摘要
The Jurassic black shales of the Qiangtang Basin,northern Tibet are known to be amongst the most important source rocks.To date,there is no detailed study on the organic geochemistry of the Bathonian/Callovian coccolith-bearing sediments and their source-rock characteristics.Here,we focus on the black limestones,marls,and shales at the Amdo 114 station section using Rock–Eval pyrolysis and biomarkers for sources,types,and thermal maturity of organic matter and the reconstructed depositional environment.All samples are characterized by(1)relatively high TOC values(0.20–2.56 wt%)and relatively low Tmax values between 428 and 452℃;(2)shortchain n-alkanes with a maximum at C_(14) to C_(18) and Pr/Ph>1;(3)31–35 homohopanes and gammacerane with lower abundance;and (4)C_(27)–C_(28)–C_(29) regular steranes with C_(29) being slightly dominant.The organic matter is TypeⅡand thus dominantly derived from phytoplankton,especially algae.Thermal maturity indicators include the Ts/(Ts+Tm),C_(31)22S/(22S+2R),C_(29)aaa20S/(20S+0R),C_(29)ββ/(ββ+aa)and C_(27)Dia/(Dia+Reg),which are regarded as immature to very early mature.Additionally,the high Pr/Ph,Pr/nC_(17),and Ph/nC_(18) ratios,Pr/Ph vs Gammacerane index suggest that these black shales were deposited in unstratified marine waters of normal salinity under dysoxic to weakly reduced conditions.
基金
supported by the National Natural Science Foundation of China (Grants Nos.41572095,42172121,41102066,and 41572089)
the Opening Foundation of the State Key Laboratory of Marine Geology,Tongji University (China) (Grant No.MGK1703)。
