Terrestrial and freshwater deposits
Two sections from Early Cretaceous lacustrine strata of the Xiagou Formation from the Changma Basin in Gansu Province, China, are correlated based on their carbon isotopic compositions of bulk sedimentary organic matter and carbonate, as well as carbonate oxygen-isotopic compositions. The samples were collected from fossiliferous strata, which contain well-preserved Cretaceous bird remains. The sections are primarily correlated based on a two-step increase in δ13Corg with an overall magnitude of c. 12.5‰. The stratigraphic variations in carbon isotopes within the two lacustrine sections are correlated with global carbon isotope variations C3–C7 based on marine carbon isotope records. This correlation places the Xiagou lacustrine strata in this locality within the early Aptian Stage, specifically, the Selli Equivalent, which is associated with Ocean Anoxic Event 1a.