试论湖相泥质岩的地球化学二分性

GEOCHEMICAL BINARY DIVISION CHARACTER OF LACUSTRINE ARGILLITE

湖相泥质岩在地球化学上具二级二分性,即首先二分为低硼低伽马蜡烷泥质岩和高硼高伽马蜡烷泥质岩,前者可再二分为低钙混合型干酪根泥岩和高钙腐泥型干酪根泥岩;后者可再二分为高伽马蜡烷奇偶均势泥岩和较高伽马蜡烷偶奇优势泥岩。湖相泥质岩地球化学二分性是其沉积时水介质的物理化学性质和在该环境中生存的生物组合,以及生物死亡后形成的分散有机质聚集与保存条件的综合反映。

Fourty lithofacies can be distinguished in lacustrine argillite according to the geochemical research of Mesozoic-Cenozoic terrestrial basins.All the lithofacies types can besimplified by using geochemical binary-division character,i.e,depending upon geochemi-cal parameters,lacustrine argillite can be divided into the argillite with lower boron andgammacerters abundance and with higher boron and gammacerane.The former can besubdivided into the argillite with lower calcium abundance mixed-type kerogen and withhigher calcium abundance sapropel-type kerogen(simply named as type-L_1 and type-L_2 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-H_1 and type-H_2 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-L_1 argillite shows higher Rb/Ca(>1),lower Ca/Mg(<5),type-Ⅱ kerogen dominant,C_(27)-sterane/C_(29)-sterane<1,oleanane+lupane/C_(30)-hopane>0.05;type-L_2 argillite shows lower Rb/Ca(<1),higher Ca/Mg (>5),type-Ⅰkerogen,C_(27)-sterane/C_(29)-sterane>1,oleanane+lupane/C_(30)-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-H_1 argillite displays high gammacerane abundance(23%on the average),a strong predominance of phytane, C_(27)-sterane/C_(29)-sterane≥1;type-H_2argillite displays higher gammacerane(7.2% on the average),relatively weak predomi-nance of phytane, C_(27)-sterane/C_(29)-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)C_(27)-sterane/C_(29)-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-L_1 argillite deposited in well circulated aero-bic,weak-reducin fresh lake with aquatic organism and terrestrial plant input;whereastype-L_2 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-H_1 accumulated in deeper,strong reducing andalkaline,relatively fresh water stage of saline lake with aquatic organism dominant,whereas type-H_2 accumulated in relatively hypersaline stage of saline lake.