TERTIARY-CARIBBEAN MOLLUSKS AND THE ANCIENT GULF OF CALIFORNIA

The earliest Gulf fossils are Middle Miocene [15.97 – 11.61 Ma, Gradstein et al., 2004] in age, mainly from subsurface sediments deposited at the end of subduction and arc volcanism (Helenes et al., 2009). The Late Miocene gulf [11.61 – 5.33 Ma] was more extensive, with more diverse Tertiary-Caribbean species living on continental platforms in shallow marine environments. When analyzed with associated dated volcanic units and both contemporary and reworked microfossils, megafossils provide finer resolution for the timing of tectonic events. Key assemblages include late Middle Miocene or early Late Miocene index fossils near Cabazon, CA, SW Isla Tiburón and the Boleo basin. Guide fossils date parts of the Imperial Formation in the Salton Trough, the Matomí Member of the Puertecitos Formation, and the lower Trinidad Formation, San José del Cabo trough as Late Miocene. These species are also found in the Tertiary-Caribbean province, in Trinidad (Springvale Formation), Venezuela (Caujaro Formation), Panamá (lower Gatún Formation), Perú (Zorritos Formation), and unnamed units in central Costa Rica and western Colombia. Refined correlation depends on taxonomy that uses the oldest valid name for a taxon rather than multiple synonyms described from different basins. Better integration of improved fossil and field data can inform tectonic models based mainly on structural and volcanic data. Fossils occurring in interfingering facies once described as sequential Miocene and Pliocene formations in the Cabo basin may be contemporary ecologic variants. Although the Trinidad Formation shares some biofacies with the Salada Formation and San Ignacio Formation of western Baja California, some species formed living populations in the Late Miocene or Early Pliocene Imperial Formation of California. Wilson and Rocha (1955) mapped the basal Boleo coquina overlying what Sawlan and Smith (1984, SEPM Book 39) called a dacite dome (K-Ar age 12.3±0.4 Ma). Holt et al. (2000) dated the entire formation from a lithic tuff 140 m up section (40Ar/39Ar age 6.76±0.9 Ma); Conley et al. (2005) regarded the basal Boleo Formation as <8 – 7.1 Ma. Miocene Caribbean index species (Leopecten gatunensis, Murexiella textilis, Turritella altilira) suggest the coquina is closer to 11 Ma in age.