Application of Biological Markers in the Assessment of Lower Paleozoic Sedimentary Palaeoenvironments of the Tarim Basin

Cambrian and Odovician marine sediments are widespread in the Keping and Kylyketake regions of the Tarim Basin. The destribution of aliphatic biomarkers of 22 samples indicate that salified lagoon sediments, freshened lagoon sediments, shallow shelf and lower slope-shelf sediments, gravity flow sediments of shallow sea slope, deep water shelf sediments and sea floor basin sediments are different in the distribution of biomarkers. The differences between salified and freshened lagoon sediments in the distribution of biomarkers reflect the difference of palaeoclimate and biological input. The formers Pr/Ph ratios 1, contents of gammacerane are 0.28-0.34. and mor abundant β- carotane, while the laters 1, 0.07-0.12, containing pregnane and no β-carotane, respectively. Shallow shelf and lower slope-shelf sediments have the Pr/Ph ratios of 0.76-1.12 and no β-carotane. and are characterized by lower carbon number n-alkanes with odd carbon predominance or even carbon predominance, the contents of hopane sieries are very lower and demethylated hopanes are present abundantly in the triterpanes. The gravity flow sediments of shallow sea slope contain β-carotane, pregnane. lower carbon number n-alkanes with odd carbon predominance or even carbon predominance and alkyl cyclohexanes with even carbon predominance. Gammacerane contents of deep water shelf sediments are 0.04-0.18. Pr/Ph ratios are 1.00-1.82, pregnane is more abundant, cholestane/ethylcholestane ratios are 1.22-1.38. Sea floor basin sediments are characterized by the Pr/Ph ratios of 0.50-0.75, β-carotane contents are more abundant, gammacerane contents are 0.18-0.27, tricyclic terpanes contents are lower, alkyl cyclohexanes have even preference. Gammacerane contents are related to salinity. The Pr/Ph ratio and c-carotane content reflect the reduction degree of sedimentary environments. Pr/Ph 1 and more abundant β-carotane content indicate strongly reducing sedimentary environment.The Pr/Ph ratios are also influenced by the zooplankton input. Lower carbon number alkanes with odd over even preference or even over odd preference, alkyl cyclonhexanes with even over odd prefrence, tricyclic terpane distribution and sterane distribution are very important for the assessment of palaeoenvironments of Lower Palaeozoic sediments. Tricyclic terpane contents are lower and the(C₂₀+C₂₁) /(C₂₃+ C₂₄) ratios of tricyclic terpanes are 1 in salified lagoon vedinents, the gravity flow sediments of shallow sea slope and sea floor basin sediments. Tricyclic terpane contents are higher and the(C₂₀+C₂₁)(C₂₃+C₂₄) ratios are 1 in freshened sediments and deep water shelf sediments.