698 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



related to the Nevadan orogenic belt (see Chapter 35). The older intru- 

 sions are overlain in Chiapas by late Paleozoic sediments and locally 

 elsewhere in the Sierra Madre del Sur by Lower Cretaceous and Jurassic 

 marine strata. A few scattered observations of the direction of foliation 

 seem random, and so no conclusions are yet justified regarding the trends 

 in the metamorphic complex (de Cserna, 1958). 



In northern Honduras and Nicaragua the metamorphic belt is labeled 

 "probably Precambrian" on the Geologic Map of Central America ( Roberts 

 and Irving, 1957), and it consists of undifferentiated schist, gneiss, phyl- 

 lite, quartzite, and marble. Sapper ( 1937 ) has emphasized that the major 

 structures here are pre-Permian. 



PERMIAN FOLD BELT 



The Geologic Map of Central America shows a belt of folded Permian 

 strata across central Guatemala. The Chochal limestone and Santa Rosa 

 limestone, conglomerate, shale, and sandstone are the formations identi- 

 fied. They are intruded by granite and serpentine, which may be Late 

 Permian or Triassic in age, and were involved in the orogeny in which 

 the Permian rocks were folded. The rocks of the Santa Rosa formation 

 become progressively more metamorphosed to the east so that shale be- 

 comes phyllite and schist in eastern Guatemala and in Honduras. The 

 Jurassic Todos Santos formation rests on the folded Permian rocks. 



The crystalline complex was widely blanketed by Upper and Lower (?) 

 Cretaceous rocks in Honduras and Nicaragua, and these have been folded 

 in the Late Cretaceous and Early Tertiary orogeny. 



An area of outcrop of the Santa Rosa formation with granitic intrusions 

 occurs north of the main Permian fold belt in British Honduras. This may 

 indicate that the original Permian fold belt was once wider than now, and 

 that the later Cretaceous fold belt largely covers it. 



LATE CRETACEOUS AND EARLY TERTIARY FOLD BELT 



General Characteristics of Mexican Fold Belt 



The strata of the Mexican geosyncline (Chapter 28) extend into 

 southern Mexico and overlap broadly southward on the crystalline belt. 



These sedimentary rocks are chiefly Jurassic and Lower Cretaceous car- 

 bonates, with some fine elastics near the base, and Upper Cretaceous 

 shales. The Albian and Cenomanian seas advanced widely over the 

 southern and southwestern crystallines and deformed Paleozoic rocks 

 which had previously been land and the source areas for the Jurassic 

 and earlies Cretaceous sediments (de Cserna, 1958). At places, however, 

 Lower Jurassic strata rest on the crystallines, as in Oaxaca. Laramide 

 compression then deformed the Jurassic and Cretaceous strata and a long 

 system of folds resulted. These are labeled on Fig. 43.1 the Late Creta- 

 ceous and Early Tertiary fold belt. For the most part the folds are asym- 

 metrical toward the northeast. The fold belt is broad in northern and 

 central Mexico (Chapter 28), but narrows southward and is marked es- 

 sentially in southern Mexico by the Sierra Madre Oriental (de Cserna, 

 1958). 



Red Conglomerates in Central and Southern Mexico 



A number of occurrences of Early Tertiary red conglomerates have 

 been noted in central and southern Mexico (Edwards, 1955). They are 

 particularly important in deciphering the Laramide and Tertiary history 

 of the fold belt. The localities where the red conglomerates are known 

 are shown on the map of Fig. 43.2. The three areas studied are noted on 

 Fig. 43.1 where they may be seen in relation to the crystalline belt and 

 fold belt. They are the Zacatecas, the Guanajuato, and the Taxco. 



The oldest rocks in the Guanajuato City area are folded, hard, black, 

 thin-bedded, marine shales which now appear in places as phyllites or 

 schists. Small quantities of limestone, sandstone, and volcanics appear in 

 the series. No fossils have been found but on the basis of lithologic 

 similarity they have been correlated with the Upper Triassic shales at 

 Zacatecas City (Edwards, 1955). Six miles northwest of Guanajuato the 

 La Luz schist occurs which contains an amygdaloidal basalt about 1000 

 feet thick. A dense, dark gray limestone, possibly of early Cretaceous age, 

 is believed to have once covered the Triassic (?) shales, but was removed 

 locally before the overlying conglomerates were deposited. 



After the full development of the Mexican geosyncline the main Lara- 

 mide orogeny occurred. Folds are the main exhibit (see Chapter 28) but 

 here in south-central Mexico, considerable plutonism occurred. A deeply 



