SOUTHERN MEXICO AND CENTRAL AMERICA 





Caribbean is shown on the Map of Mexico, and the Valle de Comayagua 

 is regarded as formed by block faulting. Parallel structures along the 

 coast of Rritish Honduras may be related to this transverse fault zone. 

 Uplift accompanied the volcanism over much of Central America ( Roberts 

 and Irving, 1957). 



Reference has been made to the numerous submarine volcanic cones 

 on the Pacific floor adjacent to Central America, and also to the Central 

 American trench, in Chapter 32. See especially Fig. 32.5. 



ISTHMIAN VOLCANIC LINK 



Southernmost Nicaragua, Costa Rica, and Panama constitute the so- 

 called Isthmian link. By it the more broad and massive foundations of 

 North and Central America are connected to South America. The rocks of 

 the link are largely igneous and stratified deposits derived mostly from 

 igneous formations. 



The Cordillera de Talmanca in central and southeastern Costa Rica 

 has been studied in considerable detail by Weyl ( 1956, 1957 ) and the 

 table of Fig. 43.5 gives the history of the isthmus there as he depicts it. 

 The Cordillera Central referred to in the table is just west of the Cordil- 

 lera Talamanca and is the eastern end of the stratovolcanic belt. 



The record in the Sierra Talamanca goes back only to the Oligocene, 

 but as may be seen, it is one of sinking and volcanism; folding, erosion, 

 and deposition; intrusive activity with much metamorphism; more fold- 

 ing; and finally uplift, faulting and more volcanism and the building of the 

 modern cones. 



The geology of Panama begins with a basement complex of Eocene 

 and possibly pre-Eocene (Terry, 1956) or Cretaceous (?) age (Wood- 

 ring, 1957) which crops out over half the country. It is made up pre- 

 dominantly of altered basic flows, agglomerates, tuffs, and diorite 

 intrusions. It is strongly deformed but little metamorphosed, although 

 argillites are known and schist float has been reported in two places. 

 The areas of outcrop of the basement complex are anticlines or horsts, 

 and their axes are shown on Fig. 43.1. 



EPOCH 



DEPOSITS 



STRUCTURAL EVENTS 



MAGMATISM 



PLEISTOCENE 



River gravel 

 and sand 



Uplift, faulting, 

 arching 



Later volcanisa 

 in the 

 Cordillera 

 Out r.i 1 



PLIOCENE 



Suretka conglomerate 



Relative quiet 

 Folding of 





MIOCENE 



U. 



Molasse sediments 

 on mountain flank 



border zone 



Late orogenic 

 intrusive 

 activity 





H. 



Gat un conglomerate 



in the mountains 



L. 



Limestone, marl 



Geosynclinal 



OLIGOCENE 



Tuff 



Chert 



Sandstone 



Marl 



Tuff 



Limestone 



sinking 



Beginning of 



basic 



volcanism 



EOCENE 



Unknown foundations 





Fig. 43.5. History of the Cordillera de Talamanca in Costa Rica. After Weyl, 1956. 



The strong deformation of the Cretaceous lavas and sediments 

 occurred in Late Cretaceous, Paleocene, or early Eocene time (Wood- 

 ring, 1957). 



The basement complex is overlain on the flanks of the uplifts by 

 sediments and volcanics of several kinds, earning fossils in many places. 

 Fossil collections indicating late Eocene, Oligocene, early, mid-, and 

 late Miocene, Pliocene, Pleistocene, and Recent have been described 

 (Terry, 1956; Woodring, 1957). A section across the isthmus at the 

 Canal Zone is shown in Fig. 43.6. 



Volcanism again reached a climax during Oligocene and early Mio- 

 cene time. The rocks have been identified as diorite, quartz diorite, 

 dacite, andesite, and basalt (Woodring, 1957). 



A number of trans-isthmian faults have been postulated by Tern 

 (1956). These are shown on Fig. 43.1. Two of them in the Canal Zone 

 and one farther west have strike slip movement, and two in eastern 

 Panama are high-angle thrusts. The Panama Canal Zone seems to be 

 the most complicated area, but this may be the result of more in- 

 tensive field work there than elsewhere. In general it is more de- 



