Hekkingen Formation shales were deposited in ∼19 Myr, from ∼157.7 Ma to ∼138.8 Ma. Re–Os ages constrain the onset and duration of regional anoxia. Radiometric ages refine the Jurassic time scale. Late Jurassic anoxia is likely related to warm climate. The long duration of anoxia required a sustained CO2 source.