SOUTHERN MEXICO AND CENTRAL AMERICA 



701 



Two distinct phases of orogeny are represented. First, the deformation 

 that resulted in the deposition of the Sepur strata, and second the involve- 

 ment of the Sepur in folding and thrusting as noted in Guatemala in the 

 Departamentos de Huehuetenango, Alta, Verapaz, and Peten. The last 

 phase must therefore be as late as Eocene or possibly post-Eocene. Mio- 

 cene strata overlap the earlier Tertiary rocks on the Peten area and are 

 hardly deformed. 



The Geologic Map of Central America (Roberts and Irving, 1957) 

 shows considerable Cretaceous strata lying on the crystallines of Hon- 

 duras and northern Nicaragua, and also that the Cretaceous strata have 

 been folded. The obvious fold axes are indicated on Fig. 43.1. It is there- 

 fore evident that the Laramide fold belt spread southward in this part 

 I of Central America and involved the crystalline belt somewhat. 



I 

 SOUTHERN GULF COASTAL PLAIN 



The Southern Gulf Coastal Plain is made up of marine Cenozoic sedi- 

 ' ments and some volcanics, and it extends as shown on Fig. 43.1 along 

 the east side of the Sierra Madre Oriental southward through the State 

 of Vera Cruz and thence in a narrow belt eastward through Tabasco to 

 the Yucatan Peninsula. It is divided into a number of basins partly for 

 the convenience of petroleum exploration, and the boundary of these 

 basins with the fold belt is not well defined nor usually agreed upon 

 (Benavides, 1956; Guzman, 1959). See Fig. 41.9. 



The strata of the Coastal Plain dip gently toward the Gulf of Mexico. 

 In the Coatzacoalcos region, or the Isthmus (of Tehuantepec) saline 

 basin salt intrusion structures are prominent, and to the east, gentle fold- 

 ing is prevalent. 



YUCATAN PENINSULA 



The terrane of the states of Tabasco, Campeche, and Yucatan is under- 

 lain by flat or very gently folded marine strata that range in age from 

 late Eocene to Pleistocene. On the whole, the Yucatan Peninsula consists 



of lowlands under 650 feet in height. As the strata are mostly limestones, 

 the country is almost destitute of rivers, and the rains sink quickl) through 

 the soluble and karsted limestone and gather in subterranean basins. The 

 water table lies at various depths down to 300 feet or more beneath the 

 land surface. 



Yucatan extends north beneath the Gulf of Mexico for at least 150 miles 

 as the Campeche or Yucatan Bank, and then descends abruptly into the 

 depths of the Gulf. Carbonaceous sediments are accumulating on the 

 shelf (Fig. 43.4). 



VOLCANIC FIELDS AND FAULTING 



Southern Mexico and Central America are particularly noted for vol- 

 canoes, and volcanic rocks cover extensive areas. The map, Fig. 33.6, 

 shows the volcanic rocks of southern Mexico, where two general ages are 

 recognized, the mid-Cenozoic and the late Cenozoic. The older volcanics 

 make up the southern end of the extensive Sierra Madre Occidental 

 province, but the younger eruptives are in the form of an east-west belt of 

 stratovolcanoes including such well-known cones as Arizaba, Popoca- 

 tepetl, Ixtaccihuatl, Paricutin, and Colima. The belt of stratovolcanoes 

 has been called the Trans-Mexican volcanic belt (de Cserna, 1958), and 

 from the map of Fig. 43.1 it can be seen to extend from the Bahia Ban- 

 deras to the Gulf of Mexico below the city of Vera Cruz. 



Beginning in Chiapas, not far from the east end of the Trans-Mexico 

 volcanic belt is another great belt of modem volcanoes which stretches 

 through southern Guatemala, El Salvador, southern Nicaragua, and 

 western and central Costa Rica. In El Salvador major faults trend north- 

 westward parallel to the volcanic chain and to the coast line. Lago de 

 Ilopango occupies a graben that developed over a long period of time 

 and was partly filled by a succession of lavas and pvroclasties during its 

 formation. Other lake basins such as Lago de Atitlan and Lago de Ama- 

 titlan in Guatemala also probably formed, at least partly, by collapse 

 ( Roberts and Irving, 1957 ) . The extrusive rocks are olivine basalt, basalt, 

 labradorite andesite, and in lesser distribution dacite (Weyl. 1956). A 

 transverse zone of faults across Honduras from the Gulf of Fonesea to the 



