关键词: 地层对比；有机碳同位素；深水相区；寒武纪早期；湖南

Abstract:

The early Cambrian is one of the critical periods during Earth evolution involving significant evolution of marine
environment and metazoans. However, the early Cambrian strata in the deep-water setting along the southeastern margin of the Yangtze Platform are still lack of systematically and accurately stratigraphic correlations. Here we conduct a high-resolution organic carbon isotope chemostratigraphy of the deep-water chert in the Liuchapo Formation and black shales in the Xiaoyanxi Formation, which werecollected from drill cores in the Huitong section of Hunan Province. Results indicate four positive δ13Corg excursions (termed P1, P2, P3 and P4 ) and two negative δ13Corg excursions (termed N1 and N2 ) in ascending order. Combined with the fossil records and zircon U-Pb dating data, we correlate the δ13Corg curve of the Huitong section with the δ13Corg and δ13Ccarb curves of other sections in Hunan and shallow-water areas including Yunnan and Three Gorges. The correlation implies that the Ediacaran-Cambrian boundary was placed at the negative δ13Corg excursion (Basal Cambrian Carbon isotope Excursion, BACE) in the upper Liuchapo Formation of Hunan Province. This negative δ13Corg excursion (BACE), however, has not been identified at the Huitong section, owning to the limited drill core depth.
The three positive δ13Corg excursions P1, P2 and P3 can be correlated with ZHUCE (ZHUjiaqing Carbon isotope Excursion, Stage 2),
CARE (Cambrian Arthropod Radiation isotope Excursion, Stage 3) and MICE (Mingxinsi Carbon Isotope Excursion, Stage 4) δ13Corg
excursions, respectively. In contrast, the two negative δ13Corg excursions N1 and N2 are likely correlated with SHICE (SHIyantou Carbon
isotope Excursion, Stage 2) and AECE (Archaeocyathid Extinction Carbon isotope Excursion, Stage 4) δ13Corg excursions, respectively.
Therefore, the upper Liuchapo and the basal Xiaoyanxi formations probably belong to the Cambrian Stage 2. The lower Xiaoyanxi
Formation belongs to the Cambrian Stage 3 and the middle-upper Xiaoyanxi Formation belongs to the Cambrian Stage 4. However, it is
still unclear whether the top of the Xiaoyanxi Formation covers the end of the Cambrian Stage 4. The negative δ13Corg excursions
probably result from the transportation of 12C-enriched anoxic bottom water through upwelling from deep to shallow water, and might be
closelyassociatedwiththemassextinctionofEdiacaranfauna,smallshellyfossilsandarchaeocyathids.Incontrast,theflourishofCambrian
faunacouldincreasetheprimaryproductivityandassociatedrateoforganicmatterburial,whichleadtothepositiveδ13Corg excursions.

Key words: stratigraphic correlation, organic carbon isotopes, deep-water facies, early Cambrian, Hunan