Abstract

Reconnaissance exploration along the Pacific coast of Mexico from the Isthmus of Tehuantepec to Puerto Vallarta in the northwest, beginning in 1977, has shown the presence of at least seven previously unreported areas with late Cenozoic sedimentary deposits. In addition marine Pleistocene deposits on the coast of Oaxaca were previously reported. In four of the newly discovered areas marine sediments occur. All of these occurrences, excepting the two southeastern ones, were encountered along the Federal coastal highway, route F 200. Beginning in the northwest these areas are: along the Rio Ameca, 10 km north of Puerto Vallarta; 10 km south of Puerto Vallarta; Tomatlan Basin (named herein), 79 km south of Puerto Vallarta, with fossil vertebrates; Campo Acosta Basin (named herein) beginning 115 km south of Puerto Vallarta, sediments at least partly marine; La Mira Basin (named herein), straddling the mouth of the Rio Balsas, with at least two marine incursions; San Nicolas Basin (named herein) about 200 km southeast of Acapulco, marine sediments encountered in water well; Colotepec Formation, marine, along the coast of Oaxaca between Puerto Angel and Puerto Escondido; marine deposits near Santa Cruz, Oaxaca, about 100 km southwest of Salina Cruz, Oaxaca. Of the seven new areas, only the La Mira Basin has been examined in more than a most cursory manner.

In the La Mira Basin four formations are recognized: Ferrotepec Formation (named herein), marine, late early to middle Miocene in age; Barranca del Perro Muerto Formation (named herein), nonmarine; El Bordonal Conglomerate (named herein), nonmarine; Abanico Brisa Mar Formation (named herein), at least partly marine, of undifferentiated Plio-Pleistocene age. If the attitudes observed in the Ferrotepec Formation continue seaward, the total thickness of the sediments in the basin might attain 1,500 m. The fossils from the Ferrotepec Formation mostly belong to the “Tertiary Caribbean Province” of Woodring, but a few show affinities to north Pacific coast Miocene faunas. The La Mira Basin sediments indicate that the Rio Balsas has existed since early Miocene time.

The few fossils from the San Nicolas Basin suggest a late Miocene to Pliocene age. The few fossils from the beds near Santa Cruz are suggestive of a Plio-Pleistocene age.

Insofar, as is known, the sediments in the areas northwest of the La Mira Basin only attain a few meters in thickness. Offshore from the San Nicolas Basin, at Deep Sea Drilling Project (DSDP) Site 493, about 670 m of early Miocene to Pleistocene sediments are reported. The Santa Cruz beds appear to be about 50 m thick, and Palmer reports a 15 m thickness for the Colotepec Formation.

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