关键词: 下寒武统, 有机地球化学, 生物标志化合物, 黑色岩系, 陕西紫阳

Abstract: The Lower Cambrian black rock series in Ziyang, Shaanxi Province is rich in organic matter and metal elements (Ba, V, Cr, Ni, Mo, Au), while there is still a lack of understanding of its organic geochemical characteristics. Based on the experimental results of polarized electron microscope, morphology under scanning electron microscope, organic carbon (TOC) and saturated hydrocarbon gas chromatography-mass spectrometry(GC-MS)，the coupling relationship of organic matter and inorganic matter, organic matter source and maturity, sedimentary environment were discussed. The results show that the content of organic matter is high in the study area, and there are two kinds of occurrence modes, dispersed organic matter and aggregated organic matter. The relationship between dispersed organic matter and clay minerals is close. The average total organic carbon (TOC) content was 2.95%, affected by lithologic differences, pyrite content and types. All the samples were rich in n-alkanes, isoprenoids, steroids and terpenes. The distribution range of carbon number was from C14 to C34, which showed obvious bimodal distribution characteristics and no obvious odd-even advantage. Isoprenoid alkenes indications showed obvious phytoalkane advantage, with low value of Pr/nC17(0.36) and Ph/nC18 (0.50); The abundance of terpenoids was tricyclic terpane > pentacyclic terpane > tetracyclic terpane, a small amount of gamma wax, and the distribution of tricyclic terpane C21-C23-C24 is asymmetric inverted “V” with low average value of Tm/ Ts ratio(1.03); The distribution of regular steranes in steroids showed an asymmetric “V” distribution of C29>C27 > C28 with low average ratio of C27 rearranged steranes / C27 regular steranes(0.18) and pregnane/ C29-20r steranes(0.84). As indicated above, the origin of organic matter is mainly marine lower organisms (such as bacteria, algae, and microorganisms) and plankton, and a small amount of terrestrial lower plants are involved in the source of organic matter in some rocks (siliceous rocks); The organic matter of the whole rock has a high maturity, but it has not reached the stage of over maturity; The black rock series comes from the semi closed-closed alkaline seawater environment with anoxic reduction and low salinity.

Key words: Lower Cambrian, organic geochemistry, biomarker compounds, black rock series, Ziyang, Shaanxi Province