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Tertiary marine mollusks in sediments associated with radiometrically dated volcanic rocks provide improved resolution for correlating upper Oligocene to Pliocene sediments in the Gulf of California and its northern extension, the Salton Trough of California. The megafossils discussed in this report came from the following formations: San Gregorio Formation, El Cien Formation, Isidro Formation, Trinidad Formation, Salada Formation, San Ignacio Formation, Boleo Formation, San Marcos Formation, Tirabuzón Formation (= Gloria Formation of Wilson, 1948), Carmen Formation, Marquer Formation, Imperial Formation, Ferrotepec Formation, and unnamed deposits at San Felipe, Isla Tiburón, Rancho el Refugio, and the La Mira basin, Michoacan.

Paleoshorelines are identified by littoral mollusks whose faunal affinities are with the Tertiary Caribbean and Pacific-Panamic provinces. By the late middle Miocene (13 Ma), the sea extended to Isla Tiburón, Sonora, where it supported a distinctive molluscan fauna. Before the late Miocene (6 Ma), some of the same mollusks lived as far north as the Salton Trough, as seen from Imperial Formation megafossils at San Gorgonio Pass, California.

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