The Xiagou Formation is the middle unit of the Xinminpu Group. It is named for its type site in Xiagou, in the Changma Basin of Gansu Province, northwestern China and is considered Early Cretaceous in age. It is known outside the specialized world of Chinese geology as the site of a lagerstätte in which the fossils were preserved of Gansus yumenensis, 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.
The age of the formation has not yet been confidently determined. The underlying Chijinpu Formation is likely the same age as the Jehol Group due to the presence of similar fossils, meaning that the Xiagou Formation is probably slightly younger than the Jehol biota, dating to around the mid Aptian age.
Read more about Xiagou Formation: Paleobiota
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