The catastrophic landslide-dam outburst flood, possibly caused by the same earthquake that destroyed the well-dated Lajia archaeological site, in the upper reaches of the Yellow River in China, may provide an accurate constraint on the age of the first Chinese dynasty (Xia Dynasty), which, according to Chinese ancient documents, has been associated with flood-control activities. The key link between the destructive earthquake and the megaflood is a flood-related blackish sand layer (BSL) covering the Lajia site on terraces high above the Yellow River channel. However, the BSL, which mainly consists of debris of local schist, may have originated from mudflows in local gullies rather than an outburst flood from the Yellow River that swept predominantly schist debris from channel slopes and/or a landslide dam. The composition of the fine particles in the BSL can help to discriminate the provenance of the BSL because an outburst flood would consist of suspended particles from the Yellow River, while a mudflow origin would incorporate fine particles from the local gullies. However, the similar geological origin between the Yellow River sediments and the Quaternary eolian loess and Tertiary reddish clay sediments that feed the local gullies precludes the application of traditional geochemical source tracers. This work shows that the 234U/238U activity ratio, which reflects the comminution age, of the fine particles in the BSL is very similar to that of the Yellow River sediments but not to the sediments in the local gullies, supporting an outburst flood origin of the BSL. Thus, the emergence of the Xia Dynasty can be constrained to be shortly after the sudden destruction of the Lajia site, if the outburst flood is the same event that initiated nationalization as described in the Chinese ancient documents.