Lower Cretaceous Red Bed Package In The Eastern North Yellow Sea Basin,and Its Coupling Relations With Magmatic Activities

The North Yellow Sea Basin,eastern China,is located in the extension part of the North China Platform.It is an offshore Mesozoic-Cenozoic rift basin with very low exploration degree.The North Yellow Sea Basin was strongly influenced by magmatic activity.It began to receive terrestrial deposits in the Early Mesozoic and developed Mesozoic to Cenozoic lacustrine strata.Among them,the Lower Cretaceous red beds are developed in the eastern part of the basin.They formed in the underwater environment and the total iron content is abnormally high,and its geological characteristics are different from Cretaceous Oceanic Red Beds?CORB?.The specific genetic mechanism remains to be further discussed.Meanwhile,the understanding of the tectonic activity and sedimentary filling characteristics of the North Yellow Sea Basin are still needed to be deepen.Combined with the multi-stage and large-scaled magmatic activity,the tectonic and sedimentary pattern of the basin are more complicated.The Lower Cretaceous red beds in the North Yellow Sea Basin were developed during the period of regional magmatism,and the sedimentary relationship between red beds and volcanic rocks needs to be further determined.This thesis uses theoretical knowledge of sequence stratigraphy,structural geology,sedimentary geology,petrology,petroleum geology and inorganic geochemistry,and utilizes geological,seismic,well drilling and logging,and test data to make the analysis of sequence stratigraphic,mineralogical,sedimentary and geochemical characteristics of the Lower Cretaceous red beds in the eastern part of the North Yellow Sea Basin.Combined with the analysis of regional tectonic movement in eastern China and the magmatism in the eastern region of the North Yellow Sea Basin,the characteristics of the deep dynamics of the magma activity in the Early Cretaceous and its influences on sedimentary system in the eastern part of the North Yellow Sea Basin are discussed.The coupling relationship of the sequence development,the deposition of red beds and the magmatic activities is summarized,and the related geological models are finally established.The main results of this thesis include:1)The Lower Cretaceous red beds in the eastern part of the North Yellow Sea Basin lack bedding,biological trances and bioclastic fragment,and the red color is uniformly evenly distributed.Thin section observation and XRD analysis confirm that the coloring mineral is hematite.The sandstones in the middle part of the red beds section display signs such as expansion,poor cementation and fragility,growth of new fibrous mineral crystals and native sulfur residue,which are the results of the swelling of volcanic ash within the sandstones.Granite debris,ductile matrix,falling volcanic clasts and tuff debris are observed,indicating that the magmatic activity affects terrigenous clastic composition.Regionally,the red beds package is evenly distributed except for the high uplift area,with an average thickness of 93.5 m.Spatially,the distribution of the red beds package is related to igneous rocks.2)The concentrations of Fe₂O₃ and TFe₂O₃ in the red beds and grey sandstones are 6.16%and 9.79%,8.42%and 9.27%,respectively,which are higher than 2.18%and 4.75%in the underlying grey mudstones.The TFe₂O₃ contents are also higher than the average concentration of the upper crust of 4.09%,and 6.22%of the Cretaceous red mudstones and 5.14%of the grey mudstones in the Songliao Basin.In addition,the concentrations of redox sensitive elements Sc,V,Co,Cu,Zn,Rb,Ba,Pb and Th in the red beds and grey mudstones are higher than the upper crust contents,and the Eu shows positively anomaly in the red beds,indicating the sedimentations are affected by hydrothermal activity.Meanwhile,the TOC contents in the red beds are extremely low,and the Ro curve shows normal evolution.Comprehensive analysis shows that the red beds are formed in the underwater environment with oxygen-rich open freshwater?shore-shallow to semi-deep lacustrine?,while grey mudstones are developed in deep water anoxic environment.3)The intrusive and eruptive igneous rocks are widely distributed in the eastern part of the North Yellow Sea Basin,including granite,peridotite,basalt,dacite,andesite and rhyolite.Through the comprehensive analysis of gravity and magnetic anomalies,seismic interpretation and igneous rocks zircon U-Pb dating,four stages magmatism including the pre-Mesozoic,Early Cretaceous,Paleogene,and Neogene were determined.Zircon U-Pb ages of the igneous rocks are concentrated in the early Cretaceous and coincides with the depositional period of red beds,displaying igneous rocks?basalt,peridotite,tuff?and red beds interbed.The magmatism is closely related to the interaction between the Izanagi plate,the Pacific plate,the Indian plate and the Eurasian plate since the Late Jurassic,and the sinistral and dextral strike-slip faulting of the Tanlu Fault.4)A total of 11 third-order sequences are divided in the Mesozoic-Cenozoic successions in the North Yellow Sea Basin.K₁sq1,the lower part sequence of the Lower Cretaceous successions,was accurately defined by the top and bottom sequence boundaries,transgressive surface and the maximum flooding surface.The system tracts including the lowstand,transgressive and highstand system tracts,as well as the internal architectures including lowstand fans,lowstand wedge,incised valley,progradational and retrogradational parasequence sets are clearly recognized in seismic profiles.Comprehensive analysis indicates that the magmatic activity affects the lacustrine sequence by changing the topography,lake level and sediment supply.In the Early Cretaceous,the magmatism produced a local uplift,and meanwhile,generated the structural slope break.Along with uplifting,relative lake level dropped sharply and lake accommodation was reduced,but with increased sediment supply.Therefore,sediment accumulated along the slope break as a lowstand systems tract.Later on,as lake level continuously rose,transgressive and highstand systems tracts were developed.5)The values of Fe₂O₃/Al₂O₃ and TFe₂O₃/Al₂O₃ are 0.34 and 0.59 in red beds,and 0.94and 1.05 in grey sandstone,respectively.These values are higher than 0.12 and 0.3 in grey mudstone,which may be due to hydrothermal contamination.Furthermore,Al/?Al+Fe+Mn?and?Fe+Mn?/Ti values,ternary diagrams of Ni-Co-Zn and Fe-Mn-?Ni+Co+Cu?*10 all show that the red beds belong to hydrothermal sediments and are similar to the mudstones with hydrothermal genesis in the Chang 7 Member of the Ordos Basin.In addition,the sandstones within the red beds section show expansion,poor cementation and fragility,growth of new fibrous mineral crystals and native sulfur residue.These may due to the cooling reaction after the hydrothermal fluid invading.The inconsistency of the redox environment discriminating results also indicate that the trace element composition may be contaminated by hydrothermal fluids.6)The Lower Cretaceous red beds have abnormally high iron ions concentration?higher than the upper crust content?.Al₂O₃-Standardized main elements composition,?REE-La/Yb diagram,the result of correlation analysis of Al₂O₃ and?REE,TiO₂ and?REE,?Eu/Eu*?and TFe₂O₃,Al₂O₃ and TFe₂O₃,Al₂O₃ and Fe₂O₃,TiO₂ and TFe₂O₃,TiO₂ and Fe₂O₃ show that the red beds are affected by the mantle substances such as Alkali basalt,and the extra iron ions are not derived from terrigenous debris,but are related to hydrothermal activity.Combined with the main elements analysis of igneous rocks that drilled in the North Yellow Sea Basin,it is believed that additional iron ions may enter the water volume with the eruption of basic igneous rocks and the flow of hydrothermal fluid.7)The Lower Cretaceous red beds are products of hydrothermal activity,rather than the climatic and ocean currents or the biological and hydrothermal origins like CORB,nor the climate and weathering,erosion and oxidation,and heating origins like other lacustrine red beds.Integrated analysis indicate that the regional extension activated the basement faults and magma conduits in the subsurface,providing conduits for upwelling hydrothermal fluids along with basalt eruption.Therefore,deep hydrothermal fluids upwelled through base faults and magma conduits,mixed with cold water and terrigenous clastics,finally forming the red beds.The hot hydrothermal fluids transported abundant iron and other related ions to the water column,changing the ionic balance and lake-floor water temperature.This relationship encouraged the oxidizing of ions in hot water by the influx of oxygen-enriched cold water.