Carbon isotopic analysis of lacustrine strata from the Chijinpu formation, Gansu Province, China




Tabbert, Colin David

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Northwest China has been an abundant source of fossils from the Late Jurassic and Cretaceous, with new discoveries rivalling the famous Jehol Biota of northeastern China. Although fossils can provide insight into paleontological questions, their use in biostratigraphy can be low resolution in many cases, particularly in continental strata. Broad age ranges for formations and groups can be delineated, but age constraint remains coarse. High resolution carbon isotope chemostratigraphy can be used to help refine age correlations of formations and fossils found in the area. This study develops a carbon isotopic record of sedimentary organic carbon from near the base of the type section of the Chijinpu Formation from the Xinminpu Group, northwest China. A total of 484 samples are used in this study, collected at 25 centimeter intervals, when possible, across 151.25 meters of section. All samples were manually crushed, decarbonated, and subsequently analyzed for δ 13 Corg (/ vs VPDB). Correlation of this section to global marine and terrestrial records is accomplished based on positive and negative excursions from an average δ13C baseline of -23.23 /, and broken down into five correlative segments. δ13 Corg of samples in the analyzed section range from -28.67 / to -19.01 /. Based on regional radiometric age dates, as well as fossil data, it is proposed that the Chijinpu Formation is likely Late Jurassic to Early Cretaceous in age. Comparison of δ13 C to global records revealed a correlation to the upper Early Valanginian (particularly the carbon isotope excursion associated with the Weissert event) to lower Early Hauterivian. This constrains the age of the Chijinpu Formation, and thus the Lower Xinminpu Group, to approximately 136 -132 Ma.


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carbon isotopes, chemostratigraphy, Chijinpu, China, Early Cretaceous, Xinminpu