Abstract:

Fourty lithofacies can be distinguished in lacustrine argillite according to the geochemical research of Mesozoic-Cenozoic terrestrial basins.All the lithofacies types can be simplified by using geochemical binary-division character,i.e,depending upon geochemical parameters,lacustrine argillite can be divided into the argillite with lower boron and gammacerters abundance and with higher boron and gammacerane.The former can be subdivided into the argillite with lower calcium abundance mixed-type kerogen and with higher calcium abundance sapropel-type kerogen(simply named as type-L₁ and type-L₂ argillite respectively);the latter can be subdivided into the argillite with high gammacerane abundance balance of even over odd carbon number n-alkanes(OEP≈1) and with higher gammacerane abundance,even numbered carbon predominance n-alkanes (OEP<1) (simply named as type-H₁ and type-H₂ argillite respectively).The concrete charactersof argillite with lower boron and gammacerane abundance are as follows:lower B/Ga(<4),Sr/Ba(<1 generally)and V/Ni(<10),gammacerane(2.5%),higher Pr/Ph<1),OEP>1,in which type-L₁ argillite shows higher Rb/Ca(>1),lower Ca/Mg(<5),type-Ⅱ kerogen dominant,C₂₇-sterane/C₂₉-sterane<1,oleanane+lupane/C₃₀-hopane>0.05;type-L₂ argillite shows lower Rb/Ca(<1),higher Ca/Mg (>5),type-Ⅰkerogen,C₂₇-sterane/C₂₉-sterane>1,oleanane+lupane/C₃₀-hopane<0.05.In contrast,the characters of argillite with higher boron and gammacerane are as follows:higher B/Ga (>4),Sr/Ba(>1)and V/Ni(>10),gammacerane(>2.5%),lower Pr/Ph(<0.5),OEP<1,in which type-H₁ argillite displays high gammacerane abundance(23%on the average),a strong predominance of phytane, C₂₇-sterane/C₂₉-sterane≥1;type-H₂argillite displays higher gammacerane(7.2% on the average),relatively weak predomi-nance of phytane, C₂₇-sterane/C₂₉-sterane<1.Element chemistry and biomarker compound molecular parameters can be used as a means for sedimentary environment.(1)Gammacerane is an indicator of higher paleosalinity and reducing condition;(2)A strong predominance of phytane may be an indicator of hypersaline environment;(3)A preference of even over odd carbon numberalkanes(OEP<1)is an indicator of reducing condition;(4)Pr/Ph is an indicator of oxidation-reduction environment;(5)Kerogen type is the indicator of primary organic matter type;(6)C₂₇-sterane/C₂₉-sterane has been used for the analysis of organism types; (7)Triterpane compound oleanane and lupane are the indicator of terrestrial plant input.Depending upon the molecular parameters mentioned above and combining depositional- physical characters,lithology mineralogy and palaeontology,the following conclutioncould be gotten:geochemical binary divition character of lacustrin argillite is mainly a re-flection of the differentiation of water-medium condition in depositional environment andorganism assemblage in it.The argillite with lower boron and gammacerane abundanceformed in fresh-brackish lake,in which type-L₁ argillite deposited in well circulated aero-bic,weak-reducin fresh lake with aquatic organism and terrestrial plant input;whereastype-L₂ argillite deposited in anaerobic,reducing,brackish intermittent stratified lake withaquatic assemblage dominant.The argillite with higher boron and gammacerane abun-dance formed in saline lake,in which type-H₁ accumulated in deeper,strong reducing andalkaline,relatively fresh water stage of saline lake with aquatic organism dominant,whereas type-H₂ accumulated in relatively hypersaline stage of saline lake.