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[BPT03-P01] Sedimentary environmental change of the Ediacaran Doushantuo Formation in Wenghui section, northeastern Guizhou Province, South China
Keywords:Ediararan period, Doushantuo Formation, Sedimentary facies analysis
The Ediacaran period is the time of drastic change in paleoenvironment and animal evolution in Earth history. The Ediacaran succession in South China include abundant fossils and therefore have been an important research target in understanding early biological evolution. They are composed of low-grade metamorphic sedimentary rocks deposited from shallow to deep setting. In the Ediacaran succession, carbonate rocks predominates in shallow settings and clastic rocks such as black shale increases at deeper sections (Zhu et al., 2003). This distribution pattern indicates that the depositional process of clastic rocks must be also considered to reconstruction of sedimentary environment of the Ediacaran succession in South China. However, sedimentary facies analysis of clastic rocks has not been applied sufficiently, and the details of the depositional environment remain ambiguous. In this study, we investigated to the Doushantuo Formation, which is the lower part of Ediacaran succession in Wenghui section, northeastern Guizhou Province and applied to sedimentary facies analysis predictively.
The Ediacaran succession in Wenghui section could be deposited at slope setting (Zhu et al., 2003). The 65 m-thick Doushantuo Formation is exposed along the road. The basal part of the Doushantuo Formation is composed of cap carbonate unit with tepee like structure and stromatactics-like cavities. This cap dolostone unit is overlain by the laminated dolostone unit unconformably. The black shale unit overlies the laminated dolostone unit. This unit is characterized by the interbedded lenticular dolostone in the lower part and thin carbonaceous bed in the upper part respectively. This thin carbonaceous bed shows grading upward with convolute lamina. The change from the laminated dolostone unit to black shale with thin calcareous sand in the lower part of the Doushantuo Formation indicates a deepening upward of depositional environment. The lower thick dolostone unit overlies the black shale unit unconformably. This unit implies that the depositional environment became shallower. Alternation unit of the thin dolostone and thin carbonaceous sandstone overlies the lower thick dolostone unit. The change of sedimentary facies from lower thick dolostone unit to alternation unit implies deepening upwards. The upper thick dolostone unit overlying the alternation unit composed of the dolomitic granule mainly in the lower part and dolostone with microcrystalline texture in the upper part. This change of sedimentary facies also implies deepening upward. The bedform unit overlies the upper thick dolostone unit. There are ripple and trough-type cross stratification in this unit. Algal black shale unit bearing fossils composed of the uppermost part of the Doushantuo Formation. These observations of sedimentary facies of the Doushantuo Formation in Wenghui section suggests 4 or more deepening upward. Also, the Doushantuo Formation in Wenghui section is considered to have been deposited on slope setting, but there was not slump structure. The repeated occurrence of relatively coarse-grained facies suggests that the Doushantuo Formation in Wenghui section could deposit in shallower water than previously proposed.
The Ediacaran succession in Wenghui section could be deposited at slope setting (Zhu et al., 2003). The 65 m-thick Doushantuo Formation is exposed along the road. The basal part of the Doushantuo Formation is composed of cap carbonate unit with tepee like structure and stromatactics-like cavities. This cap dolostone unit is overlain by the laminated dolostone unit unconformably. The black shale unit overlies the laminated dolostone unit. This unit is characterized by the interbedded lenticular dolostone in the lower part and thin carbonaceous bed in the upper part respectively. This thin carbonaceous bed shows grading upward with convolute lamina. The change from the laminated dolostone unit to black shale with thin calcareous sand in the lower part of the Doushantuo Formation indicates a deepening upward of depositional environment. The lower thick dolostone unit overlies the black shale unit unconformably. This unit implies that the depositional environment became shallower. Alternation unit of the thin dolostone and thin carbonaceous sandstone overlies the lower thick dolostone unit. The change of sedimentary facies from lower thick dolostone unit to alternation unit implies deepening upwards. The upper thick dolostone unit overlying the alternation unit composed of the dolomitic granule mainly in the lower part and dolostone with microcrystalline texture in the upper part. This change of sedimentary facies also implies deepening upward. The bedform unit overlies the upper thick dolostone unit. There are ripple and trough-type cross stratification in this unit. Algal black shale unit bearing fossils composed of the uppermost part of the Doushantuo Formation. These observations of sedimentary facies of the Doushantuo Formation in Wenghui section suggests 4 or more deepening upward. Also, the Doushantuo Formation in Wenghui section is considered to have been deposited on slope setting, but there was not slump structure. The repeated occurrence of relatively coarse-grained facies suggests that the Doushantuo Formation in Wenghui section could deposit in shallower water than previously proposed.