The correlation of clasts in Eocene river gravels in Baja California with the sources for the clasts in Sonora has indicated the necessity for the dilation of the northern Gulf of California depression prior to, or concurrent with, the 23-18 m.y. opening of the Basin Range province in Sonora. The appearance of an extensive, basin-filling seaway in Baja California and Sonora during the Miocene is consistent with a regional subsidence resulting from crustal extension. The presence of shelf-type lower Miocene marine sediments on both coasts of Baja California Sur, between Loreto and La Paz, suggests that the marine waters entered the early Gulf across a shallow, nontectonic seaway which opened to the Pacific Ocean.

The recognition of the early Miocene seaway extending into the northern Gulf, coupled with a later (4-10 m.y.) opening of the modern Gulf, supports the idea of a Gulf formed in at least 2 tectonic stages.

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