43. 



Precambrian ages. A belt of deformed Permian strata with Permian (?) 

 granitic and ultrabasic intrusives makes up part of the crystalline belt 

 through Oaxaca, Chiapas, Guatemala, and northwestern Honduras. 



A system of folds in Jurassic and Cretaceous basin-type sediments 

 (the Mexican geosyncline in Mexico) extends along and inside the crystal- 

 line belt from southern Mexico eastward through British Honduras to 

 the Caribbean and projects toward Cuba, the Yucatan basin, and the 

 Cayman trench. It is called the Late Cretaceous and Early Tertiary fold 

 belt on Fig. 43.1. 



Facing the Gulf of Mexico is a narrow coastal plain which extends to 

 the broad platform of the Yucatan Peninsula and Campeche Banks. 



The crystalline and fold belts and the Coastal Plain are referred to as 

 nuclear Central America, in contrast to the narrow volcanic province of 

 southern Nicaragua, Costa Bica, and Panama, which is called the Isthmian 

 link (Boberts and Irving, 1957). 



SOUTHERN MEXICO 

 AND CENTRAL AMERICA 



MAJOR GEOLOGIC DIVISIONS 



A great Cenozoic volcanic province, or possibly a complex of three or 

 four volcanic provinces, extends through southern Mexico and Central 

 America. The role of volcanism is most important in the geologic thinking 

 about the region. If however, the rocks older than the Cenozoic volcanics 

 are considered, a significant foundation geology becomes evident. A belt of 

 crystalline rocks extensively overlain by volcanics comprises the south- 

 western and southern coast of Mexico, of southern Guatemala, most of 

 Nicaragua, and all of El Salvador and Honduras. See map, Fig. 43.1. 

 These metamorphics are assigned variously Mesozoic, Paleozoic, and 



CRYSTALLINE BELT 



Metamorphic rocks crop out in wide areas in the Mexican State of 

 Sinaloa which borders the southern part of the Gulf of California. Other 

 occurrences are shown inland at Parral in the State of Chihuahua. See 

 the new Geologic Map of Mexico (1956). All are labeled Mesozoic and 

 are identified as marbles and slates. The same rocks crop out on Las Tres 

 Marias. 



Beginning at the Bahia Banderas at the west end of the Sierra Madre del 

 Sur (maps, Figs. 35.1 and 43.1) and extending eastward through the 

 Sierra to the Chiapas-Guatemala border is a metamorphic belt noted as 

 Paleozoic in age on the Geologic Map of Mexico. The small amount of 

 data available indicates that the rocks consist of gneisses and schists, 

 possibly of Early Paleozoic age, and greenstone conglomerates and phyl- 

 lites, possibly of Late Paleozoic age. A large batholith in eastern Oaxaca 

 and Chiapas is intrusive into the metamorphic rocks and is considered as 

 Paleozoic in age by Boberts and Irving (1957) and as Mid-Paleozoic by 

 de Cserna (1958 and 1960). The metamorphics are also intruded by 

 Mid-Cretaceous (Cenomanian) stocks and batholiths which are probably 



696 



