The lower Cambrian Niutitang Formation, a widespread black shale deposition, is of geological interest because of its polymetallic beds, Cambrian explosion, depositional ages, dramatic environmental changes and so on. Previous study focused mainly on inorganic geochemistry and few studies have investigated the organic fractions of upper Neoproterozoic-lower Cambrian strata in South China. Here we report a study of biomarkers plus organic carbon isotopes for black shales from Ganziping, Hunan Province (China). All the saturated hydrocarbon fractions have a unimodal distribution of n-alkanes, a high content of short-chain alkanes and maximize at C 19 or C 20 (C 23 for sample Gzh00-1). The C 27 /C 29 sterane ratio ranges from 0.77 to 1.20 and 4-methylsteranes are in low abundance. These parameters indicate that algae and bacteria are the important primary producers. Furthermore, biomarker maturity proxies show the samples to be higher maturity. The low Pr/Ph values (<0.7) suggest that the samples were deposited under anoxic conditions and, likely, under stratified water columns. In addition, 25-norhopanes and gammacerane are present as diagnostic indicators of normal marine salinity and dysoxic to anoxic conditions. During the Early Tommotian, known to coincide with a transgression event, small shelly fossils increased in abundance and diversity. Moreover, positive δ 13 C org excursions close to 1.4‰ occur at the base of the Tommotian stage. In summary, the Early Cambrian black shales were deposited under dramatic paleoenvironmental changes, including oceanic anoxia, higher primary productivity and sea-level rise.
CHEN LanZHONG HongHU RuizhongXIAO JiafeiTSAI Louis Loung-YieLIN Andrew Tien-ShunZOU Yanrong
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.
Guiwen XuLan ChenWeiguang ZhuXuejuan DaZhangxiong ZhuHaisheng Yi
The Biluo Co and Amdo 114 station, northern Tibet, cropping out the Early Toarcian and Middle-Late Tithonian (Jurassic) organic-rich black shales, have been a focus to petroleum geologists in discussing their oil-producing potential. This paper first reports the trace elements and rare earth elements to discuss the paleoenvironments, redox conditions and sedimentary mechanisms of those black shales. Both sections exhibit variation in trace element abundances with concentrations <0.1 ppm to 760 ppm, mostly enriched in V, Cr, Ni, Cu, Zn, Mo, Ba and U. Element ratios of Ni/Co, V/Cr, U/Th and V/(V+Ni) plus U were used to identify redox conditions. The shale-normalized rare earth element (REE) patterns are characterized by the flat-shale type with instable Ce anomalies and very weekly positive Eu anomalies. Positive Ceanom values are significant with values varying between - 0.064 and 0.029 in Biluo Co, which may be interpreted as release of REE and input of riverine terrestrial matter with rich Ce (resulting in pH change) during the anoxic conditions. In the middle parts of Amdo 114 station, distinct negative Ceanom values are observed (-0.238 to -0.111) and associated surface water warming were interpreted as being related to a major sea level rise. In contrast, the formation of the black shales in the lower and upper part of the studied succession took place during a cooler (Ceanom values >-0.10), lower surface water productivity, and lower sea-level stage. Thus, we emphasize the role of different factors that control the formation of local and regional black shales. The most important factors are sea-level fluctuations and increasing productivity.
CHEN LanYI HaishengTSAI Louis Loung-YieXU GuiwenDA XuejuanLIN Andrew Tien-Shun
