STABLE ISOTOPE COMPOSITION OF PEDOGENIC CARBONATE FROM THE ORDOS AND SICHUAN BASINS: RECORD OF TOPOGRAPHY DEGRADATION?

The stable isotope geochemical composition of pedogenic carbonate nodules are used to test the hypothesis that subduction of South China continental crust beneath the North China block during the early Mesozoic produced an elevated plateau associated with the Qinling-Dabie orogen, analogous to subduction of Indian crust beneath the Asian plate to produce the Tibet plateau. The Ordos foreland basin, which flanks the Qinling-Dabie orogen to the north, and the Sichuan basin, which flanks the orogen to the south, both contain Middle Triassic through Late Jurassic non-marine synorogenic strata with carbonate nodule-bearing paleosols. Analyses were conducted in the stable isotope laboratory at Stanford University on six samples collected from Middle Triassic, Middle Jurassic, and Lower Cretaceous strata in southern and southwestern Ordos basin and on three samples collected from Upper Jurassic and Lower Cretaceous strata in the northwestern Sichuan basin. The d¹⁸O composition of pedogenic carbonate nodules in the Ordos basin increases ~5‰ from Middle Triassic to Middle Jurassic. Values of d¹⁸O then appear to remain stable from the Middle Jurassic through the Early Cretaceous. If a topographic plateau had developed in association with subduction of South China continental crust during the Triassic, then the observed enrichment in d¹⁸O of Ordos basin pedogenic carbonate may be due to the degradation of Qinling-Dabie orogen topography during the Middle Triassic to the Middle Jurassic, such that orography no longer caused ¹⁸O depletion of airmasses carrying precipitation to the region. On the other hand, this shift may be a result of a change in precipitation provenance to a source of water vapor enriched in ¹⁸O. The d¹⁸O values of Upper Jurassic and Lower Cretaceous samples of the Sichuan basin are within 1.5‰ of d¹⁸O values for Middle Jurassic and Lower Cretaceous samples of the Ordos basin; a difference of this magnitude may not necessarily be due to variation in deuterium content in precipitation but may be due to soil temperature effects. Additional sampling to resolve temporal and spatial trends may be able to distinguish the mechanisms driving the change in stable isotopic composition of pedogenic carbonate nodules of central China during the Mesozoic.