17:15 〜 18:30
[BCG04-P08] Chemostratigraphy and paleoceanographic study of the Ediacaran Doushantuo Formation in Beidoushan, Guizhou province
★Invited Papers
キーワード：エディアカラ、ドウシャントゥオ、ウェンアン、化学層序
The animal embryos and other fossils from the Doushantuo Formation (635-551 Ma) in Weng’an area, Guizhou prefecture, South China, are important findings to understand the early animal evolution. However, the lack of reliable absolute ages makes it difficult to establish the stratigraphic correlation of the fossils form Weng’an area. This study suggests the multi-isotopic proxies that are carbonate carbon isotope (δ13Ccarb), organic carbon isotope (δ13Corg) and strontium isotopic ratio (87Sr/86Sr) from the Doushantuo Formation in Beidoushan section of Weng’an area and discusses the stratigraphic correlation.
Beidoushan section is about 50 m thick and includes the uppermost Nantuo Formation, the Doushantuo Formation and the lowermost Dengying Formation in ascending order. The Nantuo Formation composed of post-Marinoan diamictite is extensively distributed in the Ediacaran Yangtze platform. The Doushantuo Formation in Beidoushan section is mainly composed by dolostone and phosphatic rock, and the Dengying Formation mostly consists of dolostone. Based on the lithology and δ13Ccarb trend that varies from -4.8 ‰ to +2.8 ‰, we subdivided the Doushantuo Formation into 4 units that the cap dolostone unit, the thick phosphate unit, the lower alteration unit and the upper alteration unit in ascending order. The δ13Ccarb gradually decreases from +2.8 ‰ to -4.8 ‰ in the thick phosphate unit. The δ13Ccarb increases abruptly at the bottom of the lower alteration unit and sustains around +1.5 ‰ in this unit. In the upper alteration unit, the δ13Ccarb decreases to -2.5 ‰ and forms negative shift again. The δ13Corg values of the Doushantuo Formation range from -31.8 ‰ to -23.5 ‰, and show a different trend from the δ13Ccarb. Through the thick phosphate unit to the lower alteration unit, the δ13Corg shows a gradually increasing trend, from -30.5 ‰ to -27.0 ‰. Then it decreases to -31.8 ‰ and forms a negative shift in the upper alteration unit. Strontium isotopic ratios (87Sr/86Sr) of the Doushantuo Formation range from 0.7082 to 0.7188. Although there are clearly higher values, some ratios are consistent with the typical Ediacaran ratios as Ediacaran (Sawaki et al., 2010). The 87Sr/86Sr ratio is 0.7083 at the lower part of the thick phosphate unit and 0.7091 at the upper part of the lower alteration unit. In the upper alteration unit, the 87Sr/86Sr ratios are from 0.7087 to 0.7091.
We correlated our results with those of Three Gorges stratotype (Sawaki et al., 2010). The negative shift of δ13Ccarb and 87Sr/86Sr ratio of 0.7083 in the thick phosphatic unit and the constant upward trend of δ13Ccarb and 87Sr/86Sr ratio of 0.7091 in the lower alteration unit may indicate that the thick phosphatic unit and the lower alteration unit are correlated with the phase 2 and the phase 3 of Three Gorges (Sawaki et al., 2010), though it is necessary to add new strontium isotopic ratios. The stratigraphic profile from the negative shift of δ13Ccarb and δ13Corg and 87Sr/86Sr ratios around 0.7089 in the upper phosphate unit indicates that this unit could correlated to the phase 4 of the Doushantuo Formation in Three Gorges (Sawaki et al., 2010). This stratigraphic correlation could enable to constrain the emergence of the Weng’an biota.
Beidoushan section is about 50 m thick and includes the uppermost Nantuo Formation, the Doushantuo Formation and the lowermost Dengying Formation in ascending order. The Nantuo Formation composed of post-Marinoan diamictite is extensively distributed in the Ediacaran Yangtze platform. The Doushantuo Formation in Beidoushan section is mainly composed by dolostone and phosphatic rock, and the Dengying Formation mostly consists of dolostone. Based on the lithology and δ13Ccarb trend that varies from -4.8 ‰ to +2.8 ‰, we subdivided the Doushantuo Formation into 4 units that the cap dolostone unit, the thick phosphate unit, the lower alteration unit and the upper alteration unit in ascending order. The δ13Ccarb gradually decreases from +2.8 ‰ to -4.8 ‰ in the thick phosphate unit. The δ13Ccarb increases abruptly at the bottom of the lower alteration unit and sustains around +1.5 ‰ in this unit. In the upper alteration unit, the δ13Ccarb decreases to -2.5 ‰ and forms negative shift again. The δ13Corg values of the Doushantuo Formation range from -31.8 ‰ to -23.5 ‰, and show a different trend from the δ13Ccarb. Through the thick phosphate unit to the lower alteration unit, the δ13Corg shows a gradually increasing trend, from -30.5 ‰ to -27.0 ‰. Then it decreases to -31.8 ‰ and forms a negative shift in the upper alteration unit. Strontium isotopic ratios (87Sr/86Sr) of the Doushantuo Formation range from 0.7082 to 0.7188. Although there are clearly higher values, some ratios are consistent with the typical Ediacaran ratios as Ediacaran (Sawaki et al., 2010). The 87Sr/86Sr ratio is 0.7083 at the lower part of the thick phosphate unit and 0.7091 at the upper part of the lower alteration unit. In the upper alteration unit, the 87Sr/86Sr ratios are from 0.7087 to 0.7091.
We correlated our results with those of Three Gorges stratotype (Sawaki et al., 2010). The negative shift of δ13Ccarb and 87Sr/86Sr ratio of 0.7083 in the thick phosphatic unit and the constant upward trend of δ13Ccarb and 87Sr/86Sr ratio of 0.7091 in the lower alteration unit may indicate that the thick phosphatic unit and the lower alteration unit are correlated with the phase 2 and the phase 3 of Three Gorges (Sawaki et al., 2010), though it is necessary to add new strontium isotopic ratios. The stratigraphic profile from the negative shift of δ13Ccarb and δ13Corg and 87Sr/86Sr ratios around 0.7089 in the upper phosphate unit indicates that this unit could correlated to the phase 4 of the Doushantuo Formation in Three Gorges (Sawaki et al., 2010). This stratigraphic correlation could enable to constrain the emergence of the Weng’an biota.