. Bulletins of American paleontology. Baja California Stratigraphy: Carreno and Smith 103 for differences in sediment composition between the vertebrate-bearing rocks and those farther north, but detailed mapping and sedimentological studies are needed. Concliilioii Espinosa-Arrubarrena (1979) correlated the beds at Rancho Algodones with the Tirabuzon Formation of the Boleo basin on the basis of shark teeth. Moliuscan fossils from unnamed, reworked, marine sediments of the southern Cabo Trough also include a number of in common with the late Middle to Late Miocene section at southwestern
. Bulletins of American paleontology. Baja California Stratigraphy: Carreno and Smith 103 for differences in sediment composition between the vertebrate-bearing rocks and those farther north, but detailed mapping and sedimentological studies are needed. Concliilioii Espinosa-Arrubarrena (1979) correlated the beds at Rancho Algodones with the Tirabuzon Formation of the Boleo basin on the basis of shark teeth. Moliuscan fossils from unnamed, reworked, marine sediments of the southern Cabo Trough also include a number of in common with the late Middle to Late Miocene section at southwestern Isia Tihuroii and parts of the Imperial Formation in the Salton Trough (Gastil ct 1999). They include "Aequipecten" iiuiscosiis (\\lood). CliUunys mediacostata (Hanna). Aiuidara tlnniiini (Maury) [= A. currizoeiisis (Reinhart) described from the Imperial Formation], Codakia sp. cf. C. orhiciilnris (Linnaeus). Conns spuriiis Gmelin. Stroinhus ohlitcr- atiis Hanna, and Tiinitella iiupcrialis Hanna. Islas Tres Marfas. Nayarit Plate 2, Column 4.'^ (Text-figs. , Appendices 1,2) Column modified from Canefio (1985), McCloy et ul. (.1988; and Chinas-Lalo (1963). Overview The Ties Marias Islands are important to Gulf of California history because of their position 100 km offshore from Punta Mita. Nayarit. Mexico. The rocks record part of the Late Miocene to Pleistocene history al the mouth of the Gulf, although the section is considerably thinner than correlative 5- to 6-kin-thick Neogene sequences in the northern Gulf (J. T Smith. 1989). Early interpretations of a middle Pli- ocene ancient gulf must be modified to accomodate evidence of seawater in the northern gulf as much as eight million years earliei' than the onset of spreading at its mouth at 4-6 Ma (Larson. 1972; Hagstrum a 1987). Of the Islas Tres Marias, Isla Maria Madrc is the largest and has the most extensive stratigraphic Isla Maria Madre Arroyo Hondo Section ^ , â l c . â ' Caleras
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