Why are magnetic reversals less useful for dating Cretaceous rocks than Cenozoic rocks?

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Cretaceous paleogeography — Cretaceous Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cretaceous Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Magnetic reversals are less useful for dating rocks from the Cretaceous Period than those from the Cenozoic Era primarily because of a significant scarcity of reversals during the Cretaceous.1

While geomagnetic reversals occur frequently in Cenozoic rocks—providing a detailed paleomagnetic chronology—the Cretaceous record is characterized by long periods of stability. Specifically, there are only 16 reversals noted from the latest Jurassic to the Aptian stage, no reversals at all from the Aptian to the late Santonian, and only nine reversals between the late Santonian and the Cenozoic boundary. Because these reversals are infrequent, they provide fewer reference points for geochronology compared to the more dynamic magnetic history of the Cenozoic.12