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We present a new stratigraphy of the Jianchuan basin, one of the largest Cenozoic sedimentary basins in southeastern Tibet. This basin was regarded as recording sedimentation from the Eocene up to the Pliocene, and as such has been the focus of several studies aiming at constraining the environmental, tectonic and topographic evolution of the area. Within the Shuanghe and Jianchuan formations thirteen new zircon U/Pb ages and one biotite ⁴⁰Ar/³⁹Ar age of interbedded and cross-cutting ultrapotassic magmatic rocks show that a brief magmatic event occurred from ~ 35.7 to ~ 34.5 Ma (35.2 ± 0.4 Ma on average). The uppermost formation (Jianchuan Fm), supposedly Pliocene in age, is related to this magmatic event and is 35.4 ± 0.8 Ma old. All sedimentary formations are thus Eocene in age, with neither Oligocene nor Miocene sediments. The coal-bearing Shuanghe Formation yields a fossil of a large …