Volcanic strata on southwest Isla Tiburón define the age of interstratified marine rocks and, through revision of existing correlations, the age of the proto–Gulf of California marine incursion. A 5.7 ± 0.2 Ma ash flow was emplaced at the base of the marine section. A rhyodacite dike and its related lava flow, dated as 11.2 ± 1.3 Ma, 3.7 ± 0.9 Ma, and 4.2 ± 1.8 Ma, intrudes and overlies, respectively, the marine rocks. The 11.2 Ma age, which was the core datum for a middle Miocene proto–Gulf of California origin for the underlying rocks, is discordant with all other isotopic and microfossil ages. An alternative interpretation, utilizing all available geologic and geochronologic data except this discordant age, is that marine strata on Isla Tiburón are latest Miocene to early Pliocene age. Reinterpretation of these strata supports a simplified history of marine incursion into the Gulf of California. Marine rocks as old as 8.2 Ma in the southern Gulf of California indicate an early marine incursion, perhaps flooding a region of more intense proto–Gulf of California continental extension. Flooding of the entire basin by 6.5–6.3 Ma correlates to the sudden onset of significant Pacific–North American plate-boundary motion within the Gulf of California.