Xiagou Formation

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The Xiagou Formation named for its type site in Xiagou, in the Changma Basin of Gansu Province, northwestern China, is an Early Cretaceous sequence, which is known outside the specialized world of Chinese geology as the site of a lagerstätte in which the fossils were preserved of Gansu, the earliest true modern bird. The laminated yellowish mudstones of the Xiagou Formation are the lithified remnants of varves that were laid down as extremely fine silt settled to the bottom of a tranquil freshwater lake. The result was dense anoxic bottom sediment, where the lack of bacteria slowed the processes of decay, preserving uncompressed fossils in details that include feather impressions and remnants of the webbing between the bird's toes.

Other fossils from the Xiagou will permit detailed recreation of an Early Cretaceous lake ecology. There are fossils of abundant fish fauna, Charophyta and Ostracods.