Early Tertiary volcanic clasts were collected from the streambed of the Rio Morti near the village of Morti, eastern Panama, as part of a reconnaissance study. The samples range from basalts to rhyolites. K-Ar dates cluster around 58 Ma. The phenocryst mineralogy of the samples is typical of that found in arc-related volcanics: plagioclase (the dominant phase), clinopyroxene, titanomagnetite, and minor orthopyroxene. The clinopyroxenes are augites that plot in the field of orogenic lavas. The geochemistry of the rocks—high ratios of large-ion lithophile elements to high-field strength elements, negative Nb and Ta anomalies, positive Ba anomalies, and relatively low Th to U values—confirms that they are arc related (specifically the calc-alkaline series) and strongly suggests that the samples are not cogenetic. Volcanic rocks with similar ages are exposed in several other localities throughout Panama and Costa Rica (e.g., the Azuero and Sona Peninsulas in Panama and the Nicoya Peninsula in Costa Rica as well as other areas in the Darien of eastern Panama). We suggest that there may have been a more or less continuous arc from South America to the Chortis block of Nicaragua during the Paleocene-Early Eocene. The presence of the arc would imply that the breakup of the Farallon plate and its subduction below the new Caribbean plate in present-day southern Central America probably started some time close to the Cretaceous-Tertiary boundary.

Abstract The “backbone” of Colombia is the northern part of the Andes Mountains. The mountain system is here divided into the Eastern, Central, and Western Cordilleras. The Central and Eastern Cordilleras are separated by the down-faulted basin of nonmarine Cenozoic deposition through which the Magdalena River flows in its middle and upper reaches. The valleys of the upper Cauca and upper Patía Rivers mark the approximate boundary between the Western and Central Cordilleras. The Santa Marta and Perijá Mountains and the Pacific Coast Range are related to the Andean system. The structural grain is north-northeast in the southern part of the Colombian Andes. To the north, major alignments are northeast and northwest. The Sautatá arch connects the western Cordillera with the Darién Mountains of Panamá. The Guaviare arch links the Andes and the Guayana shield. The syenitic rocks and high-grade metamorphics of the Garzon-Guaviare region are the only known Precambrian west of the Guayana shield. A record of marine Paleozoic sedimentation is preserved in the Eastern Cordillera and Perijá and the Macarena Mountains. Periods and epochs represented are—Late Cambrian?, Early and Middle Ordovician, Middle Devonian, Mississippian, Pennsylvanian, and Permian. There is evidence of “Caledonian” orogeny and plutonic igneous activity. No Paleozoic marine sediments have been found west of the Central Cordillera. Interbedded marine and terrestrial sediments and volcanics of Late Triassic and Early Jurassic age crop out along the west side of the Upper Magdalena Valley, but to the northeast, only continental sediments are present. A widespread marine invasion began in latest Jurassic time and affected the Andean region and its eastern foreland during the Cretaceous. The thick marine Cretaceous deposits east of the Central Cordillera and Santa Marta Mountains are a major source of oil. The area of the Bucaramanga massif was not submerged until Aptian time. West of the Central Cordillera, Cretaceous Sediments are interbedded with porphyries and diabase flows. Orogeny began in the Maestrichtean and reached a peak in the middle or late Eocene. Basic and ultrabasic rocks were intruded in the Western Cordillera and Pacific Coast Range regions. Some of the granodiorites of the Central Cordillera may have been intruded at this time. Tertiary marine deposition was restricted to regions west and north of the Andes. A thick nonmarine Tertiary sequence elsewhere includes reservoirs containing more than 80 per cent of Colombia’s proved oil reserves. There was renewed orogeny at the close of the middle Oligocene, with extrusive igneous activity in the Central and Western Cordilleras. A thick section of marine sediments was deposited west and north of the Andes in the late Oligocene and early Miocene. Elsewhere, downwarped and down- faulted belts continued to receive continental deposits. The most recent orogenic and volcanic activity began in early Miocene and continues today, the orogeny affecting older positive belts and the Tertiary basins of marine deposition.