Element Geochemistry Of The Phosphorus Mudstone In Madingo Formation,Lower Congo Basin:Implications For Paleoceanic Environment

The lower Congo basin is one of passive continental margin basins located on the west coast of Africa.It experienced rift stage during the late Jurassic to early Cretaceous,transitional stage in Aptian?early Cretaceous?and passive continental margin stage after the Aptian?early Cretaceous?influenced by the rift in the south Atlantic.The drift-stage strata of the lower Congo basin dominated by marine sediments can be subdivided into the Sendji,Likouala,Madingo,and Paloukou Formations in ascending order.The nature of Lower Congo Basin transited to semi-enclosed marine during the sedimentary period of Madingo Formation and the phosphorus mudstone rich in organic matter was developed.This article takes the phosphorus mudstone and ordinary mudstone of Madingo Formation and the silty mudstone of Paloukou Formation as the research object.It is of important significance to study petrology and element geochemistry of these rocks to reveal paleo-ocean environment during the sedimentary period of the phosphorus mudstone in Madingo Formation and phosphorus enrichment mechanism.In this study,29 cutting samples are selected from the phosphorus mudstone and ordinary mudstone of Madingo Formation and the silty mudstone of Paloukou Formation in well A,lower Congo basin.Petrology characteristics of the phosphorus mudstone,ordinary mudstone and silty mudstone are analyzed carefully using thin sections,backscatter and energy spectrum.Redox conditions and paleoproductivity of paleo-ocean during the sedimentary period of the phosphorus mudstone,ordinary mudstone and silty mudstone are analyzed based on the test of primary,trace and rare earth elements.The phosphorus source during the sedimentary period of the phosphorus mudstone,as well as the relationship between the phosphorus enrichment of the phosphorus mudstone and sedimentary environment are evaluated by mathematical statistical methods such as correlation analysis and cluster analysis.The results of petrological analysis show that the Madingo and Paloukou Formations of well A are consisted of mudstone mainly.The lower part of the Madingo Formation is composed of ordinary mudstone,which is darker in color and contains less terrigenous clastic particles.No paleontological fossil is found in the ordinary mudstone under the microscope.The phosphorus mudstone is located at the top of the Madingo Formation.And it is darker in color and contains less terrigenous clastic particles.A large number of foraminiferal fossils and phosphate granules are found in the phosphorus mudstone under the microscope.The Paloukou Formation is composed of silty mudstone,which is lighter in color and contains lots of terrigenous clastic particles.The geochemical evidences of the trace elements especially the redox-sensitive trace elements?such as U and V?and the elemental ratios?such as U/Th,Ni/Co,V/Cr,Ce/Ce*,etc.?of the phosphorus mudstone,ordinary mudstone in the Madingo Formation and silty mudstone in the Paloukou Formation provide valuable information,regarding the water body was layered during the sedimentary period of the phosphorus mudstone and ordinary mudstone in Madingo Formation.Cerium anomaly was mainly related to phosphate minerals.The phosphate minerals with Cerium anomaly formed in the surface oxidation water and precipitated in the bottom water.The bottom water was hypoxic-anoxic,resulting in the enrichment of redox sensitive elements.And the silty mudstone was deposited in oxidation water.The geochemical evidences of Phosphorus of the phosphorus mudstone,ordinary mudstone in the Madingo Formation and silty mudstone in the Paloukou Formation provide valuable information,regarding the ancient ocean water had a high level of productivity,and there is a trend of increasing gradually during the sedimentary period of the phosphorus mudstone and ordinary mudstone in Madingo Formation.The paleo-ocean environment had undergone tremendous changes,and the productivity of ancient ocean water had plummeted during the sedimentary period of the silty mudstone in the Paloukou Formation.Phosphorus sources in marine sedimentary rocks include terrestrial sources,hydrothermal sources,biological sources,air sources and cosmic sources.The biological source was the main source of phosphorus in the phosphorus mudstone and biology played an indirect role in the phosphorus enrichment of the phosphorus mudstone in the study area by comparing the content of P₂O₅ content of the phosphorus mudstone with the contents of characteristic elements and element ratio,which can indicate source of terrigenous debris,hydrothermal fluid and biology.The seawater was layered and the upwelling activity was strong during the sedimentary period of the phosphorus mudstone in the Madingo Formation in the study area.The upwelling brought water rich in nutrients such as phosphate on the sea floor into shallow water,which promotes the massive development of organisms.Plankton absorbed phosphorus to form phosphorus-containing organic compounds,which preserved the negative anomaly of Ce in shallow oxidized water.After the biological death,a large amount of organic matter accumulated in marine sediments.At the same time,the consumption of oxygen in the decomposition process of organic matter led to a reducing environment on the surface of the sediment,which made the redox sensitive elements such as U and V rich in marine sediments.The decomposition of phosphorus-containing organic matter by microorganisms on the surface of the sediment layer can cause the release and precipitation of P,Ni and Cu.