This study describes the clay mineralogy of the Lower Cretaceous(Barremian)rocks of the Wessex Basin for paleoenvironmental interpretations.Seventy-four samples were subjected to optical microscopy,scanning electron m...This study describes the clay mineralogy of the Lower Cretaceous(Barremian)rocks of the Wessex Basin for paleoenvironmental interpretations.Seventy-four samples were subjected to optical microscopy,scanning electron microscopy/energy-dispersive X-ray spectroscopy(SEM/EDS),and quantitative evaluation of minerals by scanning electron microscopy(QEMSCAN^(R))techniques.The results revealed an illite-dominated sedimentation in most sections of the basin,with kaolinite,chlorite,smectite and glauconite occurring in subordinate quantities.Inferred paleoclimatic conditions from the clay mineral trends indicates a warm and dry climate,with seasonal precipitation.Kaolinite to illite ratios indicate that more arid climate conditions were prevalent but brief periods of warm and humid conditions were also present.A strong positive correlation between chlorite and tourmaline indicates that excess chlorite may have been contributed from tourmaline-chlorite-schists in a tourmalinedominated provenance.SEM confirms that most of the clay minerals are detrital in origin but authigenic kaolinite is also present as vermiform and mica-replacive kaolinite which formed during early diagenetic modification from flushing meteoric waters in warm humid climates.This study is significant because it demonstrates the importance of multi-proxy methods for understanding clay minerals within sedimentary basins for interpreting the paleoclimatic conditions of depositional systems.展开更多
Modelling the diagenetic history of a tectonically active basin is crucial for understanding the depositional and post-depositional processes including the evolution and development of porosity and permeability.This s...Modelling the diagenetic history of a tectonically active basin is crucial for understanding the depositional and post-depositional processes including the evolution and development of porosity and permeability.This study describes the diagenetic imprints of the inverted Lower Cretaceous Weald Basin and examines the depositional and tectonic controls on the diagenetic,porosity and permeability evolution of its sandstones.Seventy-two representative sandstone samples from the Weald Basin were subjected to detailed microscopic(optical and scanning electron)analyses.The dominant early diagenetic fabrics include grain-coating detrital clays,degraded and dissolved detrital grains,and vermiform kaolinite pore-filling cement while uplift-related or telogenetic characteristics are mainly post-compactional pore-filling goethite and hematite cement.This work is significant because it presents the first diagenetic model of the Lower Cretaceous Wealden sandstones which can be very useful for understanding basins with similar depositional and tectonic settings.展开更多
Analyses of clay minerals within the Early Cretaceous Weald Basin,Southeast England reveal kaolinite,illite and chlorite as the main detrital clay minerals while glauconite and smectite are subordinates.A kaolinite-ri...Analyses of clay minerals within the Early Cretaceous Weald Basin,Southeast England reveal kaolinite,illite and chlorite as the main detrital clay minerals while glauconite and smectite are subordinates.A kaolinite-rich assemblage which characterized the sand-dominated Ashdown and Tunbridge Wells Sand formations and an illite-dominated assemblage associated mostly with the Wadhurst Clay and Weald Clay formations are recognized.Kaolinite was enriched in the Ashdown and Tunbridge Wells Sand formations during warm and humid climate with high precipitation that encouraged chemical weathering and leaching,while cold and dry conditions favoured the concentration of illite in the Wadhurst Clay and Weald Clay formations.Rainfall patterns associated with warm climate were drastically reduced during the drier climatic conditions.Most clay minerals are detrital in origin,with chlorite being more prominent than previously recognized.Contrary to previous studies and assumptions,this study revealed that authigenic clay minerals are present in the Hastings Beds,with vermiform and mica-replacive kaolinite being the most common,consistent with humid depositional environments.Isolated authigenic illite is also present,along with a chloritized grain,providing evidence for mesodiagenesis.The absence of dickite and occurrence of kaolinite,suggest that authigenic illite formed in relatively shallow burial conditions,indicating a maximum burial depth of2500 m-3000 m,about 1000 m deeper than previous estimates of 1500 m-2000 m.Authigenic clay minerals are absent in the Weald Clay Formation possibly because of hindered flow of meteoric water and limited growth space for authigenic minerals.This study is significant in:1)reinforcing multiple methods to facilitate a robust and balanced knowledge of formation and transformation of clay minerals;2)investigating detrital and authigenic clay mineral assemblages when assessing the palaeoenvironments of sedimentary basins.展开更多
文摘This study describes the clay mineralogy of the Lower Cretaceous(Barremian)rocks of the Wessex Basin for paleoenvironmental interpretations.Seventy-four samples were subjected to optical microscopy,scanning electron microscopy/energy-dispersive X-ray spectroscopy(SEM/EDS),and quantitative evaluation of minerals by scanning electron microscopy(QEMSCAN^(R))techniques.The results revealed an illite-dominated sedimentation in most sections of the basin,with kaolinite,chlorite,smectite and glauconite occurring in subordinate quantities.Inferred paleoclimatic conditions from the clay mineral trends indicates a warm and dry climate,with seasonal precipitation.Kaolinite to illite ratios indicate that more arid climate conditions were prevalent but brief periods of warm and humid conditions were also present.A strong positive correlation between chlorite and tourmaline indicates that excess chlorite may have been contributed from tourmaline-chlorite-schists in a tourmalinedominated provenance.SEM confirms that most of the clay minerals are detrital in origin but authigenic kaolinite is also present as vermiform and mica-replacive kaolinite which formed during early diagenetic modification from flushing meteoric waters in warm humid climates.This study is significant because it demonstrates the importance of multi-proxy methods for understanding clay minerals within sedimentary basins for interpreting the paleoclimatic conditions of depositional systems.
文摘Modelling the diagenetic history of a tectonically active basin is crucial for understanding the depositional and post-depositional processes including the evolution and development of porosity and permeability.This study describes the diagenetic imprints of the inverted Lower Cretaceous Weald Basin and examines the depositional and tectonic controls on the diagenetic,porosity and permeability evolution of its sandstones.Seventy-two representative sandstone samples from the Weald Basin were subjected to detailed microscopic(optical and scanning electron)analyses.The dominant early diagenetic fabrics include grain-coating detrital clays,degraded and dissolved detrital grains,and vermiform kaolinite pore-filling cement while uplift-related or telogenetic characteristics are mainly post-compactional pore-filling goethite and hematite cement.This work is significant because it presents the first diagenetic model of the Lower Cretaceous Wealden sandstones which can be very useful for understanding basins with similar depositional and tectonic settings.
基金CGG Robertson provided collaborative funding by conducting QEMSCAN?,SEM and thin section analyses for the project。
文摘Analyses of clay minerals within the Early Cretaceous Weald Basin,Southeast England reveal kaolinite,illite and chlorite as the main detrital clay minerals while glauconite and smectite are subordinates.A kaolinite-rich assemblage which characterized the sand-dominated Ashdown and Tunbridge Wells Sand formations and an illite-dominated assemblage associated mostly with the Wadhurst Clay and Weald Clay formations are recognized.Kaolinite was enriched in the Ashdown and Tunbridge Wells Sand formations during warm and humid climate with high precipitation that encouraged chemical weathering and leaching,while cold and dry conditions favoured the concentration of illite in the Wadhurst Clay and Weald Clay formations.Rainfall patterns associated with warm climate were drastically reduced during the drier climatic conditions.Most clay minerals are detrital in origin,with chlorite being more prominent than previously recognized.Contrary to previous studies and assumptions,this study revealed that authigenic clay minerals are present in the Hastings Beds,with vermiform and mica-replacive kaolinite being the most common,consistent with humid depositional environments.Isolated authigenic illite is also present,along with a chloritized grain,providing evidence for mesodiagenesis.The absence of dickite and occurrence of kaolinite,suggest that authigenic illite formed in relatively shallow burial conditions,indicating a maximum burial depth of2500 m-3000 m,about 1000 m deeper than previous estimates of 1500 m-2000 m.Authigenic clay minerals are absent in the Weald Clay Formation possibly because of hindered flow of meteoric water and limited growth space for authigenic minerals.This study is significant in:1)reinforcing multiple methods to facilitate a robust and balanced knowledge of formation and transformation of clay minerals;2)investigating detrital and authigenic clay mineral assemblages when assessing the palaeoenvironments of sedimentary basins.
