Mid-Cretaceous Red Beds In Eastern England:Their Origin And Sedimentary Environments

In order to accurately reconstruct the evolution of earth’s life,geography,materials and climate,the"deep-time digital earth national science project"was launched,which provides an excellent opportunity for sediment research.Chalk is a fine deposit of calcium carbonate that is found mainly in the Cretaceous strata of Europe,giving it its name.The southeast coast of England,where complete Cretaceous strata were deposited,is the best place to study the sedimentary environment of chalk.Chalk is generally considered to be formed in shallow water environment,indicating that it was a long-term underwater anoxic reducing environment;but the red chalk formation on the east coast of England and the presence of mud cracks in it that indicates an oxidizing environment at the surface.At present,it is difficult to understand the two contradictory phenomena existing in red chalk.However,this provides a most direct window to explore the sedimentary environment of oceanic-continental transition facies from a whole perspective-the earth system.The genetic mechanism and sedimentary environment of the Middle Cretaceous red chalk in England have been continuously discussed.Based on the previous research results,this paper selects the Hunstanton section and Speeton section on the east coast of England as the representative sections.The red chalk samples are comprehensively studied and analyzed by means of microscope and scanning electron microscopy,environmental magnetism,diffuse reflectance spectroscopy,electron microprobe,major and minor elements,rare earth elements,carbon and oxygen isotopes,and iron isotopes.This paper attempts to reevaluate the genetic mechanism of red chalk and propose a new one.Meanwhile the sedimentary environment of red chalk is discussed as well.The main achievements and understandings are as follows:（1）There is no obvious bedding structure in the red chalk of the Middle Cretaceous in England,which indicates that the red chalk is the product of syn-sedimentary process rather than the late diagenetic stage.The red chalk is mainly composed of calcite,quartz,feldspar,chlorite,hematite,ilmenite etc..Quartz and feldspar are the important components of terrigenous detritus,which is consistent with the material source of brown gravelly sandstone in the underlying Carstone formation.（2）As a type of red beds,red chalk’s chromgenic mineral is mainly hematite and a small amount of goethite.The color depth of red chalk is uneven.First,the degree of crystallization of hematite in red chalk is different,and second,there are coarse oolitic hematite and nanoscale hematite aggregates in red chalk.These results together determine the chromgenic mechanism of red chalk.（3）The material source of Fe is one of the most important problems in the study of red chalk.The correlation of indicator elements Al₂O₃with Fe₂O₃and Mn O₂,the correlation with incompatible trace elements（such as Nb,Hf,Th and Cs）,and the relationship between Fe/Ti and Al/（Al+Fe+Mn）indicate that the material source of red chalk is mainly terrigenous clastic material and there may be some input of hydrothermal materials as well.The REE distribution pattern of the red chalk is characterized by enrichment of light REE,deficiency of heavy REE and negative Eu anomaly,which is similar to the upper crust but different from the Marine limestone.The discriminant diagrams of La/Th-Hf,La/Sc-Co/Th,La/Yb-ΣREE,Hf-Co-Th,further indicate that the non-carbonate components in the red chalk samples are derived from felsic rocks and their sedimentary derivatives in the upper crust.（4）The average volume ofδ⁵⁶Fe of red chalk in Huntington and Speeton sections is0.02‰and 0.38‰respectively,which are similar to those of bauxite.However,the volume ofδ⁵⁶Fe in submarine hydrothermal fluid and microbial red beds is obliviously negative.Based on this,the hydrothermal and microbial origins proposed by previous authors can be basically denied.What’s more,the red Cretaceous formation has mud cracks,calcareous nodules,hematite network distribution and paleosol characteristics such as secondary chalk appear at vertical fissures.Therefore,it is reasonable to first propose the pedogenesis of red chalk,which also provides valuable clues for the pedogenesis of karstic coastal environment.（5）The comparison of red chalk with continental red bed and oceanic red bed suggests that the red chalk may have been influenced by both marine and continental facies.The sedimentary environment of the red chalk was also studied based on trace elements and carbon and oxygen isotopes.The results show that the red chalk was deposited in oxidized saline water environment,and may also be affected by fresh water input,which may be similar to the current lagoon environment.Oolitic hematite dominated by concentric oolitic grains and the relationship between Al/Si and Mg/Fe indicate the initial environment of oolitic hematite formation may be similar to that of tropical or subtropical laterite formation,which also provides evidence for surface oil formation of red chalk.Based on the the above evidences and the sedimentary environment of the upper and lower strata,it is inferred that the red chalk was formed before the global transgression in the Cretaceous and experienced a certain degree of drying and pedogenic transformation in the supra-tidal environment.