Study On Proterozoic Sedimentary Facies And Diagenesis In Jizhou District, Tianjin

The Proterozoic field outcrops in Jizhou District of Tianjin City are continuous,with complete stratigraphic exposure,various rock types,and little transformation effect in the later period.It is an ideal section for the study of Proterozoic strata in China.Proterozoic sedimentary facies,sandstone diagenesis and dolomite genesis research in the study area are relatively few or blank,and the understanding is not unified.Field outcrops show that the Proterozoic sedimentary facies in the study area have a continuous change from continental facies to marine facies,various types,the rock types range from clastic rocks to carbonate rocks,and the rock types are diverse.The previous research was only aimed at a certain layer or one aspect,but there was no systematic and continuous research on the sandstone diagenesis in the study area.It is basically blank,and the research on the origin of dolomite is only in several groups,relatively few and the understanding is not unified.Aiming at the above problems,this paper conducts a detailed study of the sedimentary environment in the study area by means of field measurement,intensive sampling,microscopic thin section identification and whole-rock X-diffraction.The results show that the clastic rocks in the study area are mainly divided into four types:conglomerate,sandstone,siltstone and shale,and the sandstone is subdivided into coarse sandstone and fine sandstone.Limestone and dolomite are widely developed in the study area.Due to the different sedimentary structures,the limestones are divided into horizontal bedding limestone,massive limestone,stratified limestone,nodular limestone and molar limestone;,the dolomite is divided into horizontal bedding micrite powder dolomite,massive fine crystalline dolomite and micrite powder dolomite,corrugated bedding argillite dolomite,stromatolite argillite dolomite and mud cracked shale dolomite Five types of crystal dolomite.According to lithological changes and sedimentary structure,combined with microscopic observation and geochemical analysis data,we identified continental clastic sediments,marine clastic sediments,marine carbonate sediments,and marine carbonate sediments in the study area.There are four types of mixed sedimentary rocks and marine clastic rocks.Continental clastic sediments are mainly braided river channel sedimentary facies at the top of Changzhougou Formation,mainly channel stagnant sediments and mid-bank sediments;marine clastic sediments mainly develop littoral facies,neritic shelf facies and slope-There are three types of sedimentary facies in basin facies;marine carbonate deposits are divided into carbonate tidal flat facies(including supratidal and intertidal zones),subtidal shoal facies,open platform facies,and shale shelf facies Types of sedimentary facies;marine mixed sediments are divided into supratidal mud flat facies,intertidal sediment mixed flat facies,subtidal sand flat facies,shale limited shallow sea zone,carbonate tidal flat facies,open platform facies,slow Slope phase 7 phase bands.Three depositional models have been established,namely clastic depositional model,carbonate depositional model and clasticcarbonatonic depositional model.Through field section observation,thin section observation and geochemical analysis,it is believed that the types of sandstone diagenesis include compaction,pressure solution and cementation.