using the method of multidisciplinary association like stratigraphy,isotope geology,petrology,geochemistry,geophysics,tectonics,paleobiology and geomorphology etc.It was dissected to the basement and the filling sequence of the Mesozoic Pingzhuang rifted basin.And then the tectonic evolution of the basin was discussed.Principal understanding had been acquired as follows:The basement of Mesozoic Pingzhuang rifted basin was composed of metamorphic supracrustal rocks,metamorphic plutonic rocks and magmatic rocks:Paleoproterozoic Baoyintu Group and Chaoyanggou gneissic complex;magmatic rocks included early Triassic Daluobogou rock masses and late Jurassic Shaoguoyingzi rock masses.Developed in Mesozoic Pingzhuang rifted basin formed in early Cretaceous epoch.The base of the basin was composed of lower Cretaceous Yixian Formation.The basin was filled by strata of lower Cretaceous Jiufotang Formation,Fuxin Formation and upper Cretaceous Sunjiawan Formation in sequence.The top of the basin developed strata of Paleocene Wuzhijian Formation,Taipingdi Formation,Oligocene Zhaowuda Formation and middle to upper Pleistocene series and Holocene series sedimentary strata.According to rock association,stratigraphic sequence and contact relationships,Yixian Formation had been classified into three segments in the research area.The volcanics LA-ICP-MS isotopic dating of Yixian Formation were 127-114Ma.It was the early Cretaceous epoch.On the grounds of that the writer deduced that the period of Yixian Formation volcanics western Liaoning district activity finished times were about 114Ma.The geochemical features of Yixian Formation volcanics indicated that they had typical features of melted crustal magma.They should be a result of partial melting of lower crustal basic volcanics.During magmatic stages they also have had some level effects of upper crustal materials mixture.On basis of rock association and fossils all Jiufotang Formation shows characteristics of lacustrine sediments.Jiufotang Formation was outcome of rifted basin which was rapid subsidence to stable subsidence.Paleogeographic landscapes were combination of fan delta and lake environments.Sedimentary period of Fuxin Formation alluvial fans were always development and coacted on rivers.The factors had important effects on formation and distribution of peat swamps and coal seams.The combination of delta and fluvial facies environments was Paleogeographic landscapes.Sedimentary period of Sunjiawan Formation were arid to semiarid,to semi-humid and alternates with semiarid and finally to form humid warm paleoclimates.Upper segment of Sunjiawan Formation gray gravel strata were short-lived sedimentary products under semi-humid to humid paleoclimate conditions which were on the general background of upper Cretaceous dry and hot paleoclimates.The writer got through drilling data to analyse spatial distribution states of concealed geological bodys.Pingzhuang basin was a rifted basin that had obviously basin-controlling faults.Source regions of the northern margin of Pingzhuang basin were late Jurassic Shaoguoyingzi rock masses.Basin-controlling faults of the basin northern margin were synsedimentary structures.The evolutionary model of the basin was dustpan shape which distributing trends were getting thinner to the southeast and getting thicker to the southwest.