Origin of excess barium in the Cambrian shale of Yichang area, western Hubei, and its implication for organic matter accumulation

doi:10.13745/j.esf.sf.2022.5.34

Abstract

Abstract:

We analyzed the total organic and inorganic carbon, trace elements, and rare earth elements in shale samples from the Lower Cambrian Yanjiahe and lower Shuijingtuo Formations of well EYY1, Yichang area, western Hubei Province. Combining with already published studies and data on other 7 Cambrian sections in the middle-upper Yangtze region, we discussed the origin and source of excess barium (Ba_ex) in the organic-rich Cambrian Shuijingtuo shale and their implications for paleo-productivity. According to our analysis, there is a general barium enrichment in the organic-rich Cambrian shale of the middle-upper Yangtze region. The Ba_ex level increases from the shallow-water inner shelf to the deepwater outer shelf and slope then to the hydrothermal depositional area, which implies that the Ba_ex level increase is controlled by the distance from terrigenous source and by the paleo-geographic background. The excess Ba is of hydrothermal or biogenic origins, and the Ba_ex-Eu anomaly plot can be utilized to infer its main source. The Ba_ex levels are extremely high (>10000 μg/g) in the hydrothermal depositional regions across northern Guizhou to northwest Hunan, with obvious positive Eu anomalies in these regions. The anomaly values are positively correlated with Ba_ex levels, indicating that Ba_ex is mainly of hydrothermal origin and change in Ba_ex reflects the intensity of hydrothermal activity. In contrast, the Ba_ex levels are not as high (5000-10000 μg/g) in western Hubei and southern Guizhou and relatively low (<1000 μg/g) in the western Sichuan Basin, where no positive Eu anomalies are present in these regions, indicating Ba_ex is of biogenic origin and change in Ba_ex is related to paleo-productivity level. By comparing TOC contents, U/Th ratios, and Ba_ex levels in Cambrian shales from different areas of middle-upper Yangtze, we found the Yichang area has a relatively high paleo-productivity level and strong reducing depositional environment, which provides rich organic source and favorable preservation conditions for organic matter accumulation in the Cambrian Shuijingtuo shale formation.

Key words: middle-upper Yangtze region, Cambrian, Shuijingtuo Formation, EYY1 well, organic-rich shale, excess barium, rare earth elements

CLC Number:

P595

LUO Huan, SHAO Deyong, MENG Kang, ZHANG Yu, SONG Hui, YAN Jianping, ZHANG Tongwei. Origin of excess barium in the Cambrian shale of Yichang area, western Hubei, and its implication for organic matter accumulation[J]. Earth Science Frontiers, 2023, 30(3): 66-82.

Figures/Tables 11

Fig.1 Geological sketch map of Yichang area, western Hubei Province. Modified after [11].

Fig.2 Paleo-geographic map of the middle-upper Yangtze region in the Early Cambrian Epoch 2 (521-507 Ma). Modified after [34].

Table 1 Basic information on the studied Cambrian sections in the middle-upper Yangtze region

编号	剖面名称	剖面位置	古地理背景	采样层位	样品总数^*	寒武系第二统富有机质段厚度^***/m	富有机质段内样品数	参考文献
1	沙滩剖面	四川南江	浅水内陆棚	郭家坝组下部	26	32(28~60 )	10	[1]
2	W201	四川威远	浅水内陆棚	筇竹寺组下部	20	30(2 820~2 790 )	5	[4]
3	N206	四川长宁	资阳—长宁裂陷槽内部	筇竹寺组下部	10	31(1 890~1 859 )	7	[4]
4	EYY1	湖北宜昌	深水外陆棚	岩家河组和水井沱组下部	64	63(3 057~2 994 )	43	本研究
5	YK1	重庆酉阳	深水外陆棚 (靠近热水沉积区)	牛蹄塘组	53^**	37(62~25 )	42^*****	[2]
6	GMD1	贵州湄潭	热水沉积区	牛蹄塘组	44	110(1 709~1 599 )	37	[5]
7	松桃剖面	贵州松桃	热水沉积区	九门冲组下部	14	>16^****(44~60 )	10	[1]
8	TX1	贵州岑巩	大陆斜坡	牛蹄塘组	17	42(1 817~1 775 )	10	[5]

Table 2 Geochemical analysis results for samples from the EYY1 section

Fig.3 Stratigraphic column diagram and geochemical proxies profiles profiles for the EYY1 section

Fig.4 Stratigraphic correlation of organic-rich shale formations ( blue shaded area) of Cambrian Series 2 in the middle-upper Yangtz region

Fig.5 Log-linear plot of whole rock Ba and Th contents in shales for Baex assessment

Fig.6 Baex-Eu anomaly chart for the identification of Baex source in organic-rich shale

Fig.7 Euhedral barite in the upper Yanjiahe Formation (bed 2) of the EYY1 section

Fig.8 Vein barite in sample with abnormally high Baex level from the lower part of organic-rich interval of the Shuijingtuo Formation, EYY1 section (lower part of bed 3)

Fig.9 Box plots of TOC contents, U/Th ratios, Baex levels, and Eu anomaly values in Cambrian Stage 2 shale samples from different areas of the middle-upper Yangtze region

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