Radiometric dating
Relative age dating of the Onondaga places its formation in the Eifelian to Givetian stage of the Middle Devonian period, or 391.9 to 383.7 Ma. Radiometric dating of a sample from the bentonite at the top of the Onondaga placed it at 390 ± 0.5 Ma. The formation is time equivalent with the Floresta Formation of the Altiplano Cundiboyacense, Colombia.